1. Muscleskeletal system.pdf
3. Neurovascular system.pdf
SUBJECT OF ANATOMY, ITS METHODS AND TERMINOLOGY
Human Anatomy is the study of form, structure, origin and development of systems and organism in general. This study belongs to the biological sciences united by a common term “morphology ” (from the Greek “morpho” – form, “logos” – teaching).
The task of anatomy as a science is a systematic approach to describe the shape, structure and location (topography) parts of the body and in the unity of the functions performed taking into account age, sex and individual features of human bodies. According to this, it’s divided into systemic anatomy, topographic (surgical), age-related, comparative, plastic, typical, pathological.
Systemic anatomy studies the form and structure of body parts that are united in systems. That’s why it includes study of bones (osteology) and their compounds (Arthrology), the muscles (Myology), visceral organs (Splanchnology), cardiovascular system (Angiology), nerve system (Neurology), internal secretion (Endocrinology) and senses (Esthesiology).
Topographic (surgical) anatomy (from the Greek topos – place, grapho – write) examines the relative position of organs, blood vessels and nerves in various parts of the body that is essential for surgery.
Age-related anatomy explores age factors anatomical features of individual human development – ontogeny (from the Greek “ontos” – person, “genesis” – development). Human development before birth, particularly in the embryonic period, considering embryology (from the Greek “embryon” – embryo) and seniors studying gerontology (from the Greek “geron” – old).
Comparative anatomy studies the similarities and differences of the structure of animal and human bodies, examines the structural features of the body of animals at different stages of evolution, helping to clarify the historical development of the human body – phylogeny (from the Greek ”phylon” – family).
Plastic anatomy explores statics and dynamics of the outer body shapes. It considers internal structure mainly in order to understand the severity of the external forms of the human body (it is mostly taught in art schools in the preparation of sculptors and painters).
Pathological Anatomy (from the Greek “pathos” – illness, suffering) studies the structure of the body, changed under the influence of various diseases and injuries.
Human anatomy seen as part of anthropology ( from the Greek “anthropos” – man) – the science of human origins and development, the formation of human races and options of human structure.
Human anatomy is closely linked with a number of other morphological subjects, including cytology (from the Greek “kytos” – cell) – the science that studies the structure, functioning and development of cells. There is general cytology, which studies common to most types of cell structures, their functions, metabolism, response to damage, abnormal changes reparative processes and adapt to environmental conditions. In addition, there is special cytology – section cytology, exploring the characteristics of individual cell types due to their specialization or adaptation to habitat. Histology (compound of the Greek words: ἱστός histos “tissue”, and -λογία -logia “science”) also belongs to morphological disciplines. It’s a science of development, microscopic and ultramicroscopic structure of cells and tissues of plants and animals, their life cycle. They distinguish evolutionary histology, the division in histology that studies patterns of tissue in the phylogeny; environmental histology, the division that studies the structure and features of tissue due to the influence of living conditions and adaptation to the environment; general histology; special histology; comparative histology etc.
Modern human anatomy, as a science of the XXI century, synthesizes data of related and common to anatomy disciplines – histology, cytology, embryology, comparative anatomy, physiology as well as biology, anthropology and ecology in general. Now, anatomy considers the form and structure of organs and systems of the human body as a product of heredity, which varies depending on certain conditions, biological and social environment and the work performed by the body over time (filota ontogeny) and location (in different regions of the world).
Modern anatomy is using a large set of techniques for anatomical studies that are constantly changing, improving and supplementing according to the progress and achievements of related studies, general technical progress. The main methods of research in anatomy are macroscopic, marco-microscopic, microscopic, electron microscopic, histochemical, spectrofluorometric etc. And among the ones used during the lifetime (essentially anatomical) methods of instrumental research are X-ray, endoscopic, ultrasonic, thermographic, magnetic resonance images and more.
Macroscopic is the most common method in anatomy. It includes: 1) somatoscopy (external body review to determine its size, shape of body areas, biological signs of maturity of the body); 2) anthropometry (measurement of individual body parts according to defined rules, study their proportional relationships, determine the type of constitution of the researched person); 3) preparation (study of body structure using sections and corresponding methods of removal); 4) serial section of frozen corpse or its parts by N.I. Pirogov (to clarify the topographical location of organs, blood vessels, nerves, fascia, etc.); 5) injection of vessels with contrast or colored masses, corrosion, enlightenment (to determine the form and structure of blood vessels and hollow organs); 6) maceration (method of manufacturing drugs for bones rotting and separation of soft tissues from bones).
Macro-microscopic method (by V.P. Vorobiev) – a preparation method of total objects using microsurgical instruments utilizing optical devices with zooming in up to 5-40 times. Selective coloring of nerves (eg methylene blue) or vessels injection with colored fillings is used at the same time.
Microscopic method (as a collection of histological and histochemical methods) is often used in modern anatomy, as well as transmission electron microscope (increasing in 100-500K times) and scanning electron microscope (reproducing three-dimensional image ultrastructure).
Anatomy as science has its own conceptual material that is based on the anatomical nomenclature. Hippocrates (460-377 gg. BC), K. Galen (131-200 AD.), A . Vesalius (1514-1564) were the founders of anatomical nomenclature (scientifically proven list of anatomical terms used in medicine and biology).
A truly international status anatomical terminology acquired in 1895, when the IX Congress in Basel anatomical Union approved the International anatomical nomenclature. The list of Latin and ( partially) Greek terms is known as Basel anatomical nomenclature (Basele Nomina Anatomica, reduced BNA).
With the development of the morphology of anatomical terms needed refinements and additions. Therefore anatomical terminology was repeatedly revised. Paris the anatomical nomenclature (Parisiensia Nomina Anatomica, abbreviated as RNA) was approved in 1955 by the Federal Cartel IV International Congress of Anatomists. It was based on BNA, of which 4,286 borrowed terms; there were 1354 new titles.
Modern medicine is one of the areas of science and professional activity, which is characterized by high levels of international integration. This can be achieved only through commonly synchronized and widely used unique terminology base all around the world, as well as anatomical. Based on the needs of modern medicine, Federal Committee of Anatomical Terminology (abbreviated FCAT) adopted a new simplified and modern universal anatomical nomenclature, which has 7428 terms, in August 1997 in Sao Paulo (Brazil).
This tutorial uses precisely this latest anatomical nomenclature. Latin terms and their Ukrainian equivalents are used according to the publication “International anatomical nomenclature” by I. I. Bobrik, V.G. Koveshnikova in 2001. (designed for Latin, Ukrainian, Russian and English equivalents anatomical terms of new Latin-English (Stuttgart – New York, 1998), Ukrainian (Kyiv, 2001) and Russian (Moscow, 2003) anatomical nomenclature – International anatomical terminology (Latin, Ukrainian, Russian and English equivalents) with authors V.G. Cherkasov, I. I. Bobrik, Y. J. Huminskyy, A. Kovalchuk.
SHORT ESSAY IN HISTORY OF ANATOMY
Anatomy of the Ancient World, the Middle Ages and the Renaissance
The first anatomical knowledge originated in ancient times, long before the writing (drawings by the man of cave period as an evidence). The first written source, such as the Chinese book “Neytszyn” and the Indian book “Ayurveda “, appeared in the eleventh to the 9th century BC.
Modern medicine is based on the European medical tradition that reaches its roots to ancient Greece. During the development of ancient Greek culture scholars cutting of carcases (anatemno – divide, hence the word “anatomy”) in the way acquainted with the organs and systems of the human body, but this knowledge was fragmental and unstructured.The first work devoted to anatomy is attributed to Greek physician and philosopher Alcmaeon, who lived in the first half of 5th century BC. He was born in southern Italy, he studied under Pythagoras and was one of the founders of Croton medical school. Dissections of human bodies were banned because of religious reasons in ancient Greece. That’s why Alcmaeon of Croton was the first one to begin anatomize animals to study the structure of their organs for the needs of medicine.
An outstanding scholar of ancient Greece was Hippocrates ( 460-377 BC). For a huge contribution to the art of healing in anatomy and physiology during the life of Hippocrates was called the “father of medicine”. The doctor at that time was more of a philosopher than a naturalist, that’s why Hippocrates’ study was founded on the generalization of anatomical facts, that were accumulated as a result of the work of many other scientists, and his own materialistic views on the causes of the disease. The scientist has created a “humoral” theory. According to it, the human body involves a vital fluids such as blood, phlegm, yellow and black bile. According to the Hippocrates’ theory, the constitution and the temperament of a human being – sanguine, phlegmatic, choleric and melancholic – are determined by different ratios of these fluids. When they blended harmoniously in the body, a person is healthy. If disturbed liquids ratio inherent in the person of a certain temperament, then comes sickness or death.
Plato’s pupil, the great philosopher and lexicographer of the ancient world, Aristotle (384-322 BC), systematized and developed almost all known at that time, scientific theories and facts in the field of philosophy, logic, astronomy, history, psychology and science. In contrast to the idealist Plato (427-347 BC), Aristotle believed that the environment exists in reality, so it should be studied through the senses, observation and research. Aristotle is considered to be the founder of comparative anatomy and embryology, as he studied the anatomy of the body of animals and their embryos. The scientist came to conclusion that in embryogenesis organs do not occur immediately, but gradually, one by one, from a structureless mass. This theory later was called an epigenesis theory by a prominent British anatomist, physiologist and embryologist William Harvey.
The most prominent scientists-physicians of the ancient world after Hippocrates was the father of anatomy, Claudius Galen (131-200 AD), who was born in Pergamum and lived in Rome most of his life. The authority of Galen was so great that generations of doctors over thirteen centuries were studying by his writings on medicine and anatomy. Galen conducted anatomical experiments on animals. The obtained information he endured in man, which negatively affected the development of anatomy. Interestingly, the famous anatomist of 15th century, J. Silva (1478-1555), couldn’t find the differences between Galen’s observed anatomical facts and data, and he would rather believe that for the structure of man had changed over thirteen centuries than that Galen could be wrong.
A significant contribution to medical science was made by an outstanding physician and philosopher Abu Ali Ibn Sina (980-1037), better known in Europe as Avicenna. Abu Ali Ibn Sina wrote the famous book “The Canon of Medicine”, which was the ” Introduction to Anatomy and Physiology.” This book was a source of knowledge for doctors of East and West up to 17th century.
In the Middle Ages Medical science was completely subordinated to religion – touching the dead, except for ritual purposes was not allowed. The activities of many scientists and physicians was reduced to commenting and copying the works of Aristotle and Galen, because their anatomical achievements were considered infallible and irresistible.
The Renaissance became the beginning of the scientific study of human anatomy, when three great anatomists-reformers, Leonardo da Vinci, Vesalius A. and B. Harvey, were aware of the importance of knowledge of the body structure for medical affairs, checked on the corpses of people anatomical descriptions of the ancient Greeks, Romans, Arabs, Persians and noted the blunders that were occurred. From this period until the 21st century, the discoveries is morphology appeared one by one, and anatomists – they are doctors as well – began to perform a great job describing the new, still unknown anatomical formations, correcting old data, entering deeper into the detailed description of systems and individual organs structure.
The contribution of the great Italian artist and scientist Leonardo da Vinci (1452-1519) to the development of the science of the structure of the human body can not be overemphasized. He didn’t consider any authorities, realizing the futility of medieval scholasticism. Leonardo da Vinci was one of the first to cut the corpses of people and became a true pioneer in the study of body structure. This great scientist loved to repeat “who argues, referring to the authority, doesn’t use his own mind, but rather the memory”. Leonardo da Vinci has reached extreme accuracy in his drawings, visualising various organs of the human body, thus made a significant contribution to the development of anatomy and was the founder of the art plastic anatomy.
Great Flemish (Belgian) scientist Andrew Vesalius (1514-1564) made a revolution in anatomy – created a system of anatomical knowledge, which were based on numerous autopsy of the human body, and corrected misconceptions made by Claudius Galen about human anatomy that prevailed in medicine over 13 centuries. Realizing that the medicine may withdraw from medieval stagnation, consecrated inviolable authority of Galen, only under the conditions of progress of anatomy as a science of the structure and functions of the human body, Vesalius devoted his life to research the case. The result of hard work of this dedicated scientist was the launch of his his 7 books “On the structure of the human body” in Basle in June 1543, which were beautifully illustrated with engravings Stefan van Kalkar. This book Vesalius was the first scientific publication that contained the systematic anatomical data verified or first established during the preparation of the dead people, not animals.
The rapid development of anatomy and medicine in general began since Vesalius’ books were published. A clear understanding of the morphological basis of many clinical disciplines were followed by the first detailed descriptions of anatomical structures. Vesalius’ work was and still is today of a great scientific and educational value, it teaches bravely depart from all the outdated and reactionary in science and life, inspiringly to go forward to the true knowledge based on observation and experiment.
The eminent English physiologist, anatomist and embryologist William Harvey (1578-1657) discovered the important function of the body – blood circulation – and its scientific work created a whole epoch in science. W. Harvey studied the phenomena of nature, directly observing physiological processes experimentally investigating them based on materialistic natural law. His most outstanding discovery was circulatory functions and he published the book “Anatomical study of the motion of the heart and blood in animals” (1628 ), which finally destroyed the idea and the authority of vitalistic doctrine prevailed by Galen in the Middle Ages. Progressive teachings of W. Harvey on the circulation quickly won universal recognition and implemented the most favorable impact on the further development of medicine.
A year before B. Harvey explored the function of blood circulation, Italian anatomist Caspar Azelli described lymphatic vessels (1627 ). A little later the Italian scientist Marcello Malpighi discovered under the microscope blood capillaries (1661), whose existence is predicted W. Harvey. But it must be emphasized that in 1553 Michael Servetus described the small (pulmonary) circulation and explained the physiological sense circulation in the system. He was burned by the Inquisition for such “heretical” views. Thus, in the second half of the 17th century the concept of the structure and function of large and small circulation was finally formulated.
DEVELOPMENT OF ANATOMY IN UKRAINE
(from Kievan Rus to the present)
The beginning of studying medicine in our country is associated with the development of Kievan Rus, one of the most educated countries in Europe. There was an established organization of the help for sick and wounded, the treatment was carried out by specially trained and gifted “lyechtsy”.
Let’s recall that in Kievan Rus, starting from Scythia times was performed internal embalming of dead people. A variety of aromatic resins and vegetable oils was used for this purpose. This indicates a good anatomical knowledge that doctors had at that time. After the adoption of Christianity forced by Prince Vladimir Sviatoslavovych in 988, people autopsy was forbidden. Since then they began conducting an external embalming. The essence of this embalming was that the aromatic plant substances applied to the outside of the body from neck to feet and body tightly wrapped by the special scheme linen strips so called “lentia”. Bodies of Princess Olga, her grandson Vladimir and his sons Boris and Gleb were embalmed the same way.
After the introduction of Christianity, the expansion of ties with other countries led to the favorable conditions for the development of medicine in Ukraine. However, the church forbade doing autopsies of the dead, but those monasteries became the centers where the ancient works and manuscripts that contained medical knowledge were copied and translated. The most common in Kyivan Rus were books by Hippocrates, Aristotle, Galen, Avicenna.
Princess Anne Vsevolodovna (granddaughter of Yaroslav the Wise) opened a secular school at St. Andrew (Yanchina) monastery in Kyiv, which along with other subjects taught basics of medicine. According to historians of medicine, this school was one of the first known academic medical institutions.
The exercise of one of the principles of Christian doctrine “Faith without works is dead”, various charitable institutions functioned and shelter and hospital were founded in the monasteries of Kievan Rus. It is not surprising that “bezvozmezdnoe vrachevanye” (“medical volunteering”) was one of the objectives established in 1051 by the Kiev-Pechersk Monastery (known as Lavra). The first spreaders of medical knowledge were just monks of the monastery. First of all, it was Anthony, the founder of the monastery, which the chronicler calls “prechuden doctor”, and the monk Agapit Caves, who is also called “Ukrainian Hippocrates”. Agapit cured the sick with prayer and herbs and never took money for it. The icon labeled “Agapita bezmezdnyk” is hanging above the holy relics of St. In the Near Caves of Monastery. His students and followers are buried next to him – Damian Healer, Alimpiy Caves and Gregory Miracle Worker, whose names remain forever on the tablets of national history.
In the well-known and famous “Academy” founded by Yaroslav the Wise, who ruled in 11th century, there were many medical literature, including books Ionic Bulgarian translation for “physiologist” and “Shestydnyev”; which describes the structure of the human body and organs. Anatomical information is known in the books of the time such as The Church Arose, Izbornik of Sviatoslav, Rus Truth etc.
Science and medicine, under the influence of the Renaissance, developed in Ukraine more intense in the 14th and 15th centuries. Representatives of Ukraine studied at Jagiellonian (Cracow), Bologna and other Italian universities, the Sorbonne (Paris) since 14th century. In particular George (Yuri) Drogobych (1450-1494), originally from Western Ukraine, has studied at the Jagiellonian and Bologna Universities. The first known Ukrainian doctors of medicine, he taught anatomy and surgery at the Jagiellonian University, he works out in Latin in Rome.At the beginning of 1481, the student body of the University elected Drohobych to become the rector of the school of Medicine and Free Arts.
In the mid 17th century, an important center of Ukrainian science, education and culture, and also where he taught medicine, was the Ostroh Academy (Greek-Slavic-Latin collegium was the first institution of higher education degree in Ukraine) the so-called “Volyn Athens”. Its first rector from 1580 was known public figure and writer Gerasimos Smotrytsky. Ostroh Academy had a short life, but during its activities made a significant contribution to the development of Ukrainian medicine.
Starting from 1595, doctors were prepared at Zamostskiy Academy, about 100 km from Lviv, the city was a member of the Polish state back then. The Academy had university status with the right to grant degrees of Doctors of philosophy, Law and Medicine, as a proof was an evidenced Diploma from Pope Clement VIII (1594). Zamostska Academy lasted 190 years and was closed by the Austrian government in 1784 after the first partition of Poland .
Development of medical education in Ukraine helped to create the Kyiv Brotherhood school in 1632, at the initiative of Metropolitan Petro Mohyla, it united with Lavra school and became known as the Kiev-Brotherly or Kyiv-Mohyla Collegium (in honor of Petro Mohyla). It later won the legal rights of high school and the title “academy”. Kyiv-Mohyla Academy played a major role in training for hospital medical schools. In 1817 the Academy as a secular institution was closed. But after 175 years together with a whole statehood revival of Ukraine, this celebrated high school, now called the National University of “Kyiv-Mohyla Academy” was revived as well.
Kyiv-Mohyla Academy graduated prominent medical researchers who also worked in the field of anatomy: Epiphany Slavinetsky (1609-1675) translated Old Church Slavonic language textbook by A. Vesalius “Epitome” (1653); Konstantin Shchepin (Schepinskyy) (1728-1770) – the first who started to teach anatomy in Russian (1764) instead of Latin and Greek; Nestor Maksimovic Maksimovic – Ambodik (1744-1812) – wrote ” Anatomical and physiological Dictionary ” (1778), which was the impetus for the creation of national anatomical terminology; Nikon Karpovich Karpinski (1745-1810) – author of one of the first in the Russian Empire original anatomy textbook “Anatomy or truporoztyn “; Shumlyansky Alexander Mikhailovich (1748-1796) – before the English anatomist W. Bowman described the “membrane” which was called “boumenova capsule” (now in the world literature it is called “Shymlanskaya – Bowman capsule”).
Renowned our countryman, the famous anatomist and surgeon Ilya V. Buyal’skiy (1789-1866), who worked in St. Petersburg, made a great contribution to the development of anatomical science with his books – “Anatomiko surgical table” and “Brief general anatomy of the human body”.
An invaluable contribution to the development of anatomy did Academician Mykola Ivanovych Pyrogov (1810-1881), who created the topographic anatomy, introduced a new method in the anatomical study – consistent method of opening the frozen corpses. Works of Pirogov “Topographic anatomy to cut through frozen corpses,” “Surgical anatomy of arterial trunks and fascia”, “Full course Applied anatomy of the human body” brought him worldwide fame. Pyrogov was directly involved in the organization in 1841 of the Medical Faculty of the Imperial University of St. Vladimir in Kiev, and his disciples and followers professor M. I. Kozlov and A. P. Walter launched the famous Kiev School of Anatomists.
I. Kozlov (1814-1889) and O. Walter (1817- 1889) not only perfectly organized educational process at the department of anatomy University of St. Vladimir, who remained a model for all times, but also acted as founders of functional direction of scientific activity Kyiv Anatomical School. This trend was continued by O. P. Walter’s successor, Professor in the department Vladimir Betz (1834-1894), who brought worldwide fame to Ukrainian science by his works on the morphology of the central nervous system (in 1874 he discovered the giant pyramidal cells of the motor in the fifth layer cortex of the brain – so-called “Betz cells”). School inherent in the functional direction reflected in future: in research by professor M. Tikhomirov (1848-1902) of options arteries and veins rights; in works on the anatomy of the lymphatic system Professor F. A. Stefanisa (1865-1917), which were continued by Professor M. S. Spirovo (1892-1973), and his disciples; the study microcirculation in morphological bases functionally different organs in ontogeny, initiated by Professor I. E. Kefeli (1920-1980) and continued in our time by Professor I. I. Bobrik and his disciples. Scholarship and educational activities of such scientists as M. I. Kozlov and Walter became the foundation of the national anatomy and the key to its future development.
