Cartilage and Bone PDF

C H A P T E R HYALINE CARTILAGE 7 Cartilage 129 FIBROCARTILAGE 134 Matrix...

C H A P T E R HYALINE CARTILAGE 7 Cartilage 129 FIBROCARTILAGE 134 Matrix 131 CARTILAGE FORMATION, GROWTH, & REPAIR 134 Chondrocytes 132 SUMMARY OF KEY POINTS 136 Perichondrium 133 ELASTIC CARTILAGE 133 ASSESS YOUR KNOWLEDGE 136 C artilage is a tough, durable form of supporting con- nective tissue, characterized by an extracellular matrix (ECM) with high concentrations of GAGs and proteoglycans, interacting with collagen and elastic fibers. Structural features of its matrix make cartilage ideal for a vari- The perichondrium (Figure 7–2) is a sheath of dense connective tissue that surrounds cartilage in most places, forming an interface between the cartilage and the tissues sup- ported by the cartilage. The perichondrium harbors the blood supply serving the cartilage and a small neural component. ety of mechanical and protective roles within the adult skel- Articular cartilage, which covers the ends of bones in movable eton and elsewhere (Figure 7–1). joints and which erodes in the course of arthritic degenera- Cartilage ECM has a firm consistency that allows the tissue tion, lacks perichondrium and is sustained by the diffusion of to bear mechanical stresses without permanent distortion. In oxygen and nutrients from the synovial fluid. the respiratory tract, ears, and nose, cartilage forms the frame- As shown in Figure 7–1, variations in the composition of work supporting softer tissues. Because of its resiliency and the matrix characterize three main types of cartilage: hyaline smooth, lubricated surface, cartilage provides cushioning and cartilage, elastic cartilage, and fibrocartilage. Important fea- sliding regions within skeletal joints and facilitates bone move- tures of these are summarized in Table 7–1. ments. As described in Chapter 8, cartilage also guides devel- opment and growth of long bones, both before and after birth. Cartilage consists of cells called chondrocytes (Gr. › ›› MEDICAL APPLICATION chondros, cartilage + kytos, cell) embedded in the ECM which Many genetic conditions in humans or mice that cause defec- unlike connective tissue proper contains no other cell types. tive cartilage, joint deformities, or short limbs are due to Chondrocytes synthesize and maintain all ECM components recessive mutations in genes for collagen type II, the aggre- and are located in matrix cavities called lacunae. can core protein, the sulfate transporter, and other proteins The physical properties of cartilage depend on electro- required for normal chondrocyte function. static bonds between type II collagen fibrils, hyaluronan, and the sulfated GAGs on densely packed proteoglycans. Its semi-rigid consistency is attributable to water bound to the negatively charged hyaluronan and GAG chains extending from proteoglycan core proteins, which in turn are enclosed within a dense meshwork of thin type II collagen fibrils. The ››HYALINE CARTILAGE high content of bound water allows cartilage to serve as a Hyaline (Gr. hyalos, glass) cartilage, the most common of shock absorber, an important functional role. the three types, is homogeneous and semitransparent in the All types of cartilage lack vascular supplies and chon- fresh state. In adults hyaline cartilage is located in the articu- drocytes receive nutrients by diffusion from capillaries in lar surfaces of movable joints, in the walls of larger respira- surrounding connective tissue (the perichondrium). In some tory passages (nose, larynx, trachea, bronchi), in the ventral skeletal elements, large blood vessels do traverse cartilage ends of ribs, where they articulate with the sternum, and in to supply other tissues, but these vessels release few nutrients the epiphyseal plates of long bones, where it makes possible to the chondrocytes. As might be expected of cells in an longitudinal bone growth (Figure 7–1). In the embryo, hya- avascular tissue, chondrocytes exhibit low metabolic activity. line cartilage forms the temporary skeleton that is gradually Cartilage also lacks nerves. replaced by bone. 129 07_Mescher_ch07_p129-137.indd 129 26/04/18 9:58 am  130 CHAPTER 7 Cartilage FIGURE 7–1 Distribution of cartilage in adults. Cartilage in external ear Extracellular matrix Epiglottis Cartilages in nose Lacuna Larynx (with chondrocyte) Lung Trachea Articular cartilage Perichondrium of a joint Costal cartilage 180x b Hyaline y cartilage g Cartilage of intervertebral disc c Respiratory tract cartilages Perichondrium in the lungs, trachea, Elastic fibers and larynx Pubic symphysis is Lacunae (with chondrocytes) Extracellular matrix 80x c Elastic cartilage Meniscus (padlike fibrocartilage in Lacunae knee joint) (with chondrocytes) Extracellular matrix Collagen fibers Articular cartilage Hyaline cartilage age of a joint Fibrocartilage 80x Elastic cartilage ge a d Fibrocartilage (a) There are three types of adult cartilage distributed in many The photomicrographs show the main features of (b) hyaline areas of the skeleton, particularly in joints and where pliable sup- cartilage, (c) elastic cartilage, and (d) fibrocartilage. Dense con- port is useful, as in the ribs, ears, and nose. Cartilage support of nective tissue of perichondrium is shown here with hyaline and other tissues throughout the respiratory tract is also prominent. elastic cartilage. TABLE 7–1 Important features of the major cartilage types. Hyaline Cartilage Elastic Cartilage Fibrocartilage Main features of the Homogeneous, with type II collagen Type II collagen, aggrecan, and darker Type II collagen and large areas extracellular matrix and aggrecan elastic fibers of dense connective tissue with type I collagen Major cells Chondrocytes, chondroblasts Chondrocytes, chondroblasts Chondrocytes, fibroblasts Typical arrangement Isolated or in small isogenous groups Usually in small isogenous groups Isolated or in isogenous groups of chondrocytes arranged axially