In Ukraine (apart of Kyiv) were also formed other powerful Lviv, Kharkiv and Odessa (Novorossiysk) Anatomical Schools.
Lviv Anatomical School was founded by Anton Marghera in 1784. Its outstanding representatives – Professors Henry Kadi (1815-1912), Joseph Anton Markov (1874-1947), Thaddeus Martsinyak (1895-1966), A. P. Lyubomudrov (1895-1972), E. F. Goncharenko (1912- 1979), V. F. Vilhovyi (1918-2001), L. M. Lychkivskyy (1924-1993), Associate Professor A. Netlyuh and nowadays Associate Professor Y. Kryvko made a significant contribution to the study of the functional anatomy of heart vascular system, X-ray anatomy, the study of the formation of collateral circulation routes and a clarification of morphological changes in organs during ischemia.
I. Kozlov gives the first lecture in anatomy at the medical faculty of the Imperial University of St. Vladimir in Kiev.
Department of Anatomy of the Medical Faculty of Kharkov University was established in 1805 and its first Head was Professor L. J. Vannoti. It played an important role in the formation and development of Kharkiv Anatomical School, which anatomists (Professor I. D. Knyhin, A. Venediktov, P. A. Baranovych, D. F. lamblia, I. K. Wagner, M. O. Popov, A. K. Belousov, V. Vorobiev, G. D. Sinelnikov V. V. Bobin, V. M. Lupyr) made famous national and world science .
Leading place among scientists of Kharkiv Anatomically School holds, a student of Professor A. K. Belousov, Academician V. P. Vorobyov (1876-1937), who led the department of anatomy at the Kharkov Medical Institute. V. P. Vorobiev proposed a special method of preserving dead bodies, developed a method of macro-microscopic study of the anatomical structure (“Vorobyov makromikroskopichnyy method”) and laid the foundation for the study of the peripheral nervous system, which was continued by his many students ( F. A. Volyn, V. M . Bobin, A. A. Otelin, R. D. Sinelnikov ). V. P. Vorobyov wrote many books on anatomy and published the first in Ukraine atlas of human anatomy in three volumes (1934), and then in five volumes. Student of Academician V. P. Vorobyov, Professor G. D. Sinelnikov (1896-1981) succeeded in the department and created the “Atlas of Human Anatomy,” which became a handbook for anatomists, students, doctors and is still re-published to date.
Odessa (Novorossiysk ) Anatomical School associated with the Department of Anatomy of Medical Faculty, which was established in 1900 in connection with the opening of Novorossiysk University. The organizer of the department and its first Head was Professor M. Batuyev (1855-1917). Prominent representatives of Odessa Anatomical School (professors M. K. Lisenkov, M. S. Kondratiev, F. A. Volyn, E. M. Popovkin, and today I. I. Ilyin) made a significant contribution to the study of central and peripheral anatomy nervous systems.
Famous scientists anatomists who worked in Ukraine and created a regional school of scientists and teachers, are professors M. D. Dovhyallo (Donetsk), V. M. Bobin, V. I. Zyablov (Simferopol), V. G. Ukrainian, G . B. Terentyev, B. J. Kogan, A. Y. Romensky, P. P. Shaparenko (Vinnytsia), K. D. Filatov, S. E. Stebelsky, V. A. Kozlov (Dnepropetrovsk), M. N. Turkevych, V. M. Krutsyak (Chernivtsi), Y. P. Melman, B. V. Shutko (Ivano- Frankivsk).
Today the Department of Anatomy in Ukraine is led by highly skilled anatomists, who continue scientific traditions of their predecessors, developing new areas, adequately represent native anatomy in the global scientific and educational space. A prolific schools are working today led by Professors M. A. Voloshin (Zaporizhia), I. E. Gerasimyuk (Ternopil), A. Gоlovatskiy (Uzhgorod), J. J. Huminskiy (Vinnytsia), V. M. Lupyr (Kharkiv) G. S. Kyryakulov (Donetsk), V. G. Koveshnikov (Luhansk), Y. Kryvko (Lviv), V. A. Levitsky (Ivano-Frankivsk), B. G. Makara (Chernivtsi), V. S. Pykalyuk (Simferopol), V. Z. Sikora (MSA), O. L. Holodkova (Odessa), A. A. Sherstyuk (Poltava), V. G. Cherkasov (Kyiv).
OVERVIEW OF CELLS
Cells are the smallest living units in the body. Each cell performs all the functions necessary to sustain life. It can obtain nutrients and other essential substances from the surrounding body fluids and use these nutrients to make the molecules that it needs to survive. Each cell also disposes of its wastes and maintains its shape and integrity. Finally, cells can replicate themselves. These functions are carried out by the cell’s many subunits, most of which are called organelles (“little organs”). Although different cell types perform different functions, virtually all human cells contain the same basic parts and can be described in terms of a generalized cell.
Human cells have three main parts:
– the plasma membrane;
– the cytoplasm;
– the nucleus.
The plasma membrane is the outer boundary. Internal to this membrane is the cytoplasm (sitoplazm), which makes up the bulk of the cell, contains most of the cellular organelles, and surrounds the nucleus. The nucleus (nukleus) controls cellular activities and lies near the cell’s center. To understand the functions of a cell and its diverse cellular organelles, you can think of the cell as a manufacturing plant. The cell, like the manufacturing plant, has many divisions with specific functions. This analogy will be extended throughout this chapter.
THE PLASMA MEMBRANE
The outer cell membrane is called the plasma membrane or plasmalemma (plazmahlemah; lemma – sheath, husk). This thin, flexible layer defines the extent of the cell, thereby separating two of the body’s major fluid compartments: the intracellular fluid within the cells and the extracellular fluid that lies outside and between cells. You can think of the plasma membrane as a security fence surrounding the manufacturing plant (cell). This boundary contains specific checkpoints (receptors) that influence cellular activity in various ways.
The fluid mosaic model of membrane structure depicts the plasma membrane as a double layer, or bilayer, of lipid molecules with protein molecules dispersed within it. The most abundant lipids in the plasma membrane are phospholipids. Like a lollipop on two sticks, each phospholipid molecule has a polar “head” that is charged, and an uncharged, nonpolar “tail” made of two chains of fatty acids. The polar heads are attracted to water—the main constituent of both the cytoplasm and the fluid external to the cell—so they lie along the inner as well as the outer face of the membrane. The nonpolar tails avoid water and line up in the center of the membrane. The result is two parallel sheets of phospholipid molecules lying tail to tail, forming the membrane’s basic bilayered structure.
The inner and outer layers of the membrane differ some what in the kinds of lipids they contain. Sugar groups are attached to about 10% of the outer lipid molecules, making them “sugarfats,” or glycolipids (glikolipids). The plasma membrane also contains substantial amounts of cholesterol, another lipid. Cholesterol makes the membrane more rigid and increases its impermeability to water and water-soluble molecules.
Proteins make up about half of the plasma membrane by weight. The membrane proteins are of two distinct types: integral and peripheral. Integral proteins are firmly embedded in or strongly attached to the lipid bilayer. Some integral proteins protrude from one side of the membrane only, but most are transmembrane proteins that span the whole width of the membrane and protrude from both sides (transacross). Peripheral proteins, by contrast, are not embedded in the lipid bilayer at all. Instead, they attach rather loosely to the membrane surface. The peripheral proteins include a network of filaments that helps support the membrane from its cytoplasmic side. Without this strong, supportive base, the plasma membrane would tear apart easily.
Short chains of carbohydrate molecules attach to the integral proteins to form glycoproteins. These sugars protect from the external cell surface, forming the glycocalyx (glikokaliks; “sugar covering”), or cell coat. Also contributing to the glycocalyx are the sugars of the membrane’s glycolipids. You can therefore think of your cells as “sugar-coated.” The glycocalyx is sticky and may help cells to bind when they come together. Because every cell type has a different pattern of sugars that make up its glycocalyx, the glycocalyx is also a distinctive biological marker by which approaching cells recognize each other. For example, a sperm recognizes the ovum (egg cell) by the distinctive composition of the ovum’s glycocalyx.
The functions of the plasma membrane relate to its location at the interface between the cell’s exterior and interior:
The plasma membrane provides a protective barrier against substances and forces outside the cell.
Some of the membrane proteins act as receptors; that is, they have the ability to bind to specific molecules arriving from outside the cell. After binding to the receptor, the molecule can induce a change in the cellular activity. Membrane receptors act as part of the body’s cellular communication system.
The plasma membrane controls which substances can enter and leave the cell. The membrane is a selectively permeable barrier that allows some substances to pass between the intracellular and extracellular fluids while preventing others from doing so. The processes involved in moving substances across the plasma membrane are described next.
Small, uncharged molecules, such as oxygen, carbon dioxide, and fat-soluble molecules, can pass freely through the lipid bilayer of the plasma membrane through a process called simple diffusion. Diffusion is the tendency of molecules in a solution to move down their concentration gradient; that is, the molecules move from a region where they are more concentrated to a region where they are less concentrated. Water, like other molecules, diffuses down its concentration gradient. The diffusion of water molecules across a membrane is called osmosis (ozmosis).
Most water-soluble or charged molecules, such as glucose, amino acids, and ions, cannot pass through the lipid bilayer by simple diffusion. Such substances can cross the plasma membrane only by means of specific transport mechanisms that use the integral proteins to carry or pump molecules across the membrane or to form channels through which specific molecules pass. Some of these molecules move down their concentration gradient, diffusing through the plasma membrane by moving through a specific integral protein. This transport process is called facilitated diffusion. Other integral proteins move molecules across the plasma membrane against their concentration gradient, a process called active transport, which requires the use of energy.
The largest molecules (macromolecules) and large solid particles are transported through the plasma membrane by another set of processes, called vesicular or bulk transport. Knowledge of the two general types of bulk transport, exocytosis and endocytosis, is essential to the understanding of basic functional anatomy.
Endocytosis (endositosis; “into the cell”) is the mechanism by which large particles and macromolecules enter cells. The substance to be taken into the cell is enclosed by an infolding part of the plasma membrane. In the region of invagination, specific proteins may cover the inner surface of the plasma membrane. This protein coat aids in the selection of the substance to be transported and deforms the membrane to form a membrane-walled sac called a vesicle. The membranous vesicle is pinched off from the plasma membrane and moves into the cytoplasm, where its contents are digested. Three types of endocytosis are recognized: phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Phagocytosis (fagositosis) is literally “cell eating.” In this process, pseudopods (parts of the plasma membrane and cytoplasm) protrude and flow around some relatively large material, such as a clump of bacteria or cellular debris, and en- gulf it. The membranous vesicle thus formed is called a phagosome (fagosom; “eaten body”). In most cases, the phagosome then fuses with lysosomes (lisosomz), organelles containing digestive enzymes that break down the contents of the phagosome. Some cells—most white blood cells, for example—are experts at phagocytosis. Such cells help to police and protect the body by ingesting bacteria, viruses, and other foreign substances. They also “eat” the body’s dead and diseased cells.
Just as cells eat in a manner of speaking, they also drink. Pinocytosis (pinositosis) is “cell drinking.” In pinocytosis, a bit of infolding plasma membrane surrounds a tiny quantity of extracellular fluid containing dissolved molecules. This fluid enters the cell in a tiny membranous vesicle. Pinocytosis, a routine activity of most cells, is an un-selective way of sampling the extracellular fluid. This process is particularly important in cells that function in nutrient absorption, such as cells that line the intestines.
Some molecules, such as insulin and other hormones, enzymes, and low-density lipoproteins (LDLs, the molecules that carry cholesterol through the bloodstream to the body’s cells) are brought into cells through receptor-mediated endocytosis, an exquisitely selective transport process. These substances bind to specific receptors on the cell membrane. Upon binding, the portion of the plasma membrane bearing the molecules and attached receptors invaginates and is pinched off, bringing into the cell the membrane-bound vesicle containing the molecules. The vesicle binds with a lysosome, and the contents of the vesicle are released to be used by the cell. The receptors are recycled back to the plasma membrane. Unfortunately, harmful substances such as some toxins and viruses also use receptor-mediated endocytosis to enter and attack cells.
Exocytosis (eksositosis; “out of the cell”) is a mechanism by which substances move from the cytoplasm to the outside of the cell. Exocytosis accounts for most secretion processes, such as the release of mucus or protein hormones from the gland cells of the body. In exocytosis the substance or cell product to be released from the cell is first enclosed in a membrane-bound vesicle in the cytoplasm. The vesicle migrates to the plasma membrane. Proteins extending from the vesicle membrane, v-SNAREs (v for vesicle), bind with distinct plasma membrane proteins, t-SNAREs (t for target). These proteins cause the lipid layers from both membranes to join together, thus inserting the vesicle membrane into the plasma membrane and releasing the contents of the sac into the space outside the cell.
THE BASIC CELL STRUCTURES
Plasmallema ( the external cellular membrane)
The plasmallema strucrure
External, medium and internal layers
The plasmalemma invaginations, appendixes, microvillusses, auditory cells, ciliae, flagellums
Simple: serrated or digitiform
Complex: desmosomes, gap junctions (nexuses), synapses
Inclusions (exogenous and endogenous)
Membrane -bound organelles: mitichondrias, lysosomes,the endoplasmic reticulum, peroxisomes, golgi apparatus,
Non-Membranous-bound Organelles: ribosomes, microfilaments, Microtubules, centrosomes
Trophic: oil drops, alpha units and protein granules; pigment granules, secretory granules, crystalloids
Specialized organelles: muscular fibrillas, neurofibrillas of neuro cells.
The nuclear envelope (karyolemma)
External and internal membranes, that are divided by perinuclear cisterna; nuclear pores
Filiform and granule nucleoplasm
The main nucleoli, adittional nucleoles
Euchromatin, heterochromatin; chromatin nubbles, Barr’s chromatin body
Cytoplasm, literally “cell-forming material,” is the part of the cell that lies internal to the plasma membrane and external to the nucleus. Most cellular activities are carried out in the cytoplasm, which consists of three major elements: cytosol, organelles, and inclusions.
The cytosol (sitosol), is the jellylike, fluid-containing substance within which the other cytoplasmic elements are suspended. It consists of water, ions, and many enzymes. Some of these enzymes start the breakdown of nutrients (sugars, amino acids, and lipids) that are the raw materials and energy source for cell activities. In many cell types, the cytosol makes up about half the volume of the cytoplasm.
The cytoplasm contains about nine types of organelles, each with a different function that is essential to the survival of the cell. As separate units, the organelles compartmentalize the cell’s biochemical reactions, thus preventing reactions from interfering with one another and promoting functional efficiency. The organelles include mitochondria, ribosomes, rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, the cytoskeleton, and centrioles. As you will learn, most organelles are bounded by a membrane that is similar in composition to the plasma membrane but lacks a glycocalyx.
With very few exceptions, all cells of the human body share the same kinds of organelles. However, when a cell type performs a special body function, the organelles that contribute to that function are especially abundant in that cell. Thus, certain organelles are better developed in some cells than in others. You will see examples of this principle as you explore the organelles and their roles.
Ribosomes (ribosomz) are the assembly line of the manufacturing plant, producing proteins for cellular or extracellular function. They are small, dark-staining granules. Unlike most organelles, they are not surrounded by a membrane, but are constructed of proteins plus ribosomal RNA (RNA – ribonucleic acid). Each ribosome consists of two subunits that fit together like the body and cap of an acorn.
Almost all cells make large amounts of protein, and ribosomes are the site of protein synthesis. On the ribosomes, building blocks called amino acids are linked together to form protein molecules. This assembly process is called translation. It is dictated by the genetic material in the cell nucleus (DNA), whose instructions are carried to the ribosomes by messenger molecules called messenger RNA (mRNA).
Many ribosomes float freely within the cytosol. Such free ribosomes make the soluble proteins that function within the cytosol itself. Ribosomes attached to the membranes of the rough endoplasmic reticulum, make proteins that become part of the cell membrane or that are exported out of the cell.
The endoplasmic reticulum (endoplazmik rettikulum), or ER, is literally the “network within the cytoplasm.” It accounts for more than half of the membranous surfaces inside an average human cell. There are two distinct types of ER: rough ER and smooth ER. Either type may pre- dominate in a given cell type, depending on the specific functions of the cell.
Rough Endoplasmic Reticulum. The rough endoplasmic reticulum (rough ER) consists mainly of stacked envelopes called cisternae (sisterne; “fluid-filled cavities”). Ribosomes stud the external faces of the membranes of the rough ER, assembling proteins. The ribosomes attach to the membrane when the protein is being made, then detach when the protein is completed.
The rough ER has several functions. Its ribosomes make all proteins that are secreted from cells; thus, rough ER is especially well developed in gland cells that secrete a large amount of protein (mucous cells, for example). It makes the digestive enzymes that will be contained in lysosomes. The rough ER also makes both the integral proteins and the phospholipid molecules of the cell’s membranes. In other words, all cell membranes start out as rough ER membrane. The rough ER can therefore be considered the cell’s “membrane factory.”
Smooth Endoplasmic Reticulum. The smooth endoplasmic reticulum (smooth ER) is continuous with the rough ER. It consists of tubules arranged in a branching network. Because no ribosomes are attached to its membranes, the smooth ER is not a site of protein synthesis. It performs different functions in different cell types, but most of these relate to lipid metabolism, the making or breaking down of fats. Smooth ER is abundant in cells that make lipid steroid hormones from cholesterol and in liver cells that detoxify lipid- soluble drugs. Most cell types, however, have little smooth ER.
Another important function of smooth ER is storing calcium ions. Ionic calcium is a signal for the beginning of many cellular events, including muscle contraction and glandular secretion. The calcium concentration in the cytosol is kept low when such events are not occurring, because most calcium ions are pumped into the ER and held there until the cell needs them. The ER in muscle cells is very extensive, reflecting this essential function.
The Golgi (golje) apparatus is a stack of three to ten disc- shaped envelopes (cisternae), each bound by a membrane. It resembles a stack of hollow saucers, one cupped inside the next. The products of the rough ER move through the Golgi stack from the convex (cis) to the concave (trans) side. More specifically, the cis face receives spherical, membranous transport vesicles from the rough ER; new vesi cles bud off a trans face to leave the apparatus.
The Golgi apparatus sorts, processes, and packages the proteins and membranes made by the rough ER. For example, the Golgi apparatus distinguishes which newly made membranes will become part of the lysosomes (discussed shortly), and which ones are destined for the plasma membrane. It then sends these membranes to their correct destinations in the vesicles that leave the trans face. Thus, the Golgi apparatus is the packaging and shipping division of the manufacturing plant. It receives product produced by the rough ER, pack- ages it, and ships it to its appropriate destination.
In pathway A, which occurs in gland cells, the protein product is contained in secretory vesicles; these vesicles ultimately release their contents to the cell’s exterior by exocytosis. In pathway B, common in all cells, the membrane of the vesicle fuses to and contributes to the plasma membrane, whose components are constantly being renewed and recycled. In pathway C, also common in all cells, the vesicle leaving the Golgi apparatus is a lysosome, a sac filled with digestive enzymes, that remains inside the cell.
Lysosomes are spherical, membrane-walled sacs containing many kinds of digestive enzymes. These enzymes, called acid hydrolases, can digest almost all types of large biological molecules. Lysosomes can be considered the cell’s “demolition crew,” because they break apart and digest unwanted substances. For example, they fuse with phagosomes, emptying their enzymes into these vesicles and breaking down their contents.
When a cell’s own internal membranes, proteins, or organelles are damaged or wear out, they are encircled by a new membrane from the rough ER, forming a vesicle. Then, nearby lysosomes fuse with this vesicle to digest its contents. Within such vesicles, digestion can proceed safely, because the enclosing membrane keeps the destructive enzymes away from other cell components. Phagocytic cells, such as some white blood cells, have an exceptional number of lysosomes to degrade ingested bacteria and viruses.
Mitochondria (mitokondreah) are analogous to the power plant of the manufacturing company. These organelles produce the energy for cellular function. They usually are depicted as bean-shaped structures because of their appearance in sections under the microscope. In reality, mitochondria are long and threadlike (mitos – thread). In living cells, they squirm about and change shape as they move through the cytoplasm. Most organelles are surrounded by a membrane, but mitochondria are enclosed by two membranes: The outer membrane is smooth and featureless, and the inner membrane folds in- ward to produce shelflike cristae (krıste; “crests”). These protrude into the matrix, the jellylike substance within the mitochondrion.
Mitochondria generate most of the energy the cell uses to carry out work. They do this by systematically releasing the energy stored in the chemical bonds of nutrient molecules and then transferring this energy to produce adenosine triphosphate (ATP), the high-energy molecules that cells use to power chemical reactions. Within the mitochondrion, the ATP-generating process starts in the matrix (by a process called the citric acid cycle) and is completed on the inner membrane of the cristae (by the processes called oxidative phosphorylation and electron transport). Cell types with high energy requirements, muscle cells for example, have large numbers of mitochondria in their cytoplasm. These types of cells also have large numbers of cristae within their mitochondria.
Mitochondria are far more complex than any other organelle. They even contain some maternally inherited genetic material (DNA) and divide to form new mitochondria, as if they were miniature cells themselves. Intriguingly, mitochondria are very similar to a group of bacteria, the purple bacteria phylum. It is now widely believed that mitochondria arose from bacteria that invaded the ancient ancestors of animal and plant cells.
Peroxisomes (peroksısomz; “peroxide bodies”) are like the toxic waste removal system of the manufacturing plant. They are membrane-walled sacs that resemble small lysosomes. They contain a variety of enzymes, most importantly oxidases and catalases. Oxidases use oxy- gen to neutralize aggressively reactive molecules called free radicals, converting these to hydrogen peroxide. Free radicals are normal by-products of cellular metabolism, but if allowed to accumulate they can destroy the cell’s proteins, membranes, and DNA. Hydrogen peroxide is also reactive and dangerous, but it is converted by catalase into water and oxygen. This catalase-driven reaction breaks down poisons that have entered the cell, such as alcohol, formaldehyde, and phenol. Peroxisomes are numerous in liver and kidney cells, which play a major role in removing toxic substances from the body. Peroxisomes also perform other metabolic reactions, such as breaking down long chains of fatty acids in lipid metabolism.
The cytoskeleton, literally “cell skeleton,” is an elaborate network of rods running throughout the cytosol. This net- work acts as a cell’s “bones,” “muscles,” and “ligaments” by supporting cellular structures and generating various cell movements. The three types of rods in the cytoskeleton are microfilaments, intermediate filaments, and microtubules, none of which is covered by a membrane.
Microfilaments, the thinnest elements of the cytoskeleton, are strands of the protein actin (aktin). Also called actin filaments, they concentrate most heavily in a layer just deep to the plasma membrane. Actin filaments interact with another protein called myosin (miosin) to gen- erate contractile forces within the cell. The interaction of actin and myosin squeezes one cell into two during cell division, causes the membrane changes that accompany endocytosis and exocytosis, and enables some cells to send out and then retract extensions called pseudopods (soodopods; “false feet”), in a crawling action called amoeboid motion (ahmeboid; “changing shape”). Additionally, myosin acts as a motor protein to move some organelles within the cell. Except in muscle cells, where they are stable and permanent, the actin microfilaments are unstable, constantly breaking down and reforming from smaller subunits.
Intermediate filaments are tough, insoluble protein fibers, with a diameter between those of microfilaments and microtubules. Intermediate filaments are the most stable and permanent of the cytoskeletal elements. Their most important property is high tensile strength; that is, they act like strong guy-wires to resist pulling forces that are placed on the cell. They also function to link adjacent cells together by attaching to specific cell junctions called desmosomes .
Microtubules, the elements with the largest diameter, are hollow tubes made of spherical protein subunits called tubulins. They are stiff but bendable. All microtubules radiate from a small region of cytoplasm near the nucleus called the centrosome. This radiating pattern of stiff microtubules determines the overall shape of the cell, as well as the distribution of cellular organelles. Mitochondria, lysosomes, and secretory granules attach to the microtubules like ornaments hanging from the limbs of a Christmas tree. Organelles move within the cytoplasm, pulled along the microtubules by small motor proteins, kinesins (kinesinz) and dyneins (dineinz), that act like train engines on the microtubular railroad tracks. Microtubules are remarkably dynamic organelles, constantly growing out from the cell center, disassembling, then reassembling.
Centrosome and Centrioles
The centrosome (sentrosom) is a spherical structure in the cytoplasm near the nucleus. It contains no membranes. Instead, it consists of an outer cloud of protein called the centrosome matrix and an inner pair of centrioles (sentreolz). The matrix protein seeds the growth and elongation of microtubules, which explains why the long microtubules of the cytoskeleton radiate from the centrosome in nondividing cells, and why a mitotic spindle of microtubules radiates from it in dividing cells.
In the core of the centrosome, the two barrel-shaped centrioles lie perpendicular to one another. The wall of each centriole consists of 27 short microtubules, arranged in nine groups of three. Unlike most other microtubules, those in centrioles are stable and do not disassemble. Functionally, centrioles act in forming cilia and flagella and the mitotic spindle.
Inclusions are temporary structures in the cytoplasm that may or may not be present in a given cell type. Inclusions include pigments, crystals of protein, and food stores. The food stores, by far the most important kind, are lipid droplets and glycosomes. Lipid droplets are spherical drops of stored fat. They can have the same size and appearance as lysosomes but can be distinguished by their lack of a surrounding membrane. Only a few cell types contain lipid droplets: Small lipid droplets are found in liver cells, large ones in fat cells. Glycosomes (“sugar-containing bodies”) store sugar in the form of glycogen (glikojen), which is a long branching chain of glucose molecules, the cell’s main energy source. Glycosomes also contain enzymes that make and degrade the glycogen into its glucose subunits. Structurally, glycosomes are dense, spherical granules. They resemble ribosomes, but their diameter is twice as large.
The nucleus, literally a “little nut,” is the control center of the cell. Its genetic material, deoxyribonucleic acid (DNA), directs the cell’s activities by providing the instructions for protein synthesis. In our manufacturing analogy, the nucleus can be compared to a central library, design department, construction superintendent, and board of directors all rolled into one. Whereas most cells have only one nucleus, some, including skeletal muscle cells, have many; that is, they are multinucleate (multınukleat; multi -many). The presence of more than one nucleus usually signifies that a cell has a larger-than-usual amount of cytoplasm to regulate. One cell type in the body, the mature red blood cell, is anucleate; that is, it has no nucleus at all. Its nucleus normally is ejected before this cell first enters the bloodstream.
The nucleus, which averages 5 μm in diameter, is larger than any of the cytoplasmic organelles. Although it is usually spherical or oval, it generally conforms to the overall shape of the cell. If a cell is elongated, for ex- ample, the nucleus may also be elongated. The main parts of the nucleus are the nuclear envelope, nucleolus, and chromatin and chromosomes.
The nucleus is surrounded by a nuclear envelope that consists of two parallel membranes separated by a fluid-filled space. The outer membrane is continuous with the rough ER and has ribosomes on its external face. It forms anew from rough ER after every cell division, so it is evidently a specialized part of the rough ER. The inner membrane is lined by protein filaments, the nuclear lamina, which maintain the shape of the nucleus.
At various points, the two layers of the nuclear envelope fuse, and nuclear pores penetrate the fused regions. Each pore is formed by a bracelet-shaped complex of more than 22 proteins, and there are several thousand pores per nucleus. Like other cellular membranes, the membranes of the nuclear envelope are selectively permeable, but the pores allow large molecules to pass in and out of the nucleus as necessary. For example, protein molecules imported from the cytoplasm and RNA molecules exported from the nucleus routinely travel through the pores.
The nuclear envelope encloses a jellylike fluid called nucleoplasm (nukleoplazm), in which the chromatin and nucleolus are suspended. Like the cytosol, the nucleoplasm contains salts, nutrients, and other essential chemicals.
The nucleolus (nucleolus, “little nucleus”) is a dark-staining body in the cell nucleus. There may be one or several within a cell nucleus. A nucleolus contains parts of several different chromosomes and serves as the cell’s “ribosome- producing machine.” Specifically, it has hundreds of copies of the genes that code for ribosomal RNA and serves as the site where the large and small subunits of ribosomes are assembled. These subunits leave the nucleus through the nuclear pores and join within the cytoplasm to form complete ribosomes.
Chromatin and Chromosomes
DNA is a long double helix that resembles a spiral staircase. This double helix is in turn composed of four kinds of subunits called nucleotides, each of which contains a distinct base. These bases—thymine (T), adenine (A), cytosine (C), and guanine (G)—bind to form the “stairs” of the “staircase” and to hold the DNA helix together.
The double helix of DNA is packed with protein molecules and coiled in strands of increasing structural complexity and thickness. The DNA molecule plus the proteins form chromatin. Each two turns of the DNA helix is packed with eight disc-shaped protein molecules called histones (histonz). Each cluster of DNA and histones is called a nucleosome. In an electron micrograph of chromatin, the nucleosomes have the appearance of beads on a string. Chromatin in this form is called extended chromatin. Further coiling of the nucleosomes forms a tight helical fiber. These thick fibers of chromatin are called condensed chromatin.
During a cell’s nondividing phase, when it is performing its normal activities, the chromatin is in either its extended or condensed form. The tightly coiled DNA of condensed chromatin is inactive. The extended chromatin is the active region of the DNA, directing the synthetic activities of the cell. Specifically, extended chromatin is the site where DNA’s genetic code is copied onto messenger RNA molecules in a process called transcription. The most active cells in the body have a large amount of extended chromatin and little condensed chromatin.
During cell division, the chromatin is further packed: The helical fibers of nucleosomes are looped and then packed further into the most complex structure, the chromatid of a chromosome (kromosom; “colored body”). Each chromosome contains a single, very long molecule of DNA, and there are 46 chromosomes in a typical human cell. When a cell is dividing, its chromosomes are maximally coiled, so they appear as thick rods. Chromosomes move extensively during cell division, and their compact nature helps to keep the delicate chromatin strands from tangling and breaking as the chromosomes move. When cell division stops, many parts of the chromosome uncoil to form the extended chromatin, thereby allowing transcription to occur.
THE CELL LIFE CYCLE
The cell life cycle is the series of changes a cell undergoes from the time it forms until it reproduces itself. This cycle can be divided into two major periods : interphase, in which the cell grows and carries on its usual activities; and cell division, or the mitotic phase, during which it divides into two cells.
In addition to carrying on its life-sustaining activities, a cell in interphase prepares for the next cell division. Interphase is di- vided into G1, S, and G2 subphases. During G1, the first part of interphase, cells are metabolically active, make proteins rapidly, and grow vigorously. This is the most variable phase in terms of duration. In cells with fast division rates, G1 lasts several hours; in cells that divide slowly, it can last days or even years. Near the end of G1, the centrioles start to replicate in preparation for cell division. During the next stage, the S (synthetic) phase, DNA replicates itself, ensuring that the two daughter cells will receive identical copies of the genetic material. The final part of interphase, called G2, is brief. In this period, the enzymes needed for cell division are synthesized. Centrioles finish copying themselves at the end of G2. The cell is now ready to divide. Throughout all three subphases, the cell continues to grow, producing proteins and cytoplasmic organelles, and to carry out its normal metabolic activities.
Checkpoints that evaluate cellular activities such as cell growth, DNA replication, and mitotic spindle formation occur throughout the cell cycle. Two of these checkpoints: The G1 checkpoint assesses cell size before DNA synthesis, and the G2 checkpoint checks for DNA damage and accuracy of replication. Mitosis can be halted at these checkpoints, thus preventing damaged cells from dividing.
Cell division is essential for body growth and tissue repair. Short-lived cells that continuously wear away, such as cells of the skin and the intestinal lining, reproduce themselves al- most continuously. Others, such as liver cells, reproduce slowly (replacing those cells that gradually wear out), but can divide quickly if the organ is damaged. Cells of nervous tis- sue and for the most part skeletal muscle are unable to divide after they are fully mature; repair is carried out by scar tissue (a fibrous connective tissue).
Cells divide in the M (mitotic) phase of their life cycle, which follows interphase. In most cell types, division involves two distinct events: mitosis (mitosis), or division of the nucleus, and cytokinesis (sitokinesis), or division of the entire cell into two cells.
Mitosis is the series of events during which the replicated DNA of the original cell is parceled out into two new cells, culminating in the division of the nucleus. Throughout these events, the chromosomes are evident as thick rods or threads. Indeed, mitosis literally means “the stage of threads.” Mitosis is described in terms of four consecutive phases: prophase, metaphase, anaphase, and telophase. However, it is actually a continuous process, with each phase merging smoothly into the next. Its duration varies according to cell type, but it typically lasts about 2 hours. Mitosis is described in detail in Focus on Mitosis.
The separation of one cell into two at the end of the cell cycle is called cytokinesis, literally “cells moving (apart).” It begins during anaphase and is completed after mitosis ends. Essentially, a ring of contractile actin and myosin filaments in the center of the original cell constricts to pinch that cell in two. The two new cells, called daughter cells, then enter the interphase part of their life cycle.
DEVELOPMENTAL ASPECTS OF CELLS
All humans begin life as a single cell, the fertilized egg, from which all the cells in the body arise. Early in embryonic development, the cells begin to specialize: Some become liver cells; some become nerve cells; others become the transparent lens of the eye. Every cell in the body carries the same genes. (A gene, simply speaking, is a segment of DNA that dictates a specific cell function, usually by coding for a specific protein.) If all our cells have identical genes, how do cells differentiate and take on specialized structures and functions?
Cells in various regions of the developing embryo are exposed to different chemical signals that channel the cells into specific pathways of development. The cytoplasm of a fertilized egg contains gradients of maternally produced messenger RNA (mRNA) molecules and proteins. In the early days of development as the fertilized egg divides, the cytoplasm of each daughter cell receives a different composition of these molecules. These maternally derived molecules in the cytoplasm influence the activity of the embryonic genome. In this way, different genes are activated in each cell, leading to cellular differentiation. Once the cell-specific gene expression begins, a cell may produce signaling molecules that influence the development of neighboring cells by switching some of their genes “on” or “off.” Some genes are active in all cells; for example, all cells must carry out protein synthesis and make ATP. However, the genes for the synthesis of specialized proteins, such as hormones or mucus, are activated only in certain cell populations. The key to cell specialization lies in the kinds of proteins made and reflects differential gene activation in the different cell types.
Cell specialization, also called cell differentiation, leads to structural variation among the cell types in the body. Different organelles come to predominate in different cells. For example, muscle cells make tremendous quantities of actin and myosin proteins, and lipid accumulates in fat cells. Phagocytic cells produce more lysosomal enzymes and contain many lysosomes. There are about 200 different cell types in the body, which vary greatly in size, shape, and function. They include sphere-shaped fat cells, discshaped red blood cells, branching nerve cells, and cube-shaped cells of kidney tubules. Cells fall into these functional groups:
(a) Cells that connect body parts or cover and line organs
Fibroblast. The elongated shape of this cell extends along the cablelike fibers that it secretes. It also has an abundant rough ER and a large Golgi apparatus to make and secrete the protein components of these fibers.
Erythrocyte (red blood cell). This cell carries the respiratory gases, oxygen and carbon dioxide. Its concave disc shape provides extra surface area for the uptake of respiratory gases. This streamlined shape also allows the cell to flow easily through the bloodstream. So much oxygen- carrying pigment is packed in erythrocytes that all other organelles have been shed to make room.
Epithelial cell. The shape of these cells allows the maxi- mum number of epithelial cells to be packed together in a sheet called epithelium. An epithelial cell has abundant intermediate filaments that resist tearing when the epithelium is rubbed or pulled. Some epithelial cells are gland cells, with an abundant rough ER, Golgi apparatus, and secretory granules.
(b) Cells that produce movement and move body parts
Skeletal muscle and smooth muscle cells. These cells are elongated and filled with abundant actin and myosin filaments, so they can shorten forcefully.
Fat cell. The huge spherical shape of a fat cell is produced by a large lipid droplet in its cytoplasm.
(d) Cell that fights disease
Macrophage (a phagocytic cell). This cell extends long pseudopods to crawl through tissue to reach infection sites. The many lysosomes within the cell digest the infectious microorganisms it takes up.
(e) Cell that gathers information and controls body functions Nerve cell (neuron). This cell has long processes for receiving messages and transmitting them to other structures in the body. The processes are covered with an extensive plasma membrane, whose components are continually recycled; a large rough ER is present to synthesize membrane components.
(f) Cell of reproduction Sperm (male). This cell is long and streamlined for swimming to the egg for fertilization. The swimming tail is a motile whip called a flagellum. Most organs are well formed and functional long before birth, but the body continues to grow by forming more cells throughout childhood and adolescence. Once adult size is reached, cell division slows considerably and occurs primarily to replace short-lived cell types and to repair wounds.
An epithelium (epıtheleum; “covering”) is a sheet of cells that covers a body surface or lines a body cavity. With minor exceptions, all of the outer and inner surfaces of the body are covered by epithelia. Examples include the outer layer of the skin; the inner lining of all hollow viscera, such as the stomach and respiratory tubes; the lining of the peritoneal cavity; and the lining of all blood vessels. Epithelia also form most of the body’s glands.
Epithelia occur at the interfaces between two different environments. The epidermis of the skin, for example, lies between the inside and outside of the body. All functions of epithelia reflect their role as interface tissues and boundary layers: New stimuli, including harmful ones, are experienced at body interfaces, so epithelia both protect the underlying tissues and contain nerve endings for sensory reception. Nearly all substances that are received or given off by the body must pass across an epithelium, so epithelia function in diffusion (the movement of molecules down their concentration gradient), secretion (the release of molecules from cells), absorption (bringing small molecules into the body), and ion transport (moving ions across the interface). Furthermore, body fluids can be filtered across thin epithelia, and some epithelia form slippery surfaces along which substances move (food glides along the intestinal lining, for example).
Special Characteristics of Epithelia
Epithelial tissues have many characteristics that distinguish them from other tissue types:
Cellularity. Epithelia are composed almost entirely of cells. These cells are separated by a minimal amount of extracellular material, mainly projections of their integral membrane proteins into the narrow spaces between the cells.
Specialized contacts. Adjacent epithelial cells are directly joined at many points by special cell junctions.
Polarity. All epithelia have a free upper (apical) surface and a lower (basal) surface. They exhibit polarity, a term meaning that the cell regions near the apical surface differ from those near the basal surface. As shown in Figure 4.1, the basal surface of an epithelium lies on a thin sheet called a basal lamina, which is part of a basement membrane. The free apical surface abuts the open space of a cavity, tubule, gland, or hollow organ.
Support by connective tissue. All epithelial sheets in the body are supported by an underlying layer of connective tissue.
Avascular but innervated. Whereas most tissues in the body are vascular (contain blood vessels), epithelium is avascular (avaskular), meaning it lacks blood vessels. Epithelial cells receive their nutrients from capillaries in the underlying connective tissue. Although blood vessels do not penetrate epithelial sheets, nerve endings do; that is, epithelium is innervated.
Regeneration. Epithelial tissue has a high regenerative capacity. Some epithelia are exposed to friction, and their surface cells rub off. Others are destroyed by hostile substances in the external environment such as bacteria, acids, and smoke. As long as epithelial cells receive adequate nutrition, they can replace lost cells quickly by cell division.
Classification of Epithelia
Many kinds of epithelia exist in the body. To classify them, each epithelium is given two names. The first name indicates the number of cell layers in the epithelium, and the last name describes the shape of the cells. In classification by cell layers, an epithelium is called simple if it has just one cell layer or stratified if it has more than one layer. In classification by cell shape, the cells are called squamous, cuboidal, or columnar. Squamous (sqwamus; “platelike”) cells are flat cells; cuboidal cells are shaped like cubes; and columnar cells are taller than they are wide, like columns. In each case, the shape of the nucleus conforms to that of the cell: The nucleus of a squamous cell is disc-shaped, that of a cuboidal cell is spherical, and that of a columnar cell is an oval elongated from top to bottom. The shape of the nucleus is an important feature to keep in mind when distinguishing epithelial types.
Simple epithelia are easy to classify by cell shape be-cause all cells in the layer usually have the same shape. In stratified epithelia, however, the cell shapes usually differ among the different cell layers. To avoid ambiguity, stratified epithelia are named according to the shape of the cells in the apical layer. This naming system will become clearer as you explore the specific epithelial types.
It is useful to keep in mind how tissue structure reflects tissue function. Stratified epithelial tissues function to protect. Multiple layers of cells protect underlying connective tissues in areas where abrasion is common. For simple epithelia, the shape of the cells is indicative of tissue function. Squamous cells are found where diffusion or filtration are important, because these are distance-dependent processes; the thinner the layer, the more quickly the process occurs. Columnar and cuboidal cells are found in tissues involved in secretion and absorption. Larger cells are necessary for the additional cellular machinery needed to produce and package secretions and to produce the necessary energy for these processes. Ciliated epithelia function to propel material, for example, mucus. Keep these generalizations in mind as you study each type of epithelial tissue in detail.
As you read about the types of epithelium. Using the photomicrographs, try to pick out the individual cells within each epithelium. This is not always easy, because the boundaries between epithelial cells often are indistinct. Furthermore, the nucleus of a particular cell may or may not be visible, depending on the plane of the cut made to prepare the tissue slides. To better understand this, think about slicing a pear in transverse sections; slices from the top of the pear will not contain any seeds, but slices from the middle region will. The same is true for sections through tissues: Some cells may be sliced through the nucleus, whereas others may not.
Simple Squamous Epithelium. A simple squamous epithelium is a single layer of flat cells. When viewed from above, the closely fitting cells resemble a tiled floor. When viewed in lateral section, they resemble fried eggs seen from the side. Thin and often permeable, this type of epithelium occurs wherever small molecules pass through a membrane quickly, by processes of diffusion or filtration. The walls of capillaries consist exclusively of this epithelium, whose exceptional thinness encourages efficient exchange of nutrients and wastes between the bloodstream and surrounding tissue cells. In the lungs, this epithelium forms the thin walls of the air sacs, where gas exchange occurs.
Simple Cuboidal Epithelium Simple cuboidal epithelium consists of a single layer of cube-shaped cells. This epithelium forms the secretory cells of many glands, the walls of the smallest ducts of glands, and of many tubules in the kidney. Its functions are the same as those of simple columnar epithelium.
Simple Columnar Epithelium Simple columnar epithelium is a single layer of tall cells aligned like soldiers in a row. It lines the digestive tube from the stomach to the anal canal. It functions in the active movement of molecules, namely in absorption, secretion, and ion transport. The structure of simple columnar epithelium is ideal for these functions: It is thin enough to allow large numbers of molecules to pass through it quickly, yet thick enough to house the cellular machinery needed to perform the complex processes of molecular transport.