Presence of Yes (except at epiphyses and articular Yes No perichondrium cartilage) Main locations or Many components of upper respiratory External ear, external acoustic Intervertebral discs, pubic examples tract; articular ends and epiphyseal meatus, auditory tube; epiglottis and symphysis, meniscus, and certain plates of long bones; fetal skeleton certain other laryngeal cartilages other joints; insertions of tendons Main functions Provides smooth, low-friction surfaces Provides flexible shape and support Provides cushioning, tensile in joints; structural support for of soft tissues strength, and resistance to respiratory tract tearing and compression 07_Mescher_ch07_p129-137.indd 130 26/04/18 9:58 am  Hyaline Cartilage 131 FIGURE 7–2 The structure of cartilage matrix and cells. C H A P T E R Proteoglycan Perichondrium Hyaluronan Perichondrial fibroblast Type II 7 collagen fibril Chondroblast Cartilage Hyaline Cartilage Cartilage Interterritorial matrix Hyaluronan Link protein Chondrocyte Core protein Chondroitin sulfate Territorial matrix Collagen (type II) a b (a) A schematic representation of the most abundant molecules (b) A diagram of the transitional area between the perichon- in cartilage matrix shows the interaction between type II colla- drium and the cartilage matrix. Fibroblast-like progenitor cells gen fibrils and proteoglycans linked to hyaluronan. Link proteins in the perichondrium give rise to larger chondroblasts, which noncovalently bind the protein core of proteoglycans to the linear divide and differentiate as chondrocytes. These functional cells hyaluronan molecules. The chondroitin sulfate side chains of the produce matrix components and exist in lacunae surrounded by proteoglycan electrostatically bind to the collagen fibrils, forming the matrix. The ECM immediately around each lacuna, called the a cross-linked matrix. The circled area is shown larger in the lower territorial matrix, contains mostly proteoglycans and sparse col- part of the figure. Physical properties of these matrix components lagen; that more distant from lacunae, the interterritorial matrix, produce a highly hydrated, pliable material with great strength. is richer in collagen and may be less basophilic. Approximately 75% of the wet weight of hyaline cartilage is water. › ›› MEDICAL APPLICATION make the matrix generally basophilic and the thin collagen fibrils are barely discernible. Most of the collagen in hyaline car- Osteoarthritis, a chronic condition that commonly occurs tilage is type II, although small amounts of minor collagens are during aging, involves the gradual loss or changed physical also present. properties of the hyaline cartilage that lines the articular ends Aggrecan (250 kDa), with approximately 150 GAG of bones in joints. Joints that are weight-bearing (knees, hips) side chains of chondroitin sulfate and keratan sulfate, is the or heavily used (wrist, fingers) are most prone to cartilage most abundant proteoglycan of hyaline cartilage. Hundreds degeneration. Fragments released by wear-and-tear to the of these proteoglycans are bound noncovalently by link pro- articular cartilage trigger secretion of matrix metalloprotein- teins to long polymers of hyaluronan, as shown schematically ases and other factors from macrophages in adjacent tissues, in Figure 7–2a and discussed in Chapter 5. These proteogly- which exacerbate damage and cause pain and inflammation can complexes bind further to the surface of type II collagen within the joint. fibrils (Figure 7–2a). Water bound to GAGs in the proteogly- cans constitutes up to 60%-80% of the weight of fresh hyaline cartilage. Matrix Another important component of cartilage matrix is the structural multiadhesive glycoprotein chondronectin. Like The dry weight of hyaline cartilage is nearly 40% collagen embed- fibronectin in other connective tissues, chondronectin binds ded in a firm, hydrated gel of proteoglycans and structural gly- specifically to GAGs, collagen, and integrins, mediating the coproteins. In routine histology preparations, the proteoglycans adherence of chondrocytes to the ECM. 07_Mescher_ch07_p129-137.indd 131 26/04/18 9:59 am  132 CHAPTER 7 Cartilage Staining variations within the matrix reflect local differ- shape of the chondrocytes and their retraction from the matrix. ences in its molecular composition. Immediately surrounding In living tissue chondrocytes fill their lacunae completely. each chondrocyte, the ECM is relatively richer in GAGs than Because cartilage matrix is avascular, chondrocytes collagen, often causing these areas of territorial matrix to respire under low-oxygen tension. Hyaline cartilage cells stain differently from the intervening areas of interterritorial metabolize glucose mainly by anaerobic glycolysis. Nutrients matrix (Figures 7–2b and 7–3). from the blood diffuse to all the chondrocytes from the carti- lage surface, with movements of water and solutes in the carti- lage matrix promoted by intermittent tissue compression and Chondrocytes decompression during body movements. The limits of such Cells occupy relatively little of the hyaline cartilage mass. At diffusion define the maximum thickness of hyaline cartilage, the periphery of the cartilage, young chondrocytes or chon- which usually exists as small, thin plates. droblasts have an elliptic shape, with the long axes parallel to the surface (Figure 7–3). Deeper in the cartilage, they are › ›› MEDICAL APPLICATION round and may appear in groups of up to eight cells that origi- In contrast to other forms of cartilage and most other tissues, nate from mitotic divisions of a single chondroblast and are hyaline cartilage is susceptible to partial or isolated regions called isogenous aggregates. As the chondrocytes become of calcification during aging, especially in the costal carti- more active in secreting collagens and other ECM compo- lage adjacent to the ribs. Calcification of the hyaline matrix, nents, the aggregated