Some simple columnar epithelia bear cilia (sileah;“eyelashes”), whip like bristles on the apex of epithelial cells that beat rhythmically to move substances across certain body surfaces. A simple ciliated columnar epithelium lines the inside of the uterine tube. Its cilia help move the ovum to the uterus. Cilia are considered in detail later in this chapter.
Pseudostratified Columnar Epithelium The cells of pseudostratified (soodostratıfıd) columnar epithelium are varied in height. All of its cells rest on the basement membrane, but only the tall cells reach the apical surface of the epithelium. The short cells are undifferentiated and continuously give rise to the tall cells. The cell nuclei lie at several different levels, giving the false impression that this epithelium is stratified ( pseudo-false).
Pseudostratified columnar epithelium, like simple columnar epithelium, functions in secretion or absorption. A ciliated type lines the interior of the respiratory tubes. Here, the cilia propel sheets of dust-trapping mucus out of the lungs.
Stratified epithelia contain two or more layers of cells. They regenerate from below; that is, the basal cells divide and push apically to replace the older surface cells. Stratified epithelia are more durable than simple epithelia, and their major (but not only) role is protection.
Stratified Squamous Epithelium Stratified squamous epithelium consists of many cell layers whose surface cells are squamous. In the deeper layers, the cells are cuboidal or columnar. Of all the epithelial types, this is the thickest and best adapted for protection. It covers the often-abraded surfaces of our body, forming the epidermis of the skin and the inner lining of the mouth, esophagus, and vagina. To avoid memorizing all its locations, simply remember that this epithelium forms the outermost layer of the skin and extends a certain distance into everybody opening that is directly continuous with the skin.
The epidermis of the skin is keratinized, meaning that its surface cells contain an especially tough protective protein called keratin. The other stratified squamous epithelia of the body lack keratin and are nonkeratinized.
Stratified Cuboidal and Columnar Epithelia Stratified cuboidal and stratified columnar epithelia are rare types of tissue, located in the large ducts of some glands, for example sweat glands, mammary glands, and salivary glands. Stratified columnar epithelium is also found in small amounts in the male urethra.
Transitional Epithelium Transitional epithelium lines the inside of the hollow urinary organs. Such organs (the urinary bladder, for example) stretch as they fill with urine. As the transitional epithelium stretches, it thins from about six cell layers to three, and its apical cells unfold and flatten. When relaxed, portions of the apical surface invaginate into the cell, giving this surface a scalloped appearance. Thus, this epithelium undergoes “transitions” in shape. It also forms an impermeable barrier that keeps urine from passing through the wall of the bladder.
Epithelial cells that make and secrete a product form glands. The products of glands are aqueous (water-based) fluids that usually contain proteins. Secretion is the process whereby gland cells obtain needed substances from the blood and transform them chemically into a product that is then dis- charged from the cell. More specifically, the protein product is made in the rough endoplasmic reticulum (ER), then pack- aged into secretory granules by the Golgi apparatus and ultimately released from the cell by exocytosis. These organelles are well developed in most gland cells that secrete proteins.
Glands are classified as endocrine (endokrin; “internal secretion”) or exocrine (eksokrin; “external secretion”), depending on where they release their product, and as unicellular (“one-celled”) or multicellular (“many-celled”) on the basis of cell number. Unicellular glands are scattered within epithelial sheets, whereas most multicellular glands develop by invagination of an epithelial sheet into the under- lying connective tissue.
Endocrine glands lack ducts, so they are often referred to as ductless glands. They secrete directly into the tissue fluid that surrounds them. More specifically, endocrine glands produce messenger molecules called hormones (hormonz; “exciters”), which they release into the extracellular space. These hormones then enter nearby capillaries and travel through the bloodstream to specific target organs, which are commonly far removed from the endocrine gland that produces the hormone. Each hormone signals its target organs to respond in some characteristic way. For example, endocrine cells in the intestine secrete a hormone that signals for the pancreas to release the enzymes that help digest a meal.
Although most endocrine glands derive from epithelia, some derive from other tissues.
Exocrine glands are numerous, and many of their products are familiar ones. All exocrine glands secrete their products onto body surfaces (skin) or into body cavities (like the digestive tube), and multicellular exocrine glands have ducts that carry their product to the epithelial surfaces. The activity of an exocrine secretion is local, that is, the secretion acts near the area where it is released. Exocrine glands are a diverse group: They include many types of mucus-secreting glands, the sweat glands and oil glands of the skin, salivary glands of the mouth, the liver (which secretes bile), the pancreas (which secretes digestive enzymes), mammary glands (which secrete milk), and many others.
Unicellular Exocrine Glands The only important ex- ample of a one-celled exocrine gland is the goblet cell. True to its name, a goblet cell is indeed shaped like a goblet, a drinking glass with a stem. Goblet cells are scattered within the epithelial lining of the intestines and respiratory tubes, between columnar cells with other functions. They produce mucin (musin), a glycoprotein (sugar protein) that dissolves in water when secreted. The resulting complex of mucin and water is viscous, slimy mucus. Mucus covers, protects, and lubricates many internal body surfaces.
Multicellular Exocrine Glands Each multicellular exocrine gland has two basic parts: an epithelium-walled duct and a secretory unit consisting of the secretory epithelium. Also, in all but the simplest glands, a supportive connective tissue surrounds the secretory unit, carrying with it blood vessels and nerve fibers. Often, the connective tissue forms a fibrous capsule that extends into the gland proper and partitions the gland into subdivisions called lobes (not illustrated).
Multicellular glands are classified by the structure of their ducts. Simple glands have an unbranched duct, whereas compound glands have a branched duct. The glands are further categorized by their secretory units: They are tubular if their secretory cells form tubes and alveolar (alveolar) if the secretory cells form spherical sacs (alveolus – a small, hollow cavity). Furthermore, some glands are tubuloalveolar; that is, they contain both tubular and alveolar units. Another word for alveolar is acinar (asınar; acinus – grape or berry).
Epithelial Surface Features
As previously described, epithelial tissues are composed of many cells closely joined together by special cell junctions along their lateral walls. Epithelial tissues also have distinct apical and basal regions. The basal region sits on a specialized boundary with the underlying connective tissue; the apical region of certain epithelia has modifications associated with specific functions. These special features are described next.
Lateral Surface Features: Cell Junctions
Three factors act to bind epithelial cells to one another: (1) adhesion proteins in the plasma membranes of the adjacent cells link together in the narrow extracellular space; (2) the wavy contours of the membranes of adjacent cells join in a tongue-and-groove fashion; and (3) there are special cell junctions. Cell junctions, the most important of the factors, are characteristic of epithelial tissue but are found in other tissue types as well.
In the apical region of most epithelial tissues, a beltlike junction extends around the periphery of each cell. This is a tight junction, or a zonula occludens (zonulah okloodenz; “belt that shuts off”). At tight junctions, the adjacent cells are so close that some proteins in their plasma membranes are fused. This fusion forms a seal that closes off the extracellular space; thus tight junctions prevent molecules from passing between the cells of epithelial tissue. For example, the tight junctions in the epithelium lining the digestive tract keep digestive enzymes, ions, and microorganisms in the intestine from seeping into the bloodstream. Tight junctions need not be entirely impermeable; some are more leaky than others and may let certain types of ions through.
Adhesive Belt Junctions Just below the tight junctions in epithelial tissues are adhesive belt junctions, or zonula adherens (zonulah adhirens), a type of anchoring junction. Transmembrane linker proteins attach to the actin microfilaments of the cytoskeleton and bind adjacent cells. This junction reinforces the tight junctions, particularly when the tissues are stretched. Together with tight junctions, these form the tight junctional complex around the apical lateral borders of epithelial tissues.
Desmosomes. The main junctions for binding cells together are called desmosomes (dezmosomz; “binding bodies”), or anchoring junctions. These adhesive spots are scattered along the abutting sides of adjacent cells. Desmosomes have a complex structure: On the cytoplasmic face of each plasma membrane is a circular plaque. The plaques of neighboring cells are joined by linker proteins. These project from both cell membranes and interdigitate like the teeth of a zipper in the extracellular space. In addition, intermediate filaments (the cytoskeletal elements that resist tension) insert into each plaque from its inner, cytoplasmic side. Bundles of these filaments extend across the cytoplasm and anchor at other desmosomes on the opposite side of the same cell. Overall, this arrangement not only holds adjacent cells together but also interconnects intermediate filaments of the entire epithelium into one continuous net- work of strong guy-wires. The epithelium is thus less likely to tear when pulled on, because the pulling forces are distributed evenly throughout the sheet.
Desmosomes are found in cardiac muscle tissue as well as in epithelial tissues. In general, these junctions are common in tissues that experience great mechanical stress.
Gap Junctions. A gap junction, or nexus (neksus; “bond”), is a tunnel-like junction that can occur anywhere along the lateral membranes of adjacent cells. Gap junctions function in intercellular communication by al- lowing small molecules to move directly between neighboring cells. At such junctions, the adjacent plasma membranes are very close, and the cells are connected by hollow cylinders of protein (connexons). Ions, simple sugars, and other small molecules pass through these cylinders from one cell to the next. Gap junctions are common in embryonic tissues and in many adult tissues, including connective tissues. They are also prevalent in smooth and cardiac muscle, where the pas- sage of ions through gap junctions synchronizes contraction.
Basal Feature: The Basal Lamina
At the border between the epithelium and the connective tissue deep to it is a supporting sheet called the basal lamina (lamınah; “sheet”). This thin, noncellular sheet consists of proteins secreted by the epithelial cells. Functionally, the basal lamina acts as a selective filter; that is, it determines which molecules from capillaries in the under- lying connective tissue are allowed to enter the epithelium. The basal lamina also acts as scaffolding along which regenerating epithelial cells can migrate. Luckily, infections and toxins that destroy epithelial cells usually leave the basal lamina in place, for without this lamina, epithelial regeneration is more difficult.
Directly deep to the basal lamina is a layer of reticular fibers (defined shortly) belonging to the underlying connective tissue. Together, these reticular fibers plus the basal lamina form the basement membrane. The thin basal lamina can be seen only by electron microscopy, but the thicker basement membrane is visible by light microscopy. Although this text distinguishes between basal lamina and basement membrane, many scientists use these two terms interchangeably.
Apical Surface Features: Microvilli and Cilia
Microvilli (microvıli; “little shaggy hairs”) are fingerlike extensions of the plasma membrane of apical epithelial cells. Each microvillus contains a core of actin filaments that extend into the actin microfilaments of the cytoskeleton and function to stiffen the microvillus. Microvilli occur on almost every moist epithelium in the body but are longest and most abundant on epithelia that absorb nutrients (in the small intestine) or transport ions (in the kidney). In such epithelia, microvilli maximize the surface area across which small molecules enter or leave cells. Microvilli are also abundant on epithelia that secrete mucus, where they help anchor the mucous sheets to the epithelial surface.
Cilia are whiplike, highly motile extensions of the apical surface membranes of certain epithelial cells. Each cilium contains a core of microtubules held together by cross-linking and radial proteins. The micro- tubules are arranged in pairs, called doublets, with nine outer doublets encircling one central pair. Ciliary movement is generated when adjacent doublets grip one another with side arms made of the motor protein dynein and these arms start to oscillate. This causes the doublets to slide along the length of each other, like centipedes trying to run over each other’s backs. As a result, the cilum bends.
The microtubules in cilia are arranged in much the same way as in the cytoplasmic organelles called centrioles. Indeed, cilia originate as their microtubules assemble around centrioles that have migrated from the centrosome to the apical plasma membrane. The centriole at the base of each cilium is called a basal body.
The cilia on an epithelium bend and move in coordinated waves, like waves across a field of grass on a windy day. These waves push mucus and other substances over the epithelial surface. Each cilium executes a propulsive power stroke. This sequence ensures that fluid is moved in one direction only. An extremely long, isolated cilium is called a flagellum (flahjelum; “whip”). The only flagellated cells in the human body are sperm, which use their flagella to swim through the female reproductive tract.
The second of the four basic types of tissue is connective tissue, the most diverse and abundant type of tissue. There are four main classes of connective tissue and many sub- classes. The main classes are (1) connective tissue proper, familiar examples of which are fat tissue and the fibrous tissue of ligaments; (2) cartilage; (3) bone tissue; and (4) blood. Connective tissues do far more than just connect the tissues and organs of the body together. They also form the basis of the skeleton (bone and cartilage), store and carry nutrients (fat tissue and blood), surround all the blood vessels and nerves of the body (connective tissue proper), and lead the body’s fight against infection.
In this section, we will first discuss the special characteristics of connective tissues and then describe the structural elements found in connective tissues. Finally, we will review the structure, function, and location of the specific types of connective tissues.
Special Characteristics of Connective Tissues
As different as they are, fat, bone, and blood are all connective tissues. All connective tissues share the same simple structural plan.
Relatively few cells, lots of extracellular matrix. The cells of connective tissues are separated from one another by a large amount of extracellular material called the extracellular matrix (matriks; “womb”). This differs markedly from epithelial tissue, whose cells crowd closely together.
Extracellular matrix composed of ground substance and fibers. The extracellular matrix is produced by the cells of the connective tissue. It is composed of some type of ground substance embedded with protein fibers. The ground substance varies for each class of connective tissue. In many, it is a soft, gel-like substance that holds tissue fluid. In bone, it is hard—calcified by inorganic calcium salts. The fibrous portion of the matrix provides support for the connective tissue. Three types of protein fibers are found in connective tissues: collagen fibers, reticular fibers, and elastic fibers. The types, density, and distribution of the fibers are distinctive for each type of connective tissue. The differences among the physical properties and functions of each type of connective tissue are due to differences in the composition of the extracellular matrix. The details of the matrix structure will be discussed with each type of connective tissue.
Embryonic origin. Another feature common to connective tissues is that they all originate from the embryonic tissue called mesenchyme.
Classification of connective tissues
Type of connective tissue
PROPER CONNECTIVE TISSUES
Loose connective tissue
In all organs
dense connective tissue
Dense Regular Connective Tissue
External organ and vessel coats, septums, scleral coats, periost, perichondrium, joint capsules
Dense Irregular Connective Tissue
Tendons, bands, fascias
Elastic connective tissue
Component part of arterial walls of elastic type, in particular, aorta, yellow ligamentums, elastic larynx cone and its vocal ligamentums
SPECIAL CONNECTIVE TISSUES
organs of hematopoietic and immune system
hypoderm, epiploons and oil-bags, surrounds eye-bulbs and lymph nodes
brown adipose tissue
Prevails for babies and infant children , in particular in the area of neck
Iris and own vascular uveal tract, pial membrane , skin of external genital organs and anus
SCELETAL CONNECTIVE TISSUES
Embryo skeleton, for adults: articular cartilage, costal cart, cartilages of nose and airways
An auricle, epiglottic cartilage, cuneiform and corniculata cartilages of larynx
The Intervertebral disks, pubic symphysis, fibroplates and meniscuses
In the areas of bone, where the tendons of muscles register in the sutures of skull after their accreting
membrane reticulated bone
mesenchyme (embryotic connective tissue)
Fills space between primitive tissue layers during forming of the organs
As part of funiculus
Blood and lymph
Blood and lymphatic vessels
Structural Elements of Connective Tissues
Connective tissues are composed of cells and a significant amount of extracellular matrix. Although connective tissues differ in their structural properties because of differences in the types of cells and the composition of the extracellular matrix, they all share similar structural elements. As we describe the structural features of connective tissues, we will use loose areolar connective tissue as a model to illustrate these features.
In most connective tissues, the primary cell type produces the extracellular matrix. In connective tissue proper, these cells are called fibroblasts (literally, “fiber buds” or “fiber formers”). Fibroblasts make the protein subunits of fibers, which then assemble into fibers when the fibroblast secretes them. Fibroblasts also secrete the molecules that form the ground substance of the matrix. In cartilage tissue, the cells that secrete the matrix are chondroblasts (kondroblasts; “cartilage formers”), and in bone they are osteoblasts (osteoblasts¿; “bone formers”). Once these tissue-forming cells are not actively secreting new matrix, they are termed fibrocytes, chondrocytes, and osteocytes (cyte-cell). They function to maintain the tissue matrix and keep the tissue healthy.
The cells found in blood are an exception. These cells do not produce the plasma matrix of blood. The cellular components of blood function to carry respiratory gases (red blood cells), to fight infections (white blood cells), and to aid in blood clotting (platelets).
Additional cell types are found in many connective tissues. For example, fat cells store energy; various white blood cells (neutrophils, lymphocytes, and macrophages) respond to and protect against infectious agents; and mast cells release signaling molecules that mediate the inflammation reaction and promote healing.
The extracellular matrix of a connective tissue is composed of fibers and ground substance. Three types of fibers are found in connective tissues: collagen fibers, reticular fibers, and elastic fibers. In general, the fibers function in support, yet each type of fiber contributes unique properties to the connective tissue.
Collagen fibers, the thick whitish gray fibers, are the strongest and most abundant type of fiber in connective tissues. Collagen fibers resist tension (pulling forces) and contribute strength to a connective tissue. Pulling tests show that collagen fibers are stronger than steel fibers of the same size! The thick collagen fibers that one sees with the light microscope are bundles of thinner collagen fibrils, which consist of still thinner strands that are strongly cross- linked to one another. This cross-linking is the source of collagen’s great tensile strength.
Reticular (retikular) fibers, the thin blue fibers, are bundles of a special type of collagen fibril. These short fibers cluster into a meshlike network (reticulum- network) that covers and supports the structures bordering the connective tissue. For example, capillaries are coated with fuzzy nets of reticular fibers, and these fibers form part of the basement membrane of epithelia. Individual reticular fibers glide freely across one another when the network is pulled, so they allow more give than collagen fibers do.
The last fiber type found in certain connective tissues is elastic fibers. Elastic fibers contain the rubberlike protein elastin (elastin), which allows them to function like rubber bands. While we typically think of rubber bands as being stretchy, the defining feature of an elastic material is its ability to recoil back to its original form after being stretched. Connective tissue can stretch only so much before its thick, ropelike collagen fibers become taut. When the tension is released, the elastic fibers recoil, and the stretched tissue resumes its original shape.
The other component of the matrix of connective tissue is the ground substance. The molecules that compose the ground substance are produced and secreted by the primary cell type of the connective tissue (fibroblasts, chondroblasts, or osteoblasts). The ground substance in most connective tissues is a gel-like material that consists of large sugar and sugar-protein molecules (proteoglycans and glycosaminoglycans). These molecules soak up fluid like a sponge. The fluid-filled ground substance functions to cushion and protect body structures (connective tissue proper), to withstand compressive stresses (cartilage), or to hold the tissue fluid that bathes all the cells in our body (areolar connective tissue). In bone tissue, the secreted ground substance is embedded with calcified mineral salts, which make the matrix hard and contribute to its function in supporting the body.
Blood is an exception. The ground substance of blood, plasma, is not produced by the blood cells. Blood plasma is composed of water (about 90%) and various dissolved proteins, nutrients, ions, gases, and other molecules. These molecules are either produced by cells in other organs and then secreted into blood (for example, plasma proteins and hormones) or transported into blood from an external source (water, air, and nutrients).
Classification of Connective Tissues
Connective Tissue Proper—Loose Connective Tissues
Connective tissue proper has two subclasses: loose connective tissue (areolar, adipose, and reticular) and dense connective tissue (dense irregular, dense regular, and elastic). These sub- classes are distinguished by the density of fibers: In loose connective tissues, the fibers are distributed throughout the tissue but are separated from each other by ground substance.
Areolar Connective Tissue Loose areolar connective tissue is the most widespread type of connective tissue proper. This tissue underlies almost all the epithelia of the body and surrounds almost all the small nerves and blood vessels, including the capillaries. The structure of this tissue reflects its basic functions, which are common to many other types of connective tissues:
(3) defending the body against infection;
(4) storing nutrients as fat.
The fibers of areolar connective tissue provide support. Areolar connective tissue has all three types of fibers in its extracelluar matrix: collagen fibers, reticular fibers, and elastic fibers (although the reticular fibers do not show up when the common histological stains hematoxylin and eosin are used).
The ground substance of areolar connective tissue holds fluid. All the cells of the body are bathed in tissue fluid, or interstitial fluid. This fluid is derived from leakage of fluid and small molecules from the blood as it travels through the capillaries. Nutrients and oxygen are delivered to cells and waste molecules are carried away from cells via diffusion through this fluid. Areolar connective tissue lies between the capillaries and all other cells and tissues in the body. It soaks up this tissue fluid, much like a sponge. Thus it keeps the body’s cells surrounded by fluid and facilitates the passage of nutrients, gases, waste products, and other molecules to and from the cells.
Areolar connective tissue is the body’s first line of defense against invading microorganisms, such as bacteria, viruses, fungi, and parasites. Lying just deep to the epithelial tissues that line the body surfaces and surrounding capillaries, it is in an ideal location to destroy microorganisms at their entry site—before they enter the capillaries and use the vascular system to spread to other locations. Areolar connective tissue contains a variety of defense cells.
Cellular defenses are not the only means by which areolar connective tissue fights infection. The viscous ground sub- stance and the dense networks of collagen fibers in the extra-cellular matrix slow the progress of invading microorganisms. Some bacteria, however, secrete enzymes that rapidly break down ground substance or collagen. Such matrix-degrading bacteria are highly invasive; that is, they spread rapidly through the connective tissues and are especially difficult for the body’s defenses to control. An example of such a bacterium is the Streptococcus strain responsible for strep throat.
A minor function of areolar connective tissue is to store energy reserves as fat. The large, fat-storing cells are fat cells, also called adipose (adıpos) cells, and adipocytes. Fat cells are egg-shaped, and their cytoplasm is dominated by a single, giant lipid droplet that flattens the nucleus and cytoplasm at one end of the cell. Mature fat cells are among the largest cells in the body and cannot divide. In areolar connective tissue, fat cells occur singly or in small groups.
Adipose Tissue . Adipose tissue is similar to areolar connective tissue in structure and function, but its nutrient-storing function is much greater. Correspondingly, adipose tissue is crowded with fat cells, which account for 90% of its mass. These fat cells are grouped into large clusters called lobules. Adipose tissue is richly vascularized, reflecting its high metabolic activity. It removes lipids from the blood- stream after meals and later releases them into the blood, as needed. Without the fat stores in our adipose tissue, we could not live for more than a few days without eating.
Much of the body’s adipose tissue occurs in the layer beneath the skin called the hypodermis. Adipose tissue also is abundant in the mesenteries, which are sheets of serous membranes that hold the stomach and intestines in place. Fat in this location is called visceral fat. Additionally, fat forms cushioning pads around the kidneys and behind the eyeballs in the orbits.
Whereas the abundant fat beneath the skin serves the general nutrient needs of the entire body, smaller depots of fat serve the local nutrient needs of highly active organs. Such depots occur around the hard-working heart and around lymph nodes (where cells of the immune system are furiously fighting infection), within some muscles, and as individual fat cells in the bone marrow (where new blood cells are produced at a frantic rate). Many of these local depots offer special lipids that are highly enriched.
The typical, nutrient-storing fat is white adipose tissue or white fat. Another type, called brown adipose tissue, produces heat and is a nutrient consumer. Brown fat, thought to occur only in babies to aid in thermoregulation, has also been identified in adults. It is located in the hypodermis between the two scapulae (shoulder blades) in the center of the back, on the side of the anterior neck, and on the anterior abdominal wall. It is even more richly vascularized than white fat. Each brown-fat cell contains many lipid droplets and numerous mitochondria, which use the lipid fuel to heat the bloodstream rather than to produce ATP molecules.
Reticular Connective. Tissue Reticular connective tissue resembles areolar tissue, but the only fibers in its matrix are reticular fibers. These fine fibers form a broad, three-dimensional network like the frame of a house. The spaces in the framework create a labyrinth of caverns that hold many free cells. The bone marrow, spleen, and lymph nodes, which house many free blood cells outside their capillaries, consist largely of reticular connective tissue. Fibroblasts called reticular cells lie along the reticular net- work of this tissue.
Connective Tissue Proper—Dense
Connective Tissue. Dense connective tissue, or fibrous connective tissue, contains more collagen than areolar connective tissue does. With its thick collagen fibers, it can resist extremely strong pulling (tensile) forces. There are three types of dense connective tissue: irregular, regular, and elastic.
Dense Irregular Connective. Tissue Dense irregular connective tissue is similar to areolar tissue, but its collagen fibers are much thicker. These fibers run in different planes, allowing this tissue to resist strong tensions from different directions. This tissue dominates the leathery dermis of the skin, which is commonly stretched, pulled, and hit from various angles. This tissue also makes up the fibrous capsules that surround certain organs in the body, such as kidneys, lymph nodes, and bones. Its cellular and matrix elements are the same as in areolar connective tissue.
Dense irregular connective tissue is confusing, even for experts. The dermis obviously fits the name because its fibers run randomly, as one would expect for a tissue called “irregular.” However, the fibrous capsules of organs consist of two layers, with all the fibers in a layer running parallel to each other but perpendicular to the fibers in the other layer. That is, two perpendicular “regular” layers can make the tissue “irregular.”
Dense Regular Connective Tissue . All collagen fibers in dense regular connective tissue usually run in the same direction, parallel to the direction of pull. Crowded between the collagen fibers are rows of fibroblasts, which continuously manufacture the fibers and a scant ground substance. When this tissue is not under tension, its collagen fibers are slightly wavy. Unlike areolar connective tissue, dense regular connective tissue is poorly vascularized and contains no fat cells or defense cells.
With its enormous tensile strength, dense regular connective tissue is the main component of ligaments, bands or sheets that bind bones to one another. It also is the main tissue in tendons, which are cords that attach muscles to bones, and aponeuroses (aponuroses), which are sheet like tendons.
Dense regular connective tissue also forms fascia (fasheah; “a band”), a fibrous membrane that wraps around muscles, muscle groups, large vessels, and nerves. Many sheets of fascia occur throughout the body, binding structures together like plastic sandwich wrap. When the word fascia is used alone, it is understood to mean deep fascia. Superficial fascia, something else entirely, is the fatty hypodermis below the skin.
Elastic Connective Tissue. In elastic connective tissue, elastic fibers are the predominant type of fiber, and bundles of elastic fibers outnumber the bundles of collagen fibers. This tissue is located in structures where re- coil from stretching is important: within the walls of arteries, in certain ligaments (ligamentum nuchae and ligamentum flavum, which connect successive vertebrae), and surrounding
Cartilage. As you have seen, connective tissue proper has the ability to resist tension (pulling). Cartilage and bone are the firm connective tissues that resist compression (pressing) as well as tension. Like all connective tissues, they consist of cells separated by a matrix containing fibers, ground substance, and tissue fluid. However, these skeletal tissues exaggerate the supportive functions of connective tissue and play no role in fat storage or defense against disease.
Cartilage a firm but flexible tissue, occurs in several parts of the skeleton. For example, it forms the supporting rings of the trachea (windpipe) and gives shape to the nose and ears. Like nearly all connective tissues, cartilage consists of cells separated by an abundant extracellular matrix. This matrix contains thin collagen fibrils, a ground substance, and an exceptional quantity of tissue fluid; in fact, cartilage consists of up to 80% water! The arrangement of water in its matrix enables cartilage to spring back from compression.
Cartilage is simpler than other connective tissues: It contains no blood vessels or nerves and just one kind of cell, the chondrocyte. Each chondrocyte resides within a cavity in the matrix called a lacuna. Immature chondrocytes are chondroblasts, cells that actively secrete the matrix during cartilage growth. Cartilage is found in three varieties, each dominated by a particular fiber type: hyaline cartilage, elastic cartilage, and fibrocartilage.
Because of its rocklike hardness, bone tissue has a tremendous ability to support and protect body structures. Bone matrix contains inorganic calcium salts (bone salts), which enable bone to resist compression, and an abundance of collagen fibers, which allow bone to withstand strong tension.
Immature bone cells, called osteoblasts, secrete the collagen fibers and ground substance of the matrix. Then bone salts precipitate on and between the collagen fibers, hardening the matrix. The mature bone cells, called osteocytes, inhabit cavities (lacunae) in this hardened matrix. Bone is a living and dynamic tissue, well supplied with blood vessels.
Blood, the fluid in the blood vessels, is the most atypical connective tissue. It does not bind things together or give mechanical support. It is classified as a connective tissue because it develops from mesenchyme and consists of blood cells surrounded by a nonliving matrix, the liquid blood plasma. Its cells and matrix are very different from those in other connective tissues. Blood functions as the transport vehicle for the cardiovascular system, carrying defense cells, nutrients, wastes, respiratory gases, and many other substances throughout the body.
Covering and Lining Membranes
Now that you have learned about connective and epithelial tissues, you can consider the covering and lining membranes that combine these two tissue types. These membranes, which cover broad areas within the body, consist of an epithelial sheet plus the underlying layer of connective tissue proper. These membranes are of three types: cutaneous, mucous, and serous.
The cutaneous membrane (kutaneus; “skin”) is the skin, covering the outer surface of the body. Its outer epithelium is the thick epidermis, and its inner connective tissue is the dense dermis. It is a dry membrane.
A mucous membrane, or mucosa (mukosah), lines the inside of every hollow internal organ that opens to the outside of the body. More specifically, mucous membranes line the tubes of the respiratory, digestive, reproductive, and urinary systems. Although different mucous membranes vary widely in the types of epithelia they contain, all are wet or moist. As their name implies, many mucous membranes secrete mucus. Not all of them do so, however.
All mucous membranes consist of an epithelial sheet directly underlain by a layer of loose areolar connective tis- sue called the lamina propria (lamınah propreah; “one’s own layer”). In the mucous membranes of the digestive sys- tem, the lamina propria rests on a layer of smooth muscle cells. Later chapters discuss the specific mucous membranes of the body.
Serous membranes, or serosae
A serous membrane consists of a simple squamous epithelium, called mesothelium, lying on a thin layer of areolar connective tissue. This membrane produces a slippery serous fluid, beginning as a filtrate of the blood in capillaries in the connective tissue, with the addition of lubricating molecules by the mesothelium.
The two remaining tissue types are muscle and nervous tissues. These are sometimes called composite tissues because, along with their own muscle or nerve cells, they contain small amounts of areolar connective tissue. (Areolar connective tissue surrounds all blood vessels, and both muscle and nervous tissue are richly vascularized.)
Muscle tissues bring about most kinds of body movements. Most muscle cells are called muscle fibers. They have an elongated shape and contract forcefully as they shorten. These cells contain many myofilaments (miofilahments; “muscle filaments”), cellular organelles filled with the actin and myosin filaments that bring about contraction in all cell type. There are three kinds of muscle tissue: skeletal, cardiac, and smooth.
Skeletal muscle tissue is the major component of organs called skeletal muscles, which pull on bones to cause body movements. Skeletal muscle cells are long, large cylinders that contain many nuclei. Their obvious striated, or banded, appearance reflects a highly organized arrangement of their myofilaments.
Cardiac muscle tissue occurs in the wall of the heart. It contracts to propel blood through the blood vessels. Like skeletal muscle cells, cardiac muscle cells are striated. However, they differ in two ways: (1) Each cardiac cell has just one nucleus, and (2) cardiac cells branch and join at special cellular junctions called intercalated (interkahlated) discs.
Smooth muscle tissue is so named because there are no visible striations in its cells. These cells are elongated with tapered ends and contain one centrally located nucleus. Smooth muscle primarily occurs in the walls of hollow viscera such as the digestive and urinary organs, uterus, and blood vessels. It generally acts to squeeze substances through these organs by alternately contracting and relaxing.
Nervous tissue is the main component of the nervous organs—the brain, spinal cord, and nerves—which regulate and control body functions. It contains two types of cells, neurons and supporting cells. Neurons are the highly specialized nerve cells that generate and conduct electrical impulses. They have extensions, or processes, that allow them to transmit impulses over substantial distances within the body. Dendrites are cell processes that extend from the cell body of a neuron like branches of a tree. The dendrites, the receptive region of the neuron, transmit signals toward the cell body. A neuron also has a singular, long cell process extending from its cell body, the axon, that generates nerve impulses and transmits them away from the cell body. The supporting cells, called neuroglia, are noncom-ducting cells that nourish, insulate, and protect the delicate neurons.
The skeletal system is composed of the bones, cartilages, and joints that form the internal framework. Bones provide support and shape to the body, attachment sites for muscle, and a storage depot for essential minerals. Cartilage, a component of many joints, aids in support and movement as it cushions abutting bone surfaces. Cartilage and bone are also linked developmentally; most bones are first formed in cartilage tissue, which is then replaced by bone tissue during prenatal and childhood growth.
The skeletal system also contains clues to our personal story. The bones within you are not like the dead, dried bones you are examining in the laboratory; rather, they are living, dynamic organs that reflect many aspects of your life: age, gender, ethnicity, height, and health and nutrition status. Clues to these details can be understood only by using knowledge of the structure and growth of bone tissue.
Location and Basic Structure
Cartilaginous structures are found throughout the adult human body. These cartilages, include (1) cartilage in the external ear; (2) cartilages in the nose; (3) articular cartilages, which cover the ends of most bones at movable joints; (4) costal cartilages, which connect the ribs to the sternum (breastbone); (5) cartilages in the larynx (voice box), including the epiglottis, a flap that keeps food from entering the larynx and the lungs; (6) cartilages that hold open the air tubes of the respiratory system; (7) cartilage in the discs between the vertebrae; (8) cartilage in the pubic symphysis; and (9) cartilages that form the articular discs within certain movable joints, the meniscus in the knee for example. Cartilage is far more abundant in the embryo than in the adult: Most of the skeleton is initially formed by fast-growing cartilage, which is subsequently replaced by bone tissue in the fetal and childhood periods. This process is described later in the chapter, pp. 135–136. A typical cartilaginous structure in the skeleton is com- posed of the connective tissue cartilage, which contains no nerves or blood vessels. The structure is surrounded by a layer of dense irregular connective tissue, the perichondrium (perıkondreum; “around the cartilage”), which acts like a girdle to resist outward expansion when the cartilage is subjected to pressure. The perichondrium also functions in the growth and repair of cartilage. Cartilage tissue consists primarily of water (60% to 80%) and is very resilient; that is, it has the ability to spring back to its original shape after being compressed. For more information on the unique properties of cartilage tissue.
Types of Cartilage
Three types of cartilage tissue occur in the body: hyaline cartilage, elastic cartilage, and fibrocartilage. While reading about these, keep in mind that cartilage is a connective tissue that consists of cells called chondrocytes and an abundant extracellular matrix. In contrast to connective tissue proper, in cartilage tissue each chondrocyte is located in a space in the matrix called a lacuna (lahkoonah), literally a “lake” or “cavity.” The matrix contains fibers and a jellylike ground substance of complex sugar molecules that attract and hold water.
Hyalinecartilage (hiahln; “glass” ) , which looks like frosted glass when viewed by the unaided eye, is the most abundant kind of cartilage. When viewed under the light microscope, its chondrocytes appear spherical. The only type of fiber in the matrix is a collagen unit fibril, which forms networks that are too thin to be seen with a light microscope. The gelatinous ground substance holds large amounts of water; thus, this tissue resists compression well. Hyaline cartilage provides support through flexibility and resilience. It makes up the articular cartilage that covers the ends of adjoining bones in movable joints. It also forms the cartilaginous attachments of the ribs to the sternum, accounts for most of the cartilage found in the respiratory structures, and forms the embryonic skeleton.
Elastic cartilage is similar to hyaline cartilage, but its matrix contains many elastic fibers along with the delicate collagen fibrils. This cartilage is more elastic than hyaline cartilage and better able to tolerate repeated bending. The epiglottis, which bends down to cover the glottis (opening) of the larynx each time we swallow, is made of elastic cartilage, as is the highly bendable cartilage in the outer ear.
Fibrocartilage is an unusual tissue that resists both strong compression and strong tension (pulling) forces. It occurs in certain ligaments and certain cartilages that experience both of these forces. It is a perfect structural intermediate between hyaline cartilage and dense regular connective tissue. Microscopically, it consists of thick collagen fibers (as in dense regular connective tissue) surrounding the chondrocytes within lacunae. Two specific locations of fibrocartilage are in the anulus fibrosus portion of the discs between the vertebrae and in the articular discs of some joints, for example the menisci of the knee.
Growth of Cartilage
A piece of cartilage grows in two ways. In appositional (apozishunal) growth, “growth from outside,” cartilage- forming cells (chondroblasts) in the surrounding perichondrium produce the new cartilage tissue by actively secreting matrix. In interstitial (interstishal) growth, “growth from within,” the chondrocytes within the cartilage divide and secrete new matrix. Cartilage stops growing in the late teens when the skeleton itself stops growing, and chondrocytes do not divide again. As a result, cartilage regenerates poorly in adults. The limited healing that occurs within cartilage in adults reflects the ability of the surviving chondrocytes to secrete more extracellular matrix, and typically cartilage is repaired with fibrocartilage.
Under certain conditions, crystals of calcium phosphate precipitate in the matrix of cartilage. Such calcification is a sign of aging in an adult, but in a child it is a normal stage in the growth of most bones. Note, however, that calcified cartilage is not bone: Bone and cartilage are always distinct tissues.
The bones of the skeleton are organs because they contain several different tissues. Although bone tissue predominates, bones also contain nervous tissue in nerves, blood tissue in blood vessels, cartilage in articular cartilages, and epithelial tissue lining the blood vessels. In our discussion of bone, we describe the functions of bone and then examine the composition of bone tissue. We then look at bone as an organ, examining the macroscopic and microscopic structure of bones, bone development and growth, and bone repair.
Functions of Bones
The functions of the skeletal system were briefly mentioned in the introductory paragraph of this chapter. More specifically, bone carries out the following functions:
Support. Bones provide a hard framework that supports the weight of the body. For example, the bones of the legs are pillars that support the trunk of the body in the standing person.
Movement. Skeletal muscles attach to the bones by tendons and use the bones as levers to move the body and its parts. As a result, humans can walk, grasp objects, and move the rib cage to breathe. The arrangement of the bones and the structure of the joints determine the types of movement that are possible. Support and movement are mutually dependent functions: The supportive frame- work is necessary for movement, and the skeletal muscles contribute significantly to the support of body weight.
Protection. The bones of the skull form a protective case for the brain. The vertebrae surround the spinal cord, and the rib cage helps protect the organs of the thorax.
Mineral storage. Bone serves as a reservoir for minerals, the most important of which are calcium and phosphate. The stored minerals are released into the bloodstream as ions for distribution to all parts of the body as needed.
Blood cell formation and energy storage. Bones contain red and yellow bone marrow. Red marrow makes the blood cells, and yellow marrow is a site of fat storage, with little or no role in blood cell formation.
Energy metabolism. The role of bone cells in regulating energy metabolism has just recently been identified. Bone-producing cells, osteoblasts (described in the next section), secrete a hormone that influences blood sugar regulation. This hormone, osteocalcin, stimulates pancreatic secretions that reduce blood sugar levels (insulin). Osteocalcin also influences fat cells, causing them to store less fat and to secrete a hormone that increases the insulin sensitivity of cells. These results have clinical implications for the treatment of metabolic disorders related to blood sugar regulation.
Like other connective tissues, bone tissue consists of cells separated by an extracellular matrix. Unlike other connective tissues, bone has both organic and inorganic components. The organic components are the cells, fibers, and ground substance. The inorganic components are the mineral salts that invade the bony matrix, making bone tissue hard. Bone does contains a small amount of tissue fluid, although bone contains less water than other connective tissues.
As with other connective tissues, it is the unique composition of the matrix that gives bone its exceptional physical properties. The organic components of bone tissue account for 35% of the tissue mass. These organic substances, particularly collagen, contribute the flexibility and tensile strength that allow bone to resist stretching and twisting. Collagen is remarkably abundant in bone tissue.
The balance of bone tissue, 65% by mass, consists of in- organic hydroxyapatites (hidrokseapahtıtz), or mineral salts, primarily calcium phosphate. These mineral salts are present as tiny crystals that lie in and around the collagen fibrils in the extracellular matrix. The crystals pack tightly, providing bone with its exceptional hardness, which enables it to resist compression. These mineral salts also explain how bones can endure for hundreds of millions of years, providing information on the sizes, shapes, lifestyles, and even some of the diseases (for example, arthritis) of ancient vertebrates.
Soaking a long bone in a weak acid for several weeks dissolves away the bone’s mineral salts, leaving only the organic component, mainly collagen. The demineralized bone can be tied in a knot, demonstrating the great flexibility provided by the collagen in bone. It also confirms that without its mineral content, bone bends too easily to support weight. Baking a bone at high temperature destroys the organic portion of bone. What remains is the mineral component, stiff but extremely brittle. The proper combination of organic and inorganic elements allows bones to be exceedingly durable, strong, and resilient without being brittle.
The composite nature of bone can be compared to reinforced concrete: The collagen fibers, like the steel rods, provide tensile strength; the mineral salts, like the sand and rock in the concrete, provide compressional strength. In fact, bone is stronger than reinforced concrete in resisting compression and almost equal in tensile strength. Neither bone nor reinforced concrete resist torsional forces well. Indeed, most fractures to limb bones are caused by torsional forces.
Three types of cells in bone tissue produce or maintain the tis- sue: osteogenic cells, osteoblasts, and osteocytes. Osteogenic cells (osteojenik; osteo -bone, genic- producing) are stem cells that differentiate into bone-forming osteoblasts. Osteoblasts (osteoblasts; blast -bud, sprout) are cells that actively produce and secrete the organic components of the bone matrix: the ground substance and the collagen fibers. This bone matrix secreted by osteoblasts is called osteoid (osteoid, “bonelike”). Within a week, inorganic calcium salts crystallize within the osteoid. Once osteoblasts are completely surrounded by bone matrix and are no longer producing new osteoid, they are called osteocytes. Osteocytes (cyte -cell) function to keep the bone matrix healthy. In fact, if osteocytes die or are destroyed, the bone matrix is resorbed.
The cells responsible for the resorption of bone are the fourth type of cell found within bone tissue, osteoclasts. Osteoclasts (clast- break) are derived from a lineage of white blood cells. These multinucleated cells break down bone by secreting hydrochloric acid, which dissolves the mineral component of the matrix, and lysosomal enzymes, which digest the organic components.
Creating and destroying bone tissue is an ongoing process in normal, healthy bones. Destroying old bone tissue and replacing it with new tissue helps to keep our bones strong and enables them to respond to changing stresses. When we are physically active, new bone is formed, strengthening our skeletal support; when we are incapacitated, as with a broken limb or bedridden with illness, bone is resorbed because it is not needed to support the body. We will return to this process, called bone remodeling, later in the chapter.
Classification of Bones
Bones come in many sizes and shapes. For example, the tiny pisiform bone of the wrist is the size and shape of a pea, whereas the femur (thigh bone) is large and elongated. The shape of each bone reflects its function and formation. The femur, for example, must withstand great weight and pressure, and its hollow cylindrical design provides maximum strength with minimum weight.
Bones are classified by their shape as long, short, flat, or irregular.
Long bones. As their name suggests, long bones are considerably longer than they are wide. A long bone has a shaft plus two distinct ends. Most bones in the limbs are long bones. These bones are named for their elongated shape, not their overall size: The bones of the fingers and toes are long bones, even though they are small.
Short bones. Short bones are roughly cube-shaped. They occur in the wrist and the ankle.
Sesamoid bones (sesahmoid; “shaped like a sesame seed”) are a special type of short bone that forms within a tendon. A good example is the kneecap, or patella. Sesamoid bones vary in size and number in different people. Some clearly act to alter the direction of pull of a tendon. Others reduce friction and modify pressure in tendons, thus reducing abrasion or tearing.
Flat bones. Flat bones are thin, flattened, and usually somewhat curved. Most cranial bones of the skull are flat, as are the ribs, sternum (breastbone), and scapula (shoulder blade).
Irregular bones. Irregular bones have various shapes that do not fit into the previous categories. Examples are the vertebrae and hip bones.
Gross Anatomy of Bones
Compact and Spongy Bone.
Almost every bone of the skeleton has a dense outer layer that looks smooth and solid to the naked eye. This external layer is compact bone. Internal to this is spongy bone, also called trabecular bone, a honeycomb of small needle-like or flat pieces called trabeculae (trahbekule; “little beams”). In this network, the open spaces between the trabeculae are filled with red or yellow bone marrow.
Structure of a Typical Long Bone
With few exceptions, all the long bones in the body have the same general structure.
Diaphysis and Epiphyses The tubular diaphysis (diafısis), or shaft, forms the long axis of a long bone; the epiphyses (epifısez) are the bone ends. The joint surface of each epiphysis is covered with a thin layer of hyaline cartilage called the articular cartilage. Between the diaphysis and each epiphysis of an adult long bone is an epiphyseal line. This line is a remnant of the epiphyseal plate, a disc of hyaline cartilage that grows during childhood to lengthen the bone.
Blood Vessels Unlike cartilage, bones are well vascularized. In fact, at any given time between 3% and 11% of the blood in the body is in the skeleton. The main vessels serving the diaphysis are a nutrient artery and a nutrient vein. Together these run through a hole in the wall of the diaphysis, the nutrient foramen (foramen; “opening”). The nutrient artery runs inward to supply the bone marrow and the spongy bone. Branches then extend outward to supply the compact bone. Several epiphyseal arteries and veins serve each epiphysis in the same way.
The Medullary Cavity The interior of all bones consists largely of spongy bone. However, the very center of the diaphysis of long bones contains no bone tissue at all and is called the medullary cavity (medulare; “middle”) or marrow cavity. As its name implies, this cavity is filled with yellow bone marrow. Recall that the spaces between the trabeculae of spongy bone are also filled with marrow.
Membranes A connective tissue membrane called the periosteum (pereosteum; “around the bone”) covers the entire outer surface of each bone except on the ends of the epiphyses, where articular cartilage occurs. This periosteal membrane has two sublayers: a superficial layer of dense irregular connective tissue, which resists tension placed on a bone during bending, and a deep layer that abuts the compact bone. This deep layer is osteogenic, containing bone-depositing cells (osteoblasts) and bone- destroying cells (osteoclasts). These cells remodel bone surfaces throughout our lives. The osteogenic cells of the deep layer of the periosteum are indistinguishable from the fibroblasts within this layer. During periods of bone growth or deposition, the osteogenic cells differentiate into osteoblasts. These osteoblasts produce the layers of bone tissue that encircle the perimeter of the bone, the circumferential lamellae.
The periosteum is richly supplied with nerves and blood vessels, which is why broken bones are painful and bleed profusely. The vessels that supply the periosteum are branches from the nutrient and epiphyseal vessels. The periosteum is secured to the underlying bone by perforating fibers (Sharpey’s fibers), thick bundles of collagen that run from the periosteum into the bone matrix. The periosteum also provides insertion points for the tendons and ligaments that attach to a bone. At these points, the perforating fibers are exceptionally dense.
Whereas periosteum covers the external surface of bones, internal bone surfaces are covered by a much thinner connective tissue membrane called endosteum (endosteum; “within the bone”). Specifically, endosteum covers the trabeculae of spongy bone; it also lines the central canals of osteons. Like periosteum, endosteum is osteogenic, containing both osteoblasts and osteoclasts.
Structure of Short, Irregular, and Flat Bones
Short, irregular, and flat bones have much the same composition as long bones: periosteum-covered compact bone externally and endosteum-covered spongy bone internally. However, because these bones are not cylindrical, they have no diaphysis. They contain bone marrow (between the trabeculae of their spongy bone), but no marrow cavity is present. In flat bones, the internal spongy bone is called diploë (diploe; “double”). A flat bone might be likened to a reinforced sandwich in structure.
Bone Design and Stress
The anatomy of each bone reflects the stresses most commonly placed on it. Bones are subjected to compression as weight bears down on them or as muscles pull on them. The loading usually is applied off center, however, and threatens to bend the bone. Bending compresses the bone on one side and stretches it (subjects it to tension) on the other. Both compression and tension are greatest at the external bone surfaces. To resist these maximal stresses, the strong, compact bone tis- sue occurs in the external portion of the bone. Internal to this region, however, tension and compression forces tend to cancel each other out, resulting in less overall stress. Thus, compact bone is not found in the bone interiors; spongy bone is sufficient. Because no stress occurs at the bone’s center, the lack of bone tissue in the central medullary cavity does not impair the strength of long bones. In fact, a hollow cylinder is stronger than a solid rod of equal weight, thus this design is efficient from a biological as well as a mechanical perspective. The spongy bone and marrow cavities lighten the heavy skeleton and provide room for the bone marrow.
Spongy bone is not a random network of bone fragments. Instead, the trabeculae of spongy bone seem to align along stress lines in an organized pattern of tiny struts as crucially positioned as the flying buttresses that support the walls of a Gothic cathedral.
The surfaces of bones also reflect the stresses that are applied to the bone. The superficial surfaces have distinct bone markings that fitin to three categories:
(1) projections that are the attachment sites for muscles and ligaments;
(2) srfaces that form joints;
(3) depressions and openings.
These bone markings provide a wealth of information about the functions of bone and muscles, and on the relationship of bones to their associated soft structures.
Microscopic Structure of Bone
Viewed by the unaided eye, compact bone looks solid. However, microscopic examination reveals that it is riddled with passageways for blood vessels and nerves. An important structural component of compact bone is the osteon (osteon; “bone”), or Haversian (havershan) system. Osteons are long, cylindrical structures oriented parallel to the long axis of the bone and to the main compression stresses. Functionally, osteons can be viewed as miniature weight-bearing pillars. Structurally, an osteon is a group of concentric tubes resembling the rings of a tree trunk in cross section. Each of the tubes is a lamella (lahmelah; “little plate”), a layer of bone matrix in which the collagen fibers and mineral crystals align and run in a single direction. However, the fibers and crystals of adjacent lamellae always run in roughly opposite directions. This alternating pattern is optimal for withstanding torsion, or twisting, stresses. The lamellae of bone also inhibit crack propagation. When a crack reaches the edge of a lamella, the forces causing the crack are dispersed around the lamellar boundaries, thus preventing the crack from progressing into deeper parts of the bone and causing fracture.
Through the core of each osteon runs a canal called the central canal, or Haversian canal. Like all internal bone cavities, it is lined by endosteum. The central canal contains its own blood vessels, which supply nutrients to the bone cells of the osteon, and its own nerve fibers. The endosteum that lines the central canal is an osteogenic layer. Unlike the growth rings in trees, lamellae of bone tissue are added to the inner surface of the osteon, thus decreasing the diameter of the central canal. Perforating canals, also called Volkmann’s (folkmahnz) canals, lie at right angles to the central canals and connect the blood and nerve supply of the periosteum to that of the central canals and the marrow cavity.
The mature bone cells, the osteocytes, are spider-shaped. Their bodies occupy small cavities in the solid matrix called lacunae (“little lakes”), and their “spider legs” occupy thin tubes called canaliculi (kanahlikuli). These “little canals” run through the matrix, connecting neighboring lacunae to one another and to the nearest capillaries, such as those in the central canals. Within the canaliculi, the extensions of neighboring osteocytes touch each other and form gap junctions. Nutrients diffusing from capillaries in the central canal pass across these gap junctions, from one osteocyte to the next, throughout the entire osteon. This direct transfer from cell to cell is the only way to supply the osteocytes with the nutrients they need, because the intervening bone matrix is too solid and impermeable to act as a diffusion medium.
Not all lamellae in compact bone occur within osteons. Lying between the osteons are groups of incomplete lamellae called interstitial (interstishal) lamellae. These are simply the remains of old osteons that have been cut through by bone remodeling. Additionally, circumferential lamellae occur in the external and internal surfaces of the layer of compact bone; each of these lamellae extends around the entire circumference of the diaphysis. Functioning like an osteon but on a much larger scale, the circumferential lamellae effectively resist twisting of the entire long bone.
The microscopic anatomy of spongy bone is less complex than that of compact bone. Each trabecula contains several layers of lamellae and osteocytes but is too small to contain osteons or vessels of its own. The osteocytes receive their nutrients from capillaries in the endosteum surrounding the trabecula via connections through the canaliculi.
Bone Development and Growth
Osteogenesis (osteojenesis) and ossification are both names for the process of bone-tissue formation. Osteogenesis begins in the embryo, proceeds through childhood and adolescence as the skeleton grows, and then occurs at a slower rate in the adult as part of a continual remodeling of the full-grown skeleton. Before week 8, the skeleton of the human embryo consists only of hyaline cartilage and some membranes of mesenchyme, an embryonic connective tissue. Bone tissue first appears in week 8 and eventually replaces most cartilage and mesenchymal membranes in the skeleton. Some bones, called membrane bones, develop from a mesenchymal membrane through a process called intramembranous ossification (intra-inside). Other bones develop as hyaline cartilage, which is replaced through a process called endochondral ossification (endo-within; chondro-cartilage). These bones are called endochondral bones or cartilage replacement bones. Intramembranous Ossification Membrane bones form directly from mesenchyme without first being modeled in cartilage. All bones of the skull, except a few at the base of the skull, are of this category. The clavicles (collarbones) are the only bones formed by intramembranous ossification that are not in the skull. Intramembranous ossification proceeds in the following way: During week 8 of embryonic development, mesenchymal cells cluster within the connective tissue membrane and become bone-forming osteoblasts. These cells begin secreting the organic part of bone matrix, called osteoid, which then becomes mineralized.
Once surrounded by their own matrix, the osteoblasts are called osteocytes. The new bone tissue forms between embryonic blood vessels, which are woven in a random network. The result is woven bone tissue, with trabeculae arranged in networks. This embryonic tissue lacks the lamellae that occur in mature spongy bone. During this same stage, more mesenchyme condenses just external to the developing membrane bone and becomes the periosteum.
To complete the development of a membrane bone, the trabeculae at the periphery grow thicker until plates of com- pact bone are present on both surfaces. In the center of the membrane bone, the trabeculae remain distinct, and spongy bone results.
All bones from the base of the skull down, except for the clavicles, are endochondral bones. They are first modeled in hyaline cartilage, which then is gradually replaced by bone tissue. Endochondral ossification begins late in the second month of development and is not completed until the skeleton stops growing in early adulthood. Growing endochondral bones increase both in length and in width. The following stages outline only the increase in length, using a large long bone as an example.
A bone collar forms around the diaphysis. In the late embryo (week 8), the endochondral bone begins as a piece of cartilage called a cartilage model. Like all cartilages, it is surrounded by a perichondrium. Then, at the end of week 8 of development, the perichondrium surrounding the diaphysis is invaded by blood vessels and becomes a bone- forming periosteum. Osteoblasts in this new periosteum lay down a collar of bone tissue around the diaphysis.
Cartilage calcifies in the center of the diaphysis. At the same time the bone collar forms, the chondrocytes in the center of the diaphysis enlarge (hypertrophy) and signal the surrounding cartilage matrix to calcify. The matrix of calcified cartilage is impermeable to diffusing nutrients. Cut off from all nutrients, the chondrocytes die and disintegrate, leaving cavities in the cartilage. No longer maintained by chondrocytes, the cartilage matrix starts to deteriorate. This does not seriously weaken the diaphysis, which is well stabilized by the bone collar around it. These changes affect only the center of the diaphysis. Elsewhere, the cartilage remains healthy and continues to grow, causing the entire endochondral bone to elongate.
The periosteal bud invades the diaphysis, and the first bone trabeculae form. In the third month of development, the cavities within the diaphysis are invaded by a collection of elements called the periosteal bud. This bud consists of a nutrient artery and vein, along with the cells that will form the bone marrow. Most important, the bud contains bone-forming and bone-destroying cells (osteogenic stem cells and osteoclasts). The entering osteoclasts partly erode the matrix of calcified cartilage, and the osteogenic cells differentiate into osteoblasts, which secrete osteoid around the remaining fragments of this matrix, forming bone- covered trabeculae. In this way, the earliest version of spongy bone appears within the diaphysis.
By the third month of development, bone tissue continues to form around the diaphysis from the periosteum and has begun to appear in the center of the diaphysis. This bone tissue of the diaphysis makes up the primary ossification center.
Diaphysis elongates, and the medullary cavity forms. Throughout the rest of the fetal period, the cartilage of the epiphysis continues to grow rapidly, with the part nearest the diaphysis continually calcifying and being replaced by the bone trabeculae, thus elongating the diaphysis.
Osteoclasts in turn break down the ends of these bone trabeculae to form a central, boneless medullary cavity.
Shortly before or after birth, the epiphyses gain bone tissue: First, the cartilage in the center of each epiphysis calcifies and degenerates. Then, a bud containing the epiphyseal vessels invades each epiphysis. Bone trabeculae appear, just as they appeared earlier in the primary ossification center. The areas of bone formation in the epiphyses are called secondary ossification centers. The larger long bones of the body can have several ossification centers in each epiphysis.
Epiphyses ossify, and cartilaginous epiphyseal plates separate diaphysis and epiphyses. After the secondary ossification centers have appeared and epiphyses have largely ossified, hyaline cartilage remains at only two places:
(1) on the epiphyseal surfaces, where it forms the articular cartilages;
(2) between the diaphysis and epiphysis, where it forms the epiphyseal plates.
The epiphyseal plates, also called growth plates, are responsible for lengthening the bones during the two decades following birth.
Anatomy of the Epiphyseal Plate
In both the epiphyses of the fetus and the epiphyseal plates of the child, the cartilage is organized in a way that allows it to grow exceptionally quickly and efficiently. The cartilage cells nearest the epiphysis are relatively small and inactive. This region is called the resting (quiescent) zone. Below the resting zone, the cartilage cells form tall columns, like coins in a stack. The chondroblasts at the “top” of the stack in the proliferation zone divide quickly, pushing the epiphysis away from the diaphysis, thereby causing the entire long bone to lengthen. The older chondrocytes deeper in the stack, in the hypertrophic zone, enlarge and signal the surrounding matrix to calcify. In the calcification zone the cartilage matrix becomes calcified and the chondrocytes die. This process leaves long spicules (trabeculae) of calcified cartilage on the diaphysis side of the epiphysis-diaphysis junction. These spicules are partly eroded by osteoclasts, then covered with bone tissue by osteoblasts, forming spicules of bone. This region is the ossification zone. These bony spicules are destroyed from within the diaphysis by the action of osteoclasts at the same rate that they are formed at the epiphysis; thus they stay a constant length and the marrow cavity grows longer as the long bone lengthens.
Postnatal Growth of Endochondral Bones
During childhood and adolescence, the endochondral bones lengthen entirely by growth of the epiphyseal plates. Because its cartilage is replaced with bone tissue on the diaphysis side about as quickly as it grows, the epiphyseal plate maintains a constant thickness while the whole bone lengthens. As adolescence draws to an end, the chondroblasts in the epiphyseal plates divide less often, and the plates become thinner. Eventually, they exhaust their sup- ply of mitotically active cartilage cells, so the cartilage stops growing and is replaced by bone tissue. Long bones stop lengthening when the bone of the epiphyses and diaphysis fuses. This process, called closure of the epiphyseal plates, occurs at about 18 years of age in women and 21 years of age in men. After the epiphyseal plates close, a person can grow no taller. The age of a child or adolescent can be estimated by measuring bone length and degree of closure of the epiphyseal plate of a long bone, as shown on an X-ray image. In adults, because no further growth in length occurs after closure of the epiphyseal plates, long bone length can be used to estimate overall height. Both of these techniques are used forensically to help identify unknown individuals.
Growing bones must also widen as they lengthen. Osteoblasts in the osteogenic layer of the periosteum add bone tissue to the external face of the diaphysis as osteoclasts in the endosteum remove bone from the internal surface of the diaphysis wall. These two processes occur at about the same rate, so that the circumference of the long bone expands and the bone widens. Growth of a bone by the addition of bone tissue to its surfaces is called appositional growth.
This section has focused on the growth and development of large long bones. The other types of endochondral bones grow in slightly different ways. Short bones, such as those in the wrist, arise from only a single ossification center. Most of the irregular bones, such as the hip bone and vertebrae, develop from several distinct ossification centers. Small long bones, such as those in the palm and fingers, form from a primary ossification center (diaphysis) plus a single secondary center; that is, they have just one epiphysis. However, regard- less of the number and location of ossification centers, all endochondral bones follow steps similar to those: calcification and deterioration of cartilage internally, invasion of a periosteal bud containing osteoclasts and osteogenic stem cells, and deposition of bone tissue by osteoblasts.
Bone growth is regulated by several hormones, primarily growth hormone (produced by the pituitary gland), which stimulates the epiphyseal plates to grow. Thyroid hormones modulate the effects of growth hormone, ensuring that the skeleton retains its proper proportions as it grows. The sex hormones (androgens and estrogens) first promote bone growth in the growth spurt at adolescence and later induce the epiphyseal plates to close, ending growth.
Bones appear to be the most lifeless of body organs when seen in the lab, and once they are formed, they seem set for life. Nothing could be further from the truth. Bone is a dynamic and active tissue. Large amounts of bone matrix and thousands of osteocytes are continuously being removed and replaced within the skeleton, and the small-scale architecture of bones constantly changes. As much as half a gram of calcium may enter or leave the adult skeleton each day.
In the adult skeleton, bone is deposited and removed primarily at the endosteal surface. Together, these two processes constitute bone remodeling. The spongy bone in the skeleton, which is covered with endosteum, is entirely replaced every 3 or 4 years. Remodeling in compact bone occurs at the endosteum lining the central canals of the osteons. This process occurs more slowly than in spongy bone; compact bone is completely replaced every 10 years.
Bone remodeling is coordinated by cohorts of adjacent osteoblasts and osteoclasts. In healthy young adults, the total mass of bone in the body stays constant, an indication that the rates of deposit and resorption are essentially equal. The remodeling process is not uniform, however. Some bones (or bone parts) are very heavily remodeled; others are not. For example, the distal region of the femur is fully replaced every 5 to 6 months, whereas the diaphysis of the femur changes much more slowly.
Bone resorption is accomplished by osteoclasts. Each of these giant cells has many nuclei. Osteoclasts crawl along bone surfaces, essentially digging pits as they break down the bone tissue. The part of their plasma membrane that touches the bone surface is highly folded, or ruffled. This expanded membrane forms a tight seal against the bone and secretes concentrated hydrochloric acid, which dissolves the mineral part of the matrix. The liberated calcium ions (Ca2+) and phosphate ions (PO43-) enter the tissue fluid and the blood- stream. Lysosomal enzymes are also released across the ruffled membrane and digest the organic part of the bone matrix. Finally, osteoclasts apparently take up collagen and dead osteocytes by phagocytosis.
Bone deposition is accomplished by osteoblasts, these cells lay down organic osteoid on bone surfaces, and calcium salts crystallize within this osteoid. This calcification process takes about a week. As stated earlier, the osteoblasts transform into osteocytes when they are surrounded by bone matrix.
Bone-forming osteoblasts derive from mesenchyme cells. In adults, osteoblasts form from mesenchyme-like stem cells located in the periosteum, the endosteum, and the connective tissues of the nearby bone marrow. Osteoclasts, which also form in the bone marrow, arise from immature blood cells called hematopoietic stem cells, and they may be related to macrophages. Many of these stem cells fuse together to form each osteoclast, thus their multinucleate structure.
The bones of the skeleton are continually remodeled for two reasons. First, bone remodeling helps maintain constant concentrations of Ca2+ and PO43- in body fluids. Ca2+ levels are strictly controlled because Ca2+ is critical for muscle con- traction. When the concentration of Ca2+ in body fluids starts to fall, a hormone is released by the parathyroid (parahthiroid) glands of the neck. This parathyroid hormone stimulates osteoclasts to resorb bone, a process that releases more Ca2+ into the blood.
Second, bone is remodeled in response to the mechanical stress it experiences. Accordingly, both the osteons of compact bone and the trabeculae of spongy bone are constantly replaced by new osteons and trabeculae that are more precisely aligned with newly experienced compressive and tensile stresses. Furthermore, bone grows thicker in response to the forces experienced during exercise and gains in weight. Conversely, in the absence of mechanical stress, bone tissue is lost, which is why the bones of bedridden people atrophy. A loss of bone under near-zero-gravity conditions is the main obstacle to long missions in outer space. To slow bone loss, astronauts perform isometric exercises during space missions.
Repair of Bone Fractures
Despite their strength, bones are susceptible to fractures, or breaks. In young people, most fractures result from trauma (sports injuries, falls, or car accidents, for example) that twists or smashes the bones. In old age, bones thin and weaken, and fractures occur more often. A fracture in which the bone breaks cleanly but does not penetrate the skin is a simple fracture. When broken ends of the bone protrude through the skin, the fracture is compound.
A fracture is treated by reduction, the realignment of the broken bone ends. In closed reduction, the bone ends are coaxed back into position by the physician’s hands. In open reduction, the bone ends are joined surgically with pins or wires. After the broken bone is reduced, it is immobilized by a cast or traction to allow the healing process to begin. Healing time is about 6 to 8 weeks for a simple fracture, but it is longer for large, weight-bearing bones and for the bones of older people.
The healing of a simple fracture occurs in several phases.
1.Hematoma formation. The fracture is usually accompanied by hemorrhaging. Blood vessels break in the periosteum and inside the bone, releasing blood that clots to form a hematoma. The stages of inflammation, are evident in and around the clot.
Fibrocartilaginous callus formation. Within a few days, new blood vessels grow into the clot. The periosteum and endosteum near the fracture site show a proliferation of bone-forming cells, which then invade the clot, filling it with repair tissue called soft callus (kalus; “hard skin”). Initially, the soft callus is a fibrous granulation tissue. As more fibers are produced, the soft callus be- comes a dense connective tissue containing fibrocartilage and hyaline cartilage. At this point, the soft callus is also called a fibrocartilaginous callus.
Bony callus formation. Within a week, trabeculae of new bone begin to form in the callus, mostly by endochondral ossification. These trabeculae span the width of the callus and unite the two fragments of the broken bone. The callus is now called a bony callus, or hard callus, and its trabeculae grow thicker and stronger and become firm about 2 months after the injury.
Bone remodeling. Over a period of many months, the bony callus is remodeled. The excess bony material is removed from both the exterior of the bone shaft and the interior of the medullary cavity. Compact bone is laid down to reconstruct the shaft walls. The repaired area resembles the original unbroken bone region, because it responds to the same set of mechanical stresses.
THE AXIAL SKELETON
The word skeleton comes from a Greek word meaning “dried-up body” or “mummy,” a rather disparaging description. Nonetheless, this internal framework is a greater triumph of design and engineering than any sky- scraper. The skeleton is strong yet light, wonderfully adapted for the weight-bearing, locomotive, protective, and manipulative functions it performs.
The skeleton consists of bones, cartilages, joints, and ligaments. Joints, also called articulations, are the junctions between skeletal elements. Ligaments connect bones and reinforce most joints. Bones are described in this and the next chapter.
The 206 named bones of the human skeleton are grouped into the axial and appendicular skeletons. The appendicular (apendikular) skeleton, consists of the bones of the upper and lower limbs, including the pectoral (shoulder) and pelvic girdles that attach the limbs to the axial skeleton.
The axial skeleton, which forms the long axis of the body, is the focus of this chapter. It has 80 named bones arranged into three major regions: the skull, vertebral column, and thoracic cage. This axial division of the skeleton supports the head, neck, and trunk, and protects the brain, spinal cord, and the organs in the thorax.
THE VERTEBRAL COLUMN
The vertebral column, also called the spinal column or spine, consists of 26 bones connected into a flexible, curved structure. The main support of the body axis, the vertebral column extends from the skull to the pelvis, where it transmits the weight of the trunk to the lower limbs. It also surrounds and protects the delicate spinal cord and pro- vides attachment points for the ribs and for muscles of the neck and back. In the fetus and infant, the vertebral column consists of 33 separate bones, or vertebrae (vertebre). Inferiorly, nine of these eventually fuse to form two composite bones, the sacrum and the tiny coccyx (tailbone). The remaining 24 bones persist as individual vertebrae separated by intervertebral discs (discussed shortly).
Regions and Normal Curvatures
The vertebral column, which is about 70 cm (28 inches) long in an average adult, has five major regions. The 7 vertebrae of the neck are the cervical vertebrae, the next 12 are the thoracic vertebrae, and the 5 that support the lower back are the lumbar vertebrae. To remember the number of vertebrae in these three regions, think of the usual meal times of 7:00 A.M., 12:00 noon, and 5:00 P.M. The vertebrae become progressively larger from the cervical to the lumbar region as the weight they must support progressively increases. Inferior to the lumbar vertebrae is the sacrum (sakrum), which articulates with the hip bones of the pelvis. The most inferior part of the vertebral column is the tiny coccyx (koksiks).
All people (and in fact the majority of mammals) have seven cervical vertebrae. Variations in numbers of vertebrae in the other regions occur in about 5% of people.
From a lateral view, four curvatures that give the vertebral column an S shape are visible. The cervical and lumbar curvatures are concave posteriorly, whereas the thoracic and sacral curvatures are convex posteriorly. These curvatures increase the resilience of the spine, allowing it to function like a spring rather than a straight, rigid rod.
Only the thoracic and sacral curvatures are well developed at birth. Both of these primary curvatures are convex posteriorly, so that an infant’s spine arches (is C-shaped) like that of a four-legged animal. The secondary curvatures, the cervical and lumbar curvatures, are concave posteriorly and develop during the first 2 years of childhood as the intervertebral discs are reshaped. The cervical curvature is present be- fore birth but is not pronounced until the baby starts to lift its head at 3 months, and the lumbar curvature develops when the baby begins to walk, at about 1 year. The lumbar curvature positions the weight of the upper body over the lower limbs, providing optimal balance during standing.
Ligaments of the Spine
Like a tremulous telecommunication transmitting tower, the vertebral column cannot stand upright by itself. It must be held in place by an elaborate system of supports. Serving this role are the strap like ligaments of the back and the muscles of the trunk. The major supporting ligaments are the anterior and posterior longitudinal ligaments that run vertically along the anterior and posterior surfaces of the bodies of the vertebrae, from the neck to the sacrum. The anterior longitudinal ligament is wide and attaches strongly to both the bony vertebrae and the intervertebral discs. Along with its supporting role, this thick anterior ligament prevents hyperextension of the back (bending too far backward). The posterior longitudinal ligament, which is narrow and relatively weak, attaches only to the intervertebral discs. This ligament helps to prevent hyperflexion (bending the vertebral column too sharply forward).
Several other posterior ligaments connect each vertebra to those immediately superior and inferior.
Among these is the ligamentum flavum (flavum; “yellow”), which connects the lamina of adjacent vertebrae. It contains elastic connective tissue and is especially strong: It stretches as we bend forward, then recoils as we straighten to an erect position.
Each intervertebral disc is a cushionlike pad composed of an inner sphere, the nucleus pulposus (pulposus; “pulp”), and an outer collar of about 12 concentric rings, the anulus fibrosus (anulus fibrosus; “fibrous ring”). Each nucleus pulposus is gelatinous and acts like a rubber ball, enabling the spine to absorb compressive stress. In the anulus fibrosus, the outer rings consist of ligament and the inner ones consist of fibrocartilage. The main function of these rings is to contain the nucleus pulposus, limiting its expansion when the spine is compressed. However, the rings also function like a woven strap, binding the successive vertebrae together, resisting tension on the spine, and absorbing compressive forces. Collagen fibers in adjacent rings in the anulus cross like an X, allowing the spine to withstand twisting. This arrangement creates the same antitwisting design provided by bone lamellae in osteons.
The intervertebral discs act as shock absorbers during walking, jumping, and running. At points of compression, the discs flatten and bulge out a bit between the vertebrae. The discs are thickest in the lumbar (lower back) and cervical (neck) regions of the vertebral column. Collectively, the inter- vertebral discs make up about 25% of the height of the vertebral column. As a result of compression and loss of fluid from the gelatinous nucleus pulposus, they flatten somewhat by the end of each day. So, you are probably 1 to 2 centimeters shorter at night than when you awake in the morning.
General Structure of Vertebrae
Vertebrae from all regions share a common structural pattern. A vertebra consists of a body, or centrum, anteriorly and a vertebral arch posteriorly. The disc-shaped body is the weight-bearing region. Together, the body and vertebral arch enclose an opening called the vertebral foramen. Successive vertebral foramina of the articulated vertebrae form the long vertebral canal, through which the spinal cord passes.
The vertebral arch is a composite structure formed by two pedicles and two laminae. The sides of the arch are pedicles (pedıklz; “little feet”), short bony walls that project posteriorly from the vertebral body. The laminae (lamıne; “sheets”) are flat roof plates that complete the arch posteriorly.
Seven different processes project from each vertebral arch. The spinous process, or vertebral spine, is a median, posterior projection arising at the junction of the two laminae. A transverse process projects laterally from each pedicle- lamina junction. Both the spinous and transverse processes are attachment sites for muscles that move the vertebral column and for ligaments that stabilize it. The paired superior and inferior articular processes protrude superiorly and inferiorly, respectively, from the pedicle-lamina junctions. The inferior articular processes of each vertebra form movable joints with the superior articular processes of the vertebra immediately inferior. Thus, successive vertebrae are joined by both intervertebral discs and by articular processes. The smooth joint surfaces of these processes are facets (“little faces”).
The pedicles have notches on their superior and inferior borders, forming lateral openings between adjacent vertebrae called intervertebral foramina. The spinal nerves issuing from the spinal cord pass through these foramina.
Regional Vertebral Characteristics
The different regions of the spine perform slightly different functions, so vertebral structure shows regional variation. In general, the types of movements that can occur between vertebrae are (1) flexion and extension (anterior bending and posterior straightening of the spine), (2) lateral flexion (bending the upper body to the right or left), and (3) rotation, in which the vertebrae rotate on one another in the long axis of the vertebral column.
The seven cervical vertebrae, identified as C1–C7, are the smallest, lightest vertebrae.
The first two cervical vertebrae are the atlas (C1) and the axis (C2). These two vertebrae are unusual: No intervertebral disc lies between them, and they have unique structural and functional features.
The atlas lacks a body and a spinous process. Essentially, it is a ring of bone consisting of anterior and posterior arches, plus a lateral mass on each side. Each lateral mass has articular facets on both its superior and inferior surfaces. The superior articular facets receive the occipital condyles of the skull. Thus, they “carry” the skull, just as the giant Atlas supported the heavens in Greek mythology. These joints participate in flexion and extension of the head on the neck, as when you nod “yes.” The inferior articular facets form joints with the axis.
The axis, which has a body, a spinous process, and the other typical vertebral processes, is not as specialized as the atlas. In fact, its only unusual feature is the knoblike dens (“tooth”) projecting superiorly from its body. The dens is actually the “missing” body of the atlas that fuses with the axis during embryonic development. Cradled in the anterior arch of the atlas, the dens acts as a pivot for the rotation of the atlas and skull. Hence, this joint participates in rotating the head from side to side to indicate “no.” Axis is a good name for the second cervical vertebra because its dens allows the head to rotate on the neck’s axis.
The typical cervical vertebrae, C3–C7:
The body is wider laterally than in the anteroposterior dimension.
Except in C7, the spinous process is short, projects directly posteriorly, and is bifid (bifid; “cleaved in two” or forked); that is, split at its tip.
The vertebral foramen is large and generally triangular.
Each transverse process contains a hole, a transverse foramen, through which the vertebral blood vessels pass. These vessels ascend and descend through the neck to help serve the brain.
The superior articular facets face superoposteriorly, whereas the inferior articular facets face inferoanteriorly. Thus these articulations lie in an oblique plane. The orientation of these articulations allows the neck to carry out an extremely wide range of movements: flexion and extension, lateral flexion, and rotation. The spinous process of C7 is not bifid and is much larger than those of the other cervical vertebrae. Because its large spinous process can be seen and felt through the skin, C7 is called the vertebra prominens (“prominent vertebra”) and is used as a landmark for counting the vertebrae in living people. To locate this landmark, run your fingers inferiorly along the back of your neck, in the posterior midline, where you can feel the spinous processes of the cervical vertebrae. The spine of C7 is especially prominent.
The 12 thoracic vertebrae, T1–T12, all articulate with ribs. Their other unique characteristics are:
From a superior view, the vertebral body is roughly heart-shaped. Laterally, each side of the vertebral body bears two facets, commonly referred to as demifacets (demefasets), one at the superior edge, the superior costal facet, and the other at the inferior edge, the inferior costal facet (costa-rib; facet -joint surface). The heads of the ribs articulate with these facets. In general, the head of the rib is attached to the bodies of two vertebrae, the inferior costal facet of the superior vertebra and the superior costal facet of the inferior vertebra. Vertebra T1 differs from this general pattern in that its body bears a full facet for the first rib and a demifacet for the second rib; furthermore, the bodies of T10–T12 have only single facets to receive their respective ribs.
The spinous process is long and points inferiorly.
The vertebral foramen is circular.
With the exception of T11 and T12, the transverse processes have facets that articulate with the tubercles of the ribs called transverse costal facets.
The superior and inferior articular facets, which join adjacent vertebrae, lie mainly in the frontal plane; that is, the superior articular facets face posteriorly, whereas the inferior articular facets face anteriorly. Such articulations limit flexion and extension, but they allow rotation between successive vertebrae. Much of the ability to rotate the trunk comes from the thoracic region of the vertebral column. Lateral flexion is also possible but is restricted by the ribs.
The lumbar region of the vertebral column, the area commonly referred to as the small of the back, receives the most stress. The enhanced weight-bearing function of the five lumbar vertebrae (L1–L5) is reflected in their sturdy structure: Their bodies are massive and appear kidney-shaped from a superior view. Their other characteristics are as follows:
The pedicles and laminae are shorter and thicker than those of other vertebrae.
The spinous processes are short, flat, and hatchet-shaped, and they project straight posteriorly. These processes are robust for the attachment of large back muscles.
The vertebral foramen is triangular.
The superior articular facets face posteromedially (or medially), whereas the inferior articular facets face anterolaterally (or laterally), oriented approximately in the sagittal plane. Such articulations provide stability by preventing rotation between the lumbar vertebrae. Flexion and extension are possible, however. The lumbar region flexes, for example, when you do sit-ups or bend forward to pick up a coin from the ground. Additionally, lateral flexion is allowed by this spinal region.
The curved, triangular sacrum shapes the posterior wall of the pelvis. It is formed by five fused vertebrae (S1–S5). Superiorly, it articulates with L5 through a pair of superior articular processes and an intervertebral disc. Inferiorly, it joins the coccyx.
The anterosuperior margin of the first sacral vertebra bulges anteriorly into the pelvic cavity as the sacral promontory (promontore, “a high point of land projecting into the sea”). The human body’s center of gravity lies about 1 cm posterior to this landmark. Four transverse ridges cross the anterior surface of the sacrum, marking the lines of fusion of the sacral vertebrae. The anterior sacral foramina transmit the ventral divisions (ventral rami) of the sacral spinal nerves. The large region lateral to these foramina is simply called the lateral part. This part expands superiorly into the flaring ala (alah; “wing”), which develops from fused rib elements of S1–S5. (Embryonic rib elements form in association with all vertebrae, although they only become true ribs in the thorax. Elsewhere, they fuse into the ventral surfaces of the trans- verse processes.) The alae articulate at the auricular surface with the two hip bones to form the sacroiliac (sakroileak) joints of the pelvis.
On the posterior surface, in the midline, is the bumpy median sacral crest, which represents the fused spinous processes of the sacral vertebrae. Lateral to it are the posterior sacral foramina, which transmit the dorsal rami of the sacral spinal nerves. Just lateral to these is the lateral sacral crest, representing the tips of the transverse processes of the sacral vertebrae.
The vertebral canal continues within the sacrum as the sacral canal. The laminae of the fifth (and sometimes the fourth) sacral vertebrae fail to fuse medially, leaving an enlarged external opening called the sacral hiatus (hiatus; “gap”) at the inferior end of the sacral canal.
The coccyx, or tailbone, is small and triangular. The name coccyx is from a Greek word for “cuckoo,” and the bone was so named because of a fancied resemblance to a bird’s beak. The coccyx consists of three to five vertebrae fused together. Except for the slight support it affords the pelvic organs, it is an almost useless bone. Injury to the coccyx, such as from an abrupt fall, is extremely painful. Occasionally, a baby is born with an unusually long coccyx. In most such cases, this bony “tail” is discreetly snipped off by a physician.
THE THORACIC CAGE.
The bony framework of the chest (thorax), called the thoracic cage, is roughly barrel-shaped and includes the thoracic vertebrae posteriorly, the ribs laterally, and the sternum and costal cartilages anteriorly. The thoracic cage forms a protective cage around the heart, lungs, and other organs. It also supports the shoulder girdles and upper limbs and provides attachment points for many muscles of the back, neck, chest, and shoulders. In addition, the intercostal spaces (inter – between; costa -the ribs) are occupied by the intercostal muscles, which lift and depress the thorax during breathing.
The sternum (breastbone) lies in the anterior midline of the thorax. Resembling a dagger, it is a flat bone about 15 cm long consisting of three sections: the manubrium, body, and xiphoid process. The manubrium (mahnubreum; “knife handle”), the superior section, is shaped like the knot in a necktie. Its clavicular notches articulate with the clavicles (collarbones) superolaterally. Just below this, the manubrium also articulates with the first and second ribs. The body, or midportion, makes up the bulk of the sternum. It is formed from four separate bones, one inferior to the other, that fuse after puberty. The sides of the sternal body are notched where it articulates with the costal cartilages of the second to seventh ribs. The xiphoid process (zifoid; “sword-like”) forms the inferior end of the sternum. This tongue-shaped process is a plate of hyaline cartilage in youth.. In some people, the xiphoid process projects posteriorly. Blows to the chest can push such a xiphoid into the underlying heart or liver, causing massive hemorrhage.
The sternum has three important anatomical landmarks that can be palpated: the jugular notch, the sternal angle, and the xiphisternal joint. The jugular notch, also called the suprasternal notch, is the central indentation in the superior border of the manubrium. If you slide your finger down the anterior surface of your neck, it will land in the jugular notch. The jugular notch generally lies in the same horizontal plane as the disc between the second and third thoracic vertebrae. Just inferior is the sternal angle, a horizontal ridge across the anterior suface of the sternum where the manubrium joins the body. This fibrocartilage joint acts like a hinge, allowing the sternal body to swing anteriorly when we inhale. The sternal angle is in line with the disc between the fourth and fifth thoracic vertebrae. Anteriorly, it lies at the level of the second ribs. It is a handy reference point for finding the second rib. Once the second rib is located, you can count down to identify all the other ribs (except the first and sometimes the twelfth, which are too deep to be palpated). By locating the individual ribs, you attain a series of horizontal lines of “latitude” by which to locate the underlying visceral organs of the thoracic cavity. The xiphisternal (zifısternal) joint is where the sternal body and xiphoid process fuse. It lies at the level of the ninth thoracic vertebra. Deep to this joint, the heart lies on the diaphragm.
Twelve pairs of ribs (Latin: costa) form the flaring sides of the thoracic cage. All ribs attach to the thoracic vertebrae posteriorly and run anteroinferiorly to reach the front of the chest. The superior seven pairs, which attach directly to the sternum by their costal cartilages, are the true ribs, or vertebrosternal (vertebrosternal) ribs. The inferior five pairs, ribs 8–12, are called false ribs because they attach to the sternum either indirectly or not at all. Ribs 8–10 attach to the sternum indirectly, as each joins the costal cartilage above it; these are called vertebrochondral (vertebrokondral) ribs. Ribs 11 and 12 are called floating ribs or vertebral ribs because they have no anterior attachments. Instead, their costal cartilages lie embedded in the muscles of the lateral body wall. The ribs increase in length from pair 1 to 7, then decrease in length from pair 8 to 12. The inferior margin of the rib cage, or costal margin, is formed by the costal cartilages of ribs 7 to 10. The right and left costal mar- gins diverge from the region of the xiphisternal joint, where they form the infrasternal angle (infra-below).
A typical rib is a bowed flat bone. The bulk of a rib is simply called the shaft or body. Its superior border is smooth, but its inferior border is sharp and thin and has a costal groove on its inner face. The intercostal nerves and vessels are located in the costal groove. In addition to the shaft, each rib has a head, neck, and tubercle. The wedge- shaped head articulates with the vertebral bodies by two facets: One facet joins the body of the thoracic vertebra of the same number; the other joins the body of the vertebra immediately superior. The neck of a rib is the short, constricted region just lateral to the head. Just lateral to the neck on the posterior surface, the knoblike tubercle articulates with the transverse process of the thoracic vertebra of the same number. Lateral to the tubercle, the shaft angles sharply anteriorly (at the angle of the rib) and ex- tends to the costal cartilage anteriorly. The costal cartilages provide secure but flexible attachments of ribs to the sternum and contribute to the elasticity of the thoracic cage.
The first rib is atypical because it is flattened from superior to inferior and is quite broad. The subclavian vessels, the large artery and vein servicing the upper limb, run in a groove along its superior surface. There are other exceptions to the typical rib pattern: Rib 1 and ribs 10–12 articulate with only one vertebral body, and ribs 11 and 12 do not articulate with a vertebral transverse process.
The skull is the body’s most complex bony structure. It is formed by cranial and facial bones. The cranial bones, or cranium (kraneum), enclose and protect the brain and provide attachment sites for some head and neck muscles. The facial bones (1) form the framework of the face; (2) form cavities for the sense organs of sight, taste, and smell; (3) provide openings for the passage of air and food; (4) hold the teeth; and (5) anchor the muscles of the face.
Most skull bones are flat bones and are firmly united by interlocking, immovable joints called sutures (soocherz; “seams”). The suture lines have an irregular, saw-toothed appearance. The longest sutures—the coronal, sagittal, squamous, and lambdoid sutures—connect the cranial bones. Most other skull sutures connect facial bones and are named according to the specific bones they connect.
Overview of Skull Geography
It is worth surveying basic skull “geography” before describing the individual bones. With the lower jaw removed, the skull resembles a lopsided, hollow, bony sphere. The facial bones form its anterior aspect, and the cranium forms the rest. The cranium can be divided into a vault and a base. The cranial vault, also called the calvaria (kalvareah; “bald part of skull”) or skullcap, forms the superior, lateral, and posterior aspects of the skull, as well as the forehead region. The cranial base, or floor, is the inferior part. Internally, prominent bony ridges divide the cranial base into three distinct “steps,” or fossae—the anterior, middle, and posterior cranial fossae. The brain sits snugly in these cranial fossae and is completely enclosed by the cranial vault. Overall, the brain is said to occupy the cranial cavity.
In addition to its large cranial cavity, the skull contains many smaller cavities, including the middle ear and inner ear cavities (carved into the lateral aspects of the cranial base), the nasal cavity, and the orbits. The nasal cavity lies in and posterior to the nose, and the orbits house the eyeballs. Air-filled sinuses that occur in several bones around the nasal cavity are the paranasal sinuses.
Moreover, the skull has about 85 named openings (foramina, canals, fissures). The most important of these provide passageways for the spinal cord, the major blood vessels serving the brain, and the 12 pairs of cranial nerves, which conduct impulses to and from the brain. Cranial nerves, are classified by number, using the Roman numerals I through XII.
The eight large bones of the cranium are the paired parietal and temporal bones and the unpaired frontal, occipital, sphenoid, and ethmoid bones. Together these bones form the brain’s protective “shell.” Because its superior aspect is curved, the cranium is self-bracing. This allows the bones to be thin, and, like an eggshell, the cranium is remarkably strong for its weight.
Parietal Bones and the Major Sutures
The two large parietal bones, shaped like curved rectangles, make up the bulk of the cranial vault; that is, they form most of the superior part of the skull, as well as its lateral walls (parietal-wall). The sites at which the parietal bones articulate (form a joint) with other cranial bones are the four largest sutures:
The coronal suture, running in the coronal plane, occurs anteriorly where the parietal bones meet the frontal bone.
A squamous suture occurs where each parietal bone meets a temporal bone inferiorly, on each lateral aspect of the skull.
The sagittal suture occurs where the right and left parietal bones meet superiorly in the midline of the cranium.
The lambdoid suture occurs where the parietal bones meet the occipital bone posteriorly . This suture is so named because it resembles the Greek letter lambda.
These sutures vary somewhat in appearance in different skulls. As a person ages, the sutural lines close up, making these sutures less noticeable.
Sutural bones are small bones that occur within the sutures, especially in the lambdoid suture. They are irregular in shape, size, and location, and not all people have them. They develop between the major cranial bones during the fetal period and persist throughout life. The significance of these bones is unknown.
■ Frontal Bone
The frontal bone forms the forehead and the roofs of the orbits. Just superior to the orbits, it protrudes slightly to form superciliary (soopersileare; “eyebrow”) arches, which lie just deep to our eyebrows. The supraorbital margin, or superior margin of each orbit, is pierced by a hole or by a notch, respectively called the supraorbital foramen or supraorbital notch. This opening transmits the supraorbital nerve (a branch of cranial nerve V) and artery, which supply the forehead. The smooth part of the frontal bone between the superciliary arches in the midline is the glabella (glahbelah; “smooth, without hair”). Just inferior to it, the frontal bone meets the nasal bones at the frontonasal suture. The regions of the frontal bone lateral to the glabella contain the air-filled frontal sinuses.
Internally, the frontal bone contributes to the anterior cranial fossa, which holds the large frontal lobes of the brain.
■ Occipital Bone
The occipital bone (oksipıtal; “back of the head”) makes up the posterior part of the cranium and cranial base. It articulates with the parietal bones at the lambdoid suture and with the temporal bones at the occipitomastoid sutures. Internally, it forms the walls of the posterior cranial fossa, which holds a part of the brain called the cerebellum. In the base of the occipital bone is the foramen magnum, literally, “large hole”. Through this opening, the inferior part of the brain connects with the spinal cord. The foramen magnum is flanked laterally by two rockerlike occipital condyles, which articulate with the first vertebra of the vertebral column in a way that enables the head to nod “yes.” Hidden medial and superior to each occipital condyle is a hypoglossal (hipoglosal) canal, through which runs cranial nerve XII, the hypoglossal nerve. Anterior to the foramen magnum, the occipital bone joins the sphenoid bone via the basilar part of the occipital bone.
Several features occur on the external surface of the occipital bone. The external occipital protuberance is a knob in the midline, at the junction of the base and the posterior wall of the skull. The external occipital crest extends anteriorly from the protuberance to the foramen magnum. This crest secures the ligamentum nuchae (nuke; “of the neck”), an elastic, sheet-shaped ligament that lies in the median plane of the posterior neck and connects the neck vertebrae to the skull. Extending laterally from the occipital protuberance are the superior nuchal (nukal) lines, and running laterally from a point halfway along the occipital crest are the inferior nuchal lines. The nuchal lines and the bony regions between them anchor many muscles of the neck and back. The superior nuchal line marks the upper limit of the neck.
■ Temporal Bones
The temporal bones are best viewed laterally. They lie inferior to the parietal bones and form the inferolateral region of the skull and parts of the cranial floor. The terms temporal and temple, from the Latin word for “time,” refer to the fact that gray hairs, a sign of time’s passage, appear first at the temples.
Each temporal bone has an intricate shape and is described in terms of its four major regions: the squamous, tympanic, mastoid, and petrous regions. The plate-shaped squamous region abuts the squamous suture. It has a barlike zygomatic process (zigomatik; “cheek”) that projects anteriorly to meet the zygomatic bone of the face. Together, these two bony structures form the zygomatic arch, commonly called the cheek bone. The oval mandibular (mandibular) fossa on the inferior surface of the zygomatic process receives the mandible (lower jawbone), forming the freely movable temporomandibular joint (jaw joint).
The tympanic region (timpanik; “eardrum”) surrounds the external acoustic meatus, or external ear canal. It is through this canal that sound enters the ear. The external acoustic meatus and the tympanic membrane (eardrum) at its deep end are parts of the external ear. In a dried skull, the tympanic membrane has been removed. Thus, part of the middle ear cavity deep to the tympanic region may be visible through the meatus. Projecting inferiorly from the tympanic region is the needle-like styloid process (stiloid; “stakelike”). This process is an attachment point for some muscles of the tongue and pharynx and for a ligament that connects the skull to the hyoid bone of the neck.
The mastoid region (mastoid; “breast-shaped”) has a prominent mastoid process, an anchoring site for some neck muscles. This process can be felt as a lump just posterior to the ear. The stylomastoid foramen is located between the styloid and mastoid processes. A branch of cranial nerve VII, the facial nerve, leaves the skull through this foramen.
The mastoid process is full of air sinuses called mastoid air cells, which lie just posterior to the middle ear cavity. Infections can spread from the throat to the middle ear to the mastoid cells. Such an infection, called mastoiditis, can even spread to the brain, from which the mastoid air cells are separated by only a thin roof of bone. This was a serious problem before the late 1940s, when antibiotics became available.
The petrous (petrus; “rocky”) region of the temporal bone projects medially and contributes to the cranial base. It appears as a bony wedge between the occipital bone posteriorly and the sphenoid bone anteriorly. From within the cranial cavity this very dense region looks like a mountain ridge. The posterior slope of this ridge lies in the posterior cranial fossa, whereas the anterior slope is in the middle cranial fossa, the fossa that holds the temporal lobes of the brain. Housed inside the petrous region are the cavities of the middle and inner ear, which contain the sensory apparatus for hearing and balance.
Several foramina penetrate the bone of the petrous region. The large jugular foramen is located where the petrous region joins the occipital bone. Through this foramen pass the largest vein of the head, the internal jugular vein, and cranial nerves IX, X, and XI. The carotid (karotid) canal opens in the petrous region on the skull’s inferior aspect, just anterior to the jugular foramen. The internal carotid artery, the main artery to the brain, passes through it into the cranial cavity. The foramen lacerum (laserum; “lacerated”) is a jagged opening between the medial tip of the petrous portion of the temporal bone and the sphenoid bone. This foramen is almost completely closed by cartilage in a living person, but it is so conspicuous in a dried skull that students usually ask its name. The internal acoustic meatus lies in the cranial cavity on the posterior face of the petrous region. It transmits cranial nerves VII and VIII, the facial and vestibulo- cochlear nerves.
■ Sphenoid Bone
The sphenoid bone spans the width of the cranial floor and has been said to resemble a bat with its wings spread. It is considered the key- stone of the cranium because it forms a central wedge that articulates with every other cranial bone. It is a challenging bone to study because of its complex shape and orientation: Portions of the sphenoid are viewable from most aspects of the skull. It also has a number of foramina for the passage of cranial nerves and vessels.
The sphenoid consists of a central body and three pairs of processes: the greater wings, lesser wings, and pterygoid (terıgoid) processes. The superior surface of the body bears a saddle-shaped prominence, the sella turcica (selah tersikah; “Turkish saddle”). The seat of this saddle, called the hypophyseal (hipofizeal) fossa, holds the pituitary gland, or hypophysis. Within the sphenoid body are the paired sphenoid sinuses. The greater wings project laterally from the sphenoid body, forming parts of the middle cranial fossa and the orbit. On the lateral wall of the skull, the greater wing appears as a flag-shaped area medial to the zygomatic arch. The horn-shaped lesser wings form part of the floor of the anterior cranial fossa and a part of the orbit. The trough-shaped pterygoid (“winglike”) processes project inferiorly from the greater wings. These processes, which have both medial and lateral plates, are attachment sites for the pterygoid muscles that help close the jaw in chewing.
The sphenoid bone has five important openings on each side. The optic canal lies just anterior to the sella turcica. Cranial nerve II, the optic nerve, passes through this hole from the orbit into the cranial cavity. The other four openings lie in a crescent-shaped row just lateral to the sphenoid body on each side. The most anterior of these openings, the superior orbital fissure, is a long slit between the greater and lesser wings. It transmits several structures to and from the orbit, such as the cranial nerves that control eye movements (III, IV, and VI). This fissure is best seen in an anterior view of the orbit. The foramen rotundum lies in the medial part of the greater wing. It is usually oval, despite its name, which means “round opening.” The foramen ovale (ovale) is an oval hole posterolateral to the foramen rotundum. The foramen rotundum and foramen ovale are passageways through which two large branches of cranial nerve V (the maxillary and mandibular nerves) exit the cranium. Posterior and lateral to the foramen ovale lies the small foramen spinosum (spinosum), named for a short spine that projects from its margin on the inferior aspect of the skull. Through this foramen passes the middle meningeal artery, which supplies blood to the broad inner surfaces of the parietal and the squamous temporal bones.
■ Ethmoid Bone
The ethmoid bone is the most deeply situated bone of the skull. It lies anterior to the sphenoid bone and posterior to the nasal bones, forming most of the medial bony area between the nasal cavity and the orbits. The ethmoid is a remarkably thin-walled and delicate bone. In the articulated skull, only small portions of the ethmoid are viewable.
Its superior surface is formed by paired, horizontal cribriform (kribrıform; “perforated like a sieve”) plates that contribute to the roof of the nasal cavities and the floor of the anterior cranial fossa. The cribriform plates are perforated by tiny holes called olfactory foramina. The filaments of cranial nerve I, the olfactory nerve, pass through these holes as they run from the nasal cavity to the brain. Between the two cribriform plates, in the midline, is a superior projection called the crista galli (kristah galli; “rooster’s comb”). A fibrous membrane called the falx cerebri attaches to the crista galli and helps to secure the brain within the cranial cavity.
The perpendicular plate of the ethmoid bone projects inferiorly in the median plane. It forms the superior part of the nasal septum, the vertical partition that divides the nasal cavity into right and left halves. Flanking the perpendicular plate on each side is a delicate lateral mass riddled with ethmoidal air cells (ethmoid sinuses). The ethmoid bone is named for these sinuses, as ethmos means “sieve” in Greek. Extending medially from the lateral masses are the thin superior and middle nasal conchae (kongke), which protrude into the nasal cavity. The conchae are curved like scrolls and are named after the conch shells one finds on warm ocean beaches. The lateral surfaces of the ethmoid’s lateral masses are called orbital plates because they contribute to the medial walls of the orbits.
The skeleton of the face consists of 14 bones. These are the unpaired mandible and the vomer, plus the paired maxillae, zygomatics, nasals, lacrimals, palatines, and inferior nasal conchae.
The U-shaped mandible (mandıbl), or lower jawbone, is the largest, strongest bone in the face. It has a horizontal body that forms the inferior jawline, and two upright rami (rami; “branches”). Each ramus meets the body posteriorly at a mandibular angle. At the superior margin of each ramus are two processes. The anterior coronoid process (koronoid; “crown-shaped”) is a flat, triangular projection. The temporalis muscle, which elevates the lower jaw during chewing, inserts here. The posterior condylar process enlarges superiorly to form the mandibular condyle, or head of the mandible. It articulates with the temporal bone to form the temporomandibular joint. The coronoid and condylar processes are separated by the mandibular notch.
The body of the mandible anchors the lower teeth and forms the chin. Its superior border is the alveolar (alveolar) margin. The tooth sockets, called alveoli, open onto this margin. Anteriorly, the fusion between the two halves of the mandible is called the mental protuberance.
Several openings pierce the mandible. On the medial surface of each ramus is a mandibular foramen, through which a nerve responsible for tooth sensation (inferior alveolar nerve, a branch of cranial nerve V) enters the mandibular body and supplies the roots of the lower teeth.
Dentists inject anesthetic into this foramen before working on the lower teeth. The mental (“chin”) foramen, which opens on the anterolateral side of the mandibular body, transmits blood vessels and nerves to the lower lip and the skin of the chin.
■ Maxillary Bones
The maxillary bones, or maxillae (maksile; “jaws”), form the upper jaw and the central part of the facial skeleton. They are considered the keystone bones of the face because they articulate with all other facial bones except the mandible.
Like the mandible, the maxillae have an alveolar margin that contains teeth in alveoli. The palatine (palahten) processes project medially from the alveolar margins to form the anterior region of the hard palate, or bony roof of the mouth. The frontal processes extend superiorly to reach the frontal bone, forming part of the lateral aspect of the bridge of the nose. The maxillae lie just lateral to the nasal cavity and contain the maxillary sinuses. These sinuses, the largest of the paranasal air sinuses, extend from the orbit down to the roots of the upper teeth. Laterally, the maxillae articulate with the zygomatic bones at the zygomatic processes.
The maxilla, along with several other bones, forms the borders of the inferior orbital fissure in the floor of the orbit. This fissure transmits several vessels and nerves, including the maxillary nerve (a branch of cranial nerve V) or its continuation, the infraorbital nerve. The infraorbital nerve proceeds anteriorly to enter the face through the infraorbital foramen.
■ Zygomatic Bones
The irregularly shaped zygomatic bones are commonly called the cheekbones (zygoma -cheekbone). Each joins the zygomatic process of a temporal bone posteriorly, the zygomatic process of the frontal bone superiorly, and the zygomatic process of the maxilla anteriorly. The zygomatic bones form the prominences of the cheeks and define part of the margin of each orbit.
■ Nasal Bones
The paired, rectangular nasal bones join medially to form the bridge of the nose. They articulate with the frontal bone superiorly, the maxillae laterally, and the perpendicular plate of the ethmoid bone posteriorly. Inferiorly, they attach to the cartilages that form most of the skeleton of the external nose.
■ Lacrimal Bones
The delicate, fingernail-shaped lacrimal (lakrımal) bones are located in the medial orbital walls. They articulate with the frontal bone superiorly, the ethmoid bone posteriorly, and the maxilla anteriorly. Each lacrimal bone contains a deep groove that contributes to a lacrimal fossa. This fossa contains a lacrimal sac that gathers tears, allowing the fluid to drain from the eye surface into the nasal cavity (lacrima -tear).
■ Palatine Bones
The palatine bones lie posterior to the maxillae. These paired, L-shaped bones articulate with each other at their inferior horizontal plates, which complete the posterior part of the hard palate. The perpendicular plates form the posterior part of the lateral walls of the nasal cavity and a small part of the orbits.
The slender, plow-shaped vomer (vomer; “plowshare”) lies in the nasal cavity, where it forms the inferior part of the nasal septum.
■ Inferior Nasal Conchae
The paired inferior nasal conchae are thin, curved bones in the nasal cavity. Projecting medially from the lateral walls of the nasal cavity, just inferior to the middle nasal conchae of the ethmoid bone, they are the largest of the three pairs of conchae.
Special Parts of the Skull
Next we will examine four special parts of the skull: the nasal cavity and the orbits, which are restricted regions of the skull formed from many bones; the paranasal sinuses, which are extensions of the nasal cavity; and the hyoid bone.
The nasal cavity is constructed of bone and cartilage. Its roof is the ethmoid’s cribriform plates. The floor is formed by the palatine processes of the maxillae and the horizontal plates of the palatine bones. Keep in mind that these same nasal-floor structures also form the roof of the mouth and are collectively called the hard palate. Contributing to the lateral walls of the nasal cavity are the nasal bones, the superior and middle conchae of the ethmoid, the inferior nasal conchae, a part of the maxilla, and the perpendicular plates of the palatine bones. On these lateral walls, each of the three conchae forms a roof over a groove-shaped air passageway called a meatus (meatus; “a passage”). Therefore, there are superior, middle, and inferior meatuses.
Recall that the nasal cavity is divided into right and left halves by the nasal septum. The bony part of this septum is formed by the vomer inferiorly and by the perpendicular plate of the ethmoid superiorly. A sheet of cartilage, called the septal cartilage, completes the septum anteriorly.
The walls of the nasal cavity are covered with a mucosa that moistens and warms inhaled air. This membrane also secretes mucus that traps dust, thereby cleansing the air of debris. The three pairs of scroll-shaped nasal conchae cause the air flowing through the nasal cavity to swirl. This turbulence increases the contact of inhaled air with the mucosa through- out the nasal cavity, such that the air is warmed, moistened, and filtered more efficiently.
The bones surrounding the nasal cavity—the frontal, ethmoid, sphenoid, and both maxillary bones—contain air-filled sinuses that are called paranasal sinuses ( para-near) because they cluster around the nasal cavity. In fact, they are extensions of the nasal cavity, lined by the same mucous membrane and probably serving the same function of warming, moistening, and filtering inhaled air. The paranasal sinuses also lighten the skull, giving the bones they occupy a moth-eaten appearance in an X-ray image. These sinuses connect to the nasal cavity through small openings, most of which occur at the meatuses inferior to the conchae. For more on paranasal sinuses.
The orbits are cone-shaped bony cavities that hold the eyes, the muscles that move the eyes, some fat, and the tear-producing glands. The walls of each orbit are formed by parts of seven bones—the frontal, sphenoid, zygomatic, maxillary, palatine, lacrimal, and ethmoid bones. The superior and inferior orbital fissures, optic canal, and lacrimal fossa (described earlier) are also in the orbit.
THE APPENDICULAR SKELETON
THE PECTORAL GIRDLE
The pectoral girdle, or shoulder girdle, consists of a clavicle (klavıkl) anteriorly and a scapula (skapulah) posteriorly. The paired pectoral girdles and their associated muscles form the shoulders. The term girdle implies a belt completely circling the body, but these girdles do not quite satisfy this description: Anteriorly, the medial end of each clavicle joins to the sternum and first rib, and the lateral ends of the clavicles join to the scapulae at the shoulder. However, the two scapulae fail to complete the ring posteriorly, because their me- dial borders do not join to each other or the axial skeleton. Besides attaching the upper limb to the trunk, the pectoral girdle provides attachment for many muscles that move the limb. This girdle is light and allows the upper limbs to be quite mobile. This mobility springs from two factors:
Because only the clavicle attaches to the axial skeleton, the scapula can move quite freely across the thorax, allowing the arm to move with it.
The socket of the shoulder joint—the scapula’s glenoid cavity—is shallow, so it does not restrict the movement of the humerus (arm bone). Although this arrangement is good for flexibility, it is bad for stability: Shoulder dislocations are fairly common.