cells are pushed apart and occupy sepa- accompanied by degenerative changes in the chondrocytes, rate lacunae. is a common part of the aging process and in many respects Cartilage cells and matrix may shrink slightly during resembles endochondral ossification by which bone is formed. routine histologic preparation, resulting in both the irregular FIGURE 7–3 Hyaline cartilage. P P C C M M C P a b (a) The upper part of the photo shows the perichondrium (P), an (b) The thin region of hyaline cartilage shown here has perichon- example of dense connective tissue consisting largely of type I drium (P) on both sides and shows larger lacunae containing collagen. Among the fibroblastic cells of the perichondrium are isogenous groups of chondrocytes (C) within the matrix (M). Such indistinguishable mesenchymal stem cells. There is a gradual groups of two, four, or more cells are produced by mitosis; the transition and differentiation of cells from the perichondrium to cells will separate into individual lacunae as they begin to secrete the cartilage, with some elongated fibroblast-like cells becoming matrix. Territorial matrix immediately around the chondrocytes is larger and more rounded as chondroblasts and chondrocytes (C). more basophilic than interterritorial matrix farther from the cells. These are located within lacunae surrounded by the matrix (M) (X160; H&E) which these cells secreted. (X200; H&E) 07_Mescher_ch07_p129-137.indd 132 26/04/18 9:59 am  Elastic Cartilage 133 Chondrocyte synthesis of sulfated GAGs and secretion of perichondrium, which is essential for the growth and main- proteoglycans are accelerated by many hormones and growth tenance of cartilage (Figures 7–2b and 7–3). The outer region C H A P T E R factors. A major regulator of hyaline cartilage growth is the of the perichondrium consists largely of collagen type I fibers pituitary-derived protein called growth hormone or somato- and fibroblasts, but an inner layer adjoining the cartilage tropin. This hormone acts indirectly, promoting the endocrine matrix also contains mesenchymal stem cells which provide a release from the liver of insulin-like growth factors, or somato- source for new chondroblasts that divide and differentiate into medins, which directly stimulate the cells of hyaline cartilage. chondrocytes. 7 › ›› MEDICAL APPLICATION ››ELASTIC CARTILAGE Cartilage Elastic Cartilage Cells of cartilage can give rise to either benign (chondroma) or slow-growing, malignant (chondrosarcoma) tumors in which Elastic cartilage is essentially similar to hyaline cartilage cells produce normal matrix components. Chondrosarcomas except that it contains an abundant network of elastic fibers in seldom metastasize and are generally removed surgically. addition to a meshwork of collagen type II fibrils (Figures 7–4 and 7–1c), which give fresh elastic cartilage a yellowish color. With appropriate staining the elastic fibers usually appear as Perichondrium dark bundles distributed unevenly through the matrix. Except in the articular cartilage of joints, all hyaline car- More flexible than hyaline cartilage, elastic cartilage is tilage is covered by a layer of dense connective tissue, the found in the auricle of the ear, the walls of the external auditory FIGURE 7–4 Elastic cartilage. P C M a b The chondrocytes (C) and overall organization of elastic cartilage flexibility to this type of cartilage. The section in part b includes are similar to those of hyaline cartilage, but the matrix (M) also perichondrium (P) that is also similar to that of hyaline cartilage. contains elastic fibers that can be seen as darker components (a) X160; Hematoxylin and orcein. (b) X180; Weigert resorcin and with proper staining. The abundant elastic fibers provide greater van Gieson. 07_Mescher_ch07_p129-137.indd 133 26/04/18 9:59 am  134 CHAPTER 7 Cartilage canals, the auditory (Eustachian) tubes, the epiglottis, and the upper respiratory tract. Elastic cartilage in these locations FIGURE 7–5 Fibrocartilage. includes a perichondrium similar to that of most hyaline car- tilage. Throughout elastic cartilage the cells resemble those of hyaline cartilage both physiologically and structurally. ››FIBROCARTILAGE C 同与不同。 Fibrocartilage takes various forms in different structures but C is essentially a mingling of hyaline cartilage and dense connec- tive tissue (Figures 7–5 and 7–1d). It is found in intervertebral 位置功能。 discs, in attachments of certain ligaments, and in the pubic symphysis—all places where it serves as very tough, yet cush- ioning support tissue for bone. 结构功能。。 Chondrocytes of fibrocartilage occur singly and often in 排队方式功能。 aligned 细胞。 isogenous aggregates, producing type II collagen and other ECM components, although the matrix around these chondrocytes is typically sparse. Areas with chondrocytes and hyaline matrix are separated by other regions with fibroblasts and dense bundles of type I collagen which confer extra ten- C sile strength to this tissue (Figure 7–5). The relative scarcity of proteoglycans overall makes fibrocartilage matrix more aci-分子组成和表象。 dophilic than that of hyaline or elastic cartilage. There is no distinct surrounding perichondrium in fibrocartilage. Intervertebral discs of the spinal column are composed primarily of fibrocartilage and act as lubricated cushions and shock absorbers preventing damage to adjacent vertebrae from abrasive forces or impacts. Held in place by ligaments, inter- vertebral discs are discussed further with joints in Chapter 8. Important features of the three major types of cartilage Fibrocartilage varies histologically in different structures, but are summarized in Table 7–1. is always essentially a mixture of hyaline cartilage and dense 特征汇总。 connective tissue. ››CARTILAGE FORMATION, GROWTH, In a small region of intervertebral disc, the axially arranged aggregates of chondrocytes (C) are seen to be surrounded by & REPAIR small amounts of matrix and separated by larger regions with dense collagen and scattered fibroblasts with elongated nuclei All cartilage forms from embryonic mesenchyme in the pro- (arrows). (X250; Picrosirius-hematoxylin) cess of chondrogenesis (Figure 7–6). The first indication of cell differentiation is the rounding up of the mesenchymal cells, which retract their extensions, multiply rapidly, and perichondrium (Figure 7–2b). In both cases, the synthesis of become more densely packed together. In general the terms matrix contributes greatly to the growth of the cartilage. Appo- “chondroblasts” and “chondrocytes” respectively refer to the sitional growth of cartilage is more important during postnatal cartilage cells during and after the period of rapid prolifera- development, although as described in Chapter 8, interstitial tion. At both stages the cells have basophilic cytoplasm rich growth in cartilaginous regions within long bones is important in RER for collagen synthesis (Figure 7–7). Production of the in increasing the length of these structures. In articular carti- ECM encloses the cells in their lacunae and then gradually lage, cells and matrix near the articulating surface are gradu- separates chondroblasts from one another. During embryonic ally worn away and must be replaced from within, because development, the cartilage differentiation takes place primar- there is no perichondrium to add cells by appositional growth. ily from the center outward; therefore the more central cells Except in young children, damaged cartilage undergoes have the characteristics of chondrocytes, whereas the periph- slow and often incomplete repair, primarily dependent on eral cells are typical chondroblasts. The superficial mesen- cells in the perichondrium which invade the injured area and chyme develops as the perichondrium. produce new cartilage. In damaged areas the perichondrium Once formed, the cartilage tissue enlarges both by inter- produces a scar of dense connective tissue instead of form- stitial growth, involving mitotic division of preexisting ing new cartilage. The poor capacity of cartilage for repair or chondrocytes, and by appositional growth, which involves regeneration is due in part to its avascularity and low meta- chondroblast differentiation from progenitor cells in the bolic rate. 07_Mescher_ch07_p129-137.indd 134 26/04/18 9:59 am  Cartilage Formation, Growth, & Repair 135 FIGURE 7–6 Chondrogenesis. 7 C H A P T E R Cartilage Cartilage Formation, Growth, & Repair a b c d The major stages of embryonic cartilage formation, or chondro- with water and form the very extensive ECM. (d) Multiplication genesis, are shown here. of chondroblasts within the matrix gives rise to isogenous cell (a) Mesenchyme is the precursor for all types of cartilage. (b) Mitosis aggregates surrounded by a condensation of territorial matrix. and initial cell differentiation produces a tissue with condensa- In mature cartilage, this interstitial mitotic activity ceases and all tions of rounded cells called chondroblasts. (c) Chondroblasts chondrocytes typically become more widely separated by their are then separated from one another again by their produc- production of matrix. tion of the various matrix components, which collectively swell FIGURE 7–7 Chondrocytes in growing cartilage. This TEM of fibrocartilage shows chondrocytes with abundant are both present in fibrocartilage. Chondrocytes in growing hya- RER actively secreting the collagen-rich matrix. Bundles of colla- line and elastic cartilage have more prominent Golgi complexes gen fibrils, sectioned in several orientations, are very prominent and synthesize abundant proteoglycans in addition to collagens. around the chondrocytes of fibrocartilage. Collagen types I and II (X3750) 07_Mescher_ch07_p129-137.indd 135 26/04/18 9:59 am  136 CHAPTER 7 Cartilage Cartilage SUMMARY OF KEY POINTS Cartilage is a tough, resilient type of connective tissue that struc- Elastic Cartilage turally supports certain soft tissues, notably in the respiratory tract, Elastic cartilage generally resembles hyaline cartilage in its chon- and provides cushioned, low-friction surfaces in joints. drocytes and major ECM components, but its matrix includes Cells of cartilage, chondrocytes, make up a small percentage of abundant elastic fibers, visible with special stains, which increase the tissue’s mass, which is mainly a flexible mass of extracellular the tissue’s flexibility. matrix (ECM). Elastic cartilage provides flexible support for the external ear as Chondrocytes are embedded within lacunae surrounded by the well as certain structures of the middle ear and larynx; it is always ECM. surrounded by perichondrium. Cartilage ECM typically includes collagen as well as abundant proteoglycans, notably aggrecan, which bind a large amount of Fibrocartilage water. Fibrocartilage contains varying combinations of hyaline cartilage Cartilage always lacks blood vessels, lymphatics, and nerves, but it in small amounts of dense connective tissue. is usually surrounded by a dense connective tissue perichondrium Histologically it consists of small chondrocytes in a hyaline matrix, that is vascularized. usually layered with larger areas of bundled type I collagen with There are three major forms of cartilage: (1) hyaline cartilage, (2) scattered fibroblasts. elastic cartilage, and (3) fibrocartilage. Fibrocartilage provides very tough, strong support at tendon insertions and in intervertebral discs and certain other joints. Hyaline Cartilage The ECM of hyaline cartilage is homogenous and glassy, rich Cartilage Formation, Growth, & Repair in fibrils of type II collagen and aggrecan complexes with bound All forms of cartilage form from embryonic mesenchyme. water. Cartilaginous structures grow by mitosis of existing chondroblasts The ECM has less collagen and more proteoglycan immediately in lacunae (interstitial growth) or formation of new chondro- around the lacunae, producing slight staining differences in this blasts peripherally from progenitor cells in the perichondrium territorial matrix. (appositional growth). Chondrocytes occur singly or in small, mitotically derived isog- Repair or replacement of injured cartilage is very slow and inef- enous groups. fective, due in part to the tissue’s avascularity and low metabolic Perichondrium is usually present, but not at the hyaline cartilage rate. of articular surfaces or the epiphyses of growing long bones. Cartilage ASSESS YOUR KNOWLEDGE 1. The molecular basis for the shock absorbing properties of cartilage 5. What is the source of the mesenchymal progenitor cells activated involves which of the following? for the repair of hyaline cartilage of accident-damaged costal a. Electrostatic interaction of proteoglycans with type IV collagen cartilages? b. Ability of glycosaminoglycans to bind anions a. Perichondrium c. Noncovalent binding of glycosaminoglycans to protein cores b. Adjacent loose connective tissue d. Sialic acid residues in the glycoproteins c. Bone of the adjacent rib(s) and sternum e. Hydration of glycosaminoglycans d. Chondrocytes of the injured cartilage e. Stem cells circulating with blood 2. What distinguishes cartilage from most other connective tissues? a. Its extracellular matrix is rich in collagen. 6. How does articular cartilage differ from most other hyaline b. Its predominant cell type is a mesenchymal derivative. cartilage? c. Its predominant cell type secretes both fibers and proteoglycans. a. It undergoes mainly appositional growth. d. It lacks blood vessels. b. It contains isogenous groups of chondrocytes. e. It functions in mechanical support. c. It lacks a perichondrium. d. Its matrix contains aggrecan. 3. Which feature is typical of elastic cartilage? e. It is derived from embryonic mesenchyme. a. Primary skeletal tissue in the fetus b. No identifiable perichondrium 7. Which step occurs first in chondrogenesis? c. Found in intervertebral discs a. Appositional growth d. Most widely distributed cartilage type in the body b. Conversion of chondroblasts to chondrocytes e. Collagen is mainly type II c. Formation of mesenchymal condensations d. Interstitial growth 4. Which area in cartilage is relatively collagen-poor and e. Secretion of collagen-rich and proteoglycan-rich matrix proteoglycan-rich? a. Fibrocartilage b. Territorial matrix c. Epiphyseal plate d. Interterritorial matrix e. Perichondrium 07_Mescher_ch07_p129-137.indd 136 26/04/18 9:59 am  Cartilage Formation, Growth, & Repair 137 8. Osteoarthritis is characterized by the progressive erosion of articu- 10. A 66-year-old man who suffered from severe osteoarthritis is lar cartilage. The matrix metalloproteinases involved in this ero- referred to an orthopedic surgeon for replacement of his right knee. C H A P T E R sion primarily act on which matrix component? He had been actively involved in both high school and intercolle- a. Aggrecan giate football and had continued running until about the age of 45 b. Link proteins as a form of relaxation and exercise. With the patient’s permission c. Network-forming collagen the removed joint is used by investigators performing a proteomic d. Fibril-forming collagen analysis of different joint tissues. The meniscus was found to con- e. Chondronectin tain almost exclusively type I collagen and aggrecan was undetect- able. What is the most likely explanation for this result? 9. A 28-year-old woman visits the family medicine clinic com- a. The meniscus normally consists of dense regular connective 7 plaining of loss of the sense of smell, nosebleeds, problems with tissue, which contains primarily type I collagen. Cartilage Cartilage Formation, Growth, & Repair swallowing, and hoarseness. She admits to “casual, social use” of b. The meniscus normally consists of fibrocartilage, which con- cocaine on a regular basis since her sophomore year of college. A tains only type I collagen. complete examination of her nose with a speculum and otoscope c. The meniscus had undergone repeated rounds of repair due to shows severe rhinitis (inflammation). There is also perforation and wear-and-tear during which its hyaline cartilage component collapse of the nasal cartilage resulting in a “saddle nose” defor- was replaced by dense connective tissue. mity. Erosions in the enamel of her front teeth are noted. The d. Osteoarthritic injury in the knee resulted in the chondrocytes breakdown of the nasal cartilage releases collagen fibers primarily of the meniscus switching from expression of genes for type II of which type? collagen to type I collagen. a. Type I e. Elastic cartilage is normally replaced by fibrocartilage during b. Type II aging and this process can be accelerated by exercise. c. Type III d. Type IV e. Type VII Answers: 1e, 2d, 3e, 4b, 5a, 6c, 7c, 8d, 9b, 10c 07_Mescher_ch07_p129-137.indd 137 26/04/18 9:59 am  C H A P T E R BONE CELLS Osteoblasts 8 Bone 138 138 OSTEOGENESIS Intramembranous Ossification 148 149 Osteocytes 142 Endochondral Ossification 149 Osteoclasts 143 BONE REMODELING & REPAIR 152 BONE MATRIX 143 METABOLIC ROLE OF BONE 153 PERIOSTEUM & ENDOSTEUM 143 JOINTS 155 TYPES OF BONE 143 SUMMARY OF KEY POINTS 158 Lamellar Bone 145 ASSESS YOUR KNOWLEDGE 159 Woven Bone 148 A s the main constituent of the adult skeleton, bone tis- sue (Figure 8–1) provides solid support for the body, protects vital organs such as those in the cranial and thoracic cavities, and encloses internal (medullary) cavities con- taining bone marrow where blood cells are formed. Bone (or All bones are lined on their internal and external surfaces by layers of connective tissue containing osteogenic cells— endosteum on the internal surface surrounding the marrow cavity and periosteum on the external surface. Because of its hardness, bone cannot be sectioned rou- osseous) tissue also serves as a reservoir of calcium, phosphate, tinely. Bone matrix is usually softened by immersion in a and other ions that can be released or stored in a controlled decalcifying solution before paraffin embedding, or embed- fashion to maintain constant concentrations in body fluids. ded in plastic after fixation and sectioned with a specialized In addition, bones form a system of levers that multiply microtome. the forces generated during skeletal muscle contraction and transform them into bodily movements. This mineralized tis- sue therefore confers mechanical and metabolic functions to the skeleton. ››BONE CELLS Bone is a specialized connective tissue composed of cal- Osteoblasts cified extracellular material, the bone matrix, and following Originating from mesenchymal stem cells, osteoblasts pro- three major cell types (Figure 8–2): duce the organic components of bone matrix, including type I collagen fibers, proteoglycans, and matricellular glycoproteins Osteocytes (Gr. osteon, bone + kytos, cell), which are such as osteonectin. Deposition of the inorganic components found in cavities (lacunae) between bone matrix layers of bone also depends on osteoblast activity. Active osteo- (lamellae), with cytoplasmic processes in small canaliculi blasts are located exclusively at the surfaces of bone matrix, to (L. canalis, canal) that extend into the matrix (Figure 8–1b) which they are bound by integrins, typically forming a single Osteoblasts (osteon + Gr. blastos, germ), growing cells layer of cuboidal cells joined by adherent and gap junctions which synthesize and secrete the organic components of (Figure 8–3). When their synthetic activity is completed, some the matrix osteoblasts differentiate as osteocytes entrapped in matrix- Osteoclasts (osteon + Gr. klastos, broken), which are bound lacunae, some flatten and cover the matrix surface as giant, multinucleated cells involved in removing calcified bone lining cells, and the majority undergo apoptosis. bone matrix and remodeling bone tissue During the processes of matrix synthesis and calcification, Because metabolites are unable to diffuse through the cal- osteoblasts are polarized cells with ultrastructural features cified matrix of bone, the exchanges between osteocytes and denoting active protein synthesis and secretion. Matrix com- blood capillaries depend on communication through the very ponents are secreted at the cell surface in contact with existing thin, cylindrical spaces of the canaliculi. bone matrix, producing a layer of unique collagen-rich material 138 08_Mescher_ch08_p138-160.indd 138 26/04/18 9:59 am  Bone Cells 139 FIGURE 8–1 Components of bone. C H A P T E R Concentric Nerve lamellae Vein Artery 8 Collagen fiber Canaliculi Bone Bone Cells orientation Diaphysis Central of humerus canal Central canal Osteon External circumferential Osteon lamellae Lacuna Perforating fibers Periosteum Internal Cellular circumferential Osteocyte Fibrous layer lamellae layer Canaliculi Interstitial lamellae (b) Compact bone Inner circumferential lamellae Trabeculae of cancellous bone Perforating Central Endosteum canals canal (a) Section of humerus Osteoclast Space for bone marrow Lamellae Osteocyte Trabeculae in lacuna Canaliculi opening Osteoblasts at surface aligned along trabecula of new bone Canaliculi opening at surface (c) Cancellous bone A schematic overview of the basic features of bone, including the the osteons via very small channels called canaliculi that intercon- three key cell types: osteocytes, osteoblasts, and osteoclasts; nect the lacunae. Osteoclasts are monocyte-derived cells in bone their usual locations; and the typical lamellar organization of required for bone remodeling. bone. Osteoblasts secrete the matrix that then hardens by calcifica- The periosteum consists of dense connective tissue, with a pri- tion, trapping the differentiating cells now called osteocytes in marily fibrous layer covering a more cellular layer. Bone is vascular- individual lacunae. Osteocytes maintain the calcified matrix and ized by small vessels that penetrate the matrix from the periosteum. receive nutrients from microvasculature in the central canals of Endosteum covers all trabeculae around the marrow cavities. 08_Mescher_ch08_p138-160.indd 139 26/04/18 10:00 am  140 CHAPTER 8 Bone FIGURE 8–2 Bone tissue. Newly formed bone tissue decalcified for sec- tioning and stained with trichrome in which the collagen-rich ECM appears bright blue. The tissue is a combination of mesenchymal regions (M) con- taining capillaries, fibroblasts, and osteoprogenitor stem cells and regions of normally calcified matrix Oc Oc with varying amounts of collagen and the three major cell types found in all bone tissue. Bone-forming osteoblasts (Ob) differentiate from osteoprogenitor cells in the periosteum and endosteum, and cover the surfaces of existing bone matrix. Osteoblasts secrete osteoid rich in collagen type I, but also containing proteoglycans and other molecules. As osteoid undergoes calci- fication and hardens, it entraps some osteoblasts Ob Ocl that then differentiate further as osteocytes (Oc) occupying lacunae surrounded by bony matrix. The much less numerous large, multinuclear osteoclasts (Ocl), produced by the fusion of blood M monocytes, reside on bony surfaces and erode the matrix during bone remodeling. (400X; Mallory trichrome) FIGURE 8–3 Osteoblasts, osteocytes, and osteoclasts. Osteoclast Mesenchyme Newly formed Osteoblast Osteocyte Bone matrix matrix (osteoid) Ob Os B Oc M a b (a) Diagram showing the relationship of osteoblasts to the newly cells in the adjacent mesenchyme (M), cover a thin layer of lightly formed matrix called “osteoid,” bone matrix, and osteocytes. Osteo- stained osteoid (Os) on the surface of the more heavily stained blasts and most of the larger osteoclasts are part of the endosteum bony matrix (B). Most osteoblasts that are no longer actively covering the bony trabeculae. secreting osteoid will undergo apoptosis; others differentiate (b) The photomicrograph of developing bone shows the location either as flattened bone lining cells on the trabeculae of bony and morphologic differences between active osteoblasts (Ob) and matrix or as osteocytes located within lacunae surrounded by bony osteocytes (Oc). Rounded osteoblasts, derived from progenitor matrix. (X300; H&E) 08_Mescher_ch08_p138-160.indd 140 26/04/18 10:00 am  Bone Cells 141 called osteoid between the osteoblast layer and the preexisting [Ca10(PO4)6(OH)2] crystals, the first visible step in calcifica- bone surface (Figure 8–3). This process of bone appositional tion. These crystals grow rapidly by accretion of more mineral C H A P T E R growth is completed by subsequent deposition of calcium salts and eventually produce a confluent mass of calcified material into the newly formed matrix. embedding the collagen fibers and proteoglycans (Figure 8–4). The process of matrix mineralization is not completely understood, but basic aspects of the process are shown in Figure 8–4. Prominent among the noncollagen proteins › ›› MEDICAL APPLICATION secreted by osteoblasts is the vitamin K-dependent polypep- Cancer originating directly from bone cells (a primary bone 8 tide osteocalcin, which together with various glycoproteins tumor) is fairly uncommon (0.5% of all cancer deaths), although a cancer called osteosarcoma can arise in osteo- Bone Bone Cells binds Ca2+ ions and concentrates this mineral locally. Osteo- blasts also release membrane-enclosed matrix vesicles rich progenitor cells. The skeleton is often the site of secondary, in alkaline phosphatase and other enzymes whose activity metastatic tumors, however, arising when cancer cells move raises the local concentration of PO43− ions. In the microenvi- into bones via small blood or lymphatic vessels from malig- ronment with high concentrations of both these ions, matrix nancies in other organs, most commonly the breast, lung, vesicles serve as foci for the formation of hydroxyapatite prostate gland, kidney, or thyroid gland. FIGURE 8–4 Mineralization in bone matrix. Osteoblasts release matrix vesicles Osteoblasts Released matrix vesicles and collagen fibers Osteoid layer Early mineralization around vesicles Matrix becoming confluent between vesicles Mineralized bone From their ends adjacent to the bone matrix, osteoblasts secrete and PO4- ion concentrations cause calcified nanocrystals to form type I collagen, several glycoproteins, and proteoglycans. Some of in and around the matrix vesicles. The crystals grow and mineralize these factors, notably osteocalcin and certain glycoproteins, bind further with formation of small growing masses of calcium hydroxy- Ca2+ with high affinity, raising the local concentration of these ions. apatite [Ca10(PO4)6(OH)2], which surround the collagen fibers and Osteoblasts also release very small membrane-enclosed matrix all other macromolecules. Eventually the masses of hydroxyapatite vesicles containing alkaline phosphatase and other enzymes. These merge as a confluent solid bony matrix as calcification of the matrix enzymes remove PO4- ions from various matrix macromolecules, is completed. creating a high concentration of these ions locally. The high Ca2+ 08_Mescher_ch08_p138-160.indd 141 26/04/18 10:00 am  142 CHAPTER 8 Bone Osteocytes Normally the most abundant cells in bone, osteocytes exhibit significantly less RER, smaller Golgi complexes, and more As mentioned some osteoblasts become surrounded by the condensed nuclear chromatin than osteoblasts (Figure 8–5a). material they secrete and then differentiate as osteocytes Osteocytes maintain the calcified matrix and their death is fol- enclosed singly within the lacunae spaced throughout the lowed by rapid matrix resorption. While sharing most matrix- mineralized matrix. During the transition from osteoblasts related activities with osteoblasts, osteocytes also express many to osteocytes, the cells extend many long dendritic processes, different proteins, including factors with paracrine and endo- which also become surrounded by calcifying matrix. The pro- crine effects that help regulate bone remodeling. cesses thus come to occupy the many canaliculi, 250-300 nm in diameter, radiating from each lacuna (Figures 8–5 and 8–1b). Diffusion of metabolites between osteocytes and blood vessels occurs through the small amount of interstitial fluid in › ›› MEDICAL APPLICATION the canaliculi between the bone matrix and the osteocytes and The extensive network of osteocyte dendritic processes and their processes. Osteocytes also communicate with one another other bone cells has been called a “mechanostat,” monitoring and ultimately with nearby osteoblasts and bone lining cells via mechanical loads within bones and signaling cells to adjust gap junctions at the ends of their processes. These connections ion levels and maintain the adjacent bone matrix accordingly. between osteocyte processes and nearly all other bone cells in Resistance exercise can produce increased bone density the extensive lacunar-canalicular network allow osteocytes to and thickness in affected regions, while lack of exercise (or serve as mechanosensors detecting the mechanical load on the the weightlessness experienced by astronauts) leads to bone as well as stress- or fatigue-induced microdamage and to decreased bone density, due in part to the lack of mechani- trigger remedial activity in osteoblasts and osteoclasts. cal stimulation of the bone cells. FIGURE 8–5 Osteocytes in lacunae. C C b C C a c (a) TEM showing an osteocyte in a lacuna and two dendritic pro- between these structures through which nutrients derived from cesses in canaliculi (C) surrounded by bony matrix. Many such blood vessels diffuse and are passed from cell to cell in living bone. processes are extended from each cell as osteoid is being secreted; (X400; Ground bone) this material then undergoes calcification around the processes, (c) SEM of nondecalcified, sectioned, and acid-etched bone show- giving rise to canaliculi. (X30,000) ing lacunae and canaliculi (C). (X400) (b) Photomicrograph of bone, not decalcified or sectioned, but (Figure 8-5c, used with permission from Dr Matt Allen, Indiana Uni- ground very thin to demonstrate lacunae and canaliculi. The lacu- versity School of Medicine, Indianapolis.) nae and canaliculi (C) appear dark and show the communication 08_Mescher_ch08_p138-160.indd 142 26/04/18 10:00 am  Types of Bone 143 Osteoclasts released from cells in matrix vesicles promote calcification of the matrix. Other tissues rich in type I collagen lack osteocal- C H A P T E R Osteoclasts are very large, motile cells with multiple nuclei cin and matrix vesicles and therefore do not normally become (Figure 8–6) that are essential for matrix resorption during calcified. bone growth and remodeling. The large size and multinucle- The association of minerals with collagen fibers during ated condition of osteoclasts are due to their origin from the calcification provides the hardness and resistance required for fusion of bone marrow-derived monocytes. Osteoclast devel- bone function. If a bone is decalcified by a histologist, its shape opment requires two polypeptides produced by osteoblasts: is preserved but it becomes soft and pliable like other connec- macrophage-colony-stimulating factor (M-CSF; discussed 8 tive tissues. Because of its high collagen content, decalcified with hemopoiesis, Chapter 13) and the receptor activator of Bone Types of Bone bone matrix is usually acidophilic. nuclear factor-κB ligand (RANKL). In areas of bone undergo- ing resorption, osteoclasts on the bone surface lie within enzy- matically etched depressions or cavities in the matrix known as resorption lacunae (or Howship lacunae). ››PERIOSTEUM & ENDOSTEUM In an active osteoclast, the membrane domain that con- External and internal surfaces of all bones are covered by con- tacts the bone forms a circular sealing zone that binds the nective tissue of the periosteum and endosteum, respectively cell tightly to the bone matrix and surrounds an area with (Figures 8–1a and 8–1c). The periosteum is organized much many surface projections, called the ruffled border. This cir- like the perichondrium of cartilage, with an outer fibrous layer cumferential sealing zone allows the formation of a specialized of dense connective tissue, containing mostly bundled type I microenvironment between the osteoclast and the matrix in collagen, but also fibroblasts and blood vessels. Bundles of which bone resorption occurs (Figure 8–6b). periosteal collagen, called perforating (or Sharpey) fibers, Into this subcellular pocket the osteoclast pumps protons penetrate the bone matrix and bind the periosteum to the to acidify and promote dissolution of the adjacent hydroxy- bone. Periosteal blood vessels branch and penetrate the bone, apatite, and releases matrix metalloproteinases and other carrying metabolites to and from bone cells. hydrolytic enzymes from lysosome-related secretory vesicles The periosteum’s inner layer is more cellular and includes for the localized digestion of matrix proteins. Osteoclast activ- osteoblasts, bone lining cells, and mesenchymal stem cells ity is controlled by local signaling factors from other bone referred to as osteoprogenitor cells. With the potential to cells. Osteoblasts activated by parathyroid hormone produce proliferate extensively and produce many new osteoblasts, M-CSF, RANKL, and other factors that regulate the formation osteoprogenitor cells play a prominent role in bone growth and activity of osteoclasts. and repair. Internally the very thin endosteum covers small tra- › ›› MEDICAL APPLICATION beculae of bony matrix that project into the marrow cavities (Figure 8–1). The endosteum also contains osteoprogenitor In the genetic disease osteopetrosis, which is characterized cells, osteoblasts, and bone lining cells, but within a sparse, by dense, heavy bones (“marble bones”), the osteoclasts lack delicate matrix of collagen fibers. ruffled borders and bone resorption is defective. This disorder results in overgrowth and thickening of bones, often with obliteration of the marrow cavities, depressing blood cell for- › ›› MEDICAL APPLICATION mation and causing anemia and the loss of white blood cells. Osteoporosis, frequently found in immobilized patients and The defective osteoclasts in most patients with osteopetrosis in postmenopausal women, is an imbalance in skeletal turn- have mutations in genes for the cells’ proton-ATPase pumps over so that bone resorption exceeds bone formation. This or chloride channels. leads to calcium loss from bones and reduced bone mineral density (BMD). Individuals at risk for osteoporosis are rou- ››BONE MATRIX tinely tested for BMD by dual-energy x-ray absorptiometry (DEXA scans). About 50% of the dry weight of bone matrix is inorganic mate- rials. Calcium hydroxyapatite is most abundant, but bicarbon- ate, citrate, magnesium, potassium, and sodium ions are also found. Significant quantities of noncrystalline calcium phos- ››TYPES OF BONE phate are also present. The surface of hydroxyapatite crystals Gross observation of a bone in cross section (Figure 8–7) shows are hydrated, facilitating the exchange of ions between the a dense area near the surface corresponding to compact (cor- mineral and body fluids. tical) bone, which represents 80% of the total bone mass, and The organic matter embedded in the calcified matrix is 90% deeper areas with numerous interconnecting cavities, called type I collagen, but also includes mostly small proteoglycans and cancellous (trabecular) bone, constituting about 20% of multiadhesive glycoproteins such as osteonectin. Calcium- total bone mass. Histological features and important locations binding proteins, notably osteocalcin, and the phosphatases of the major types of bone are summarized in Table 8–1. 08_Mescher_ch08_p138-160.indd 143 26/04/18 10:00 am  144 CHAPTER 8 Bone FIGURE 8–6 Osteoclasts and their activity. Ocl Ocl Oc B B a Osteoclast Bone matrix Osteoclast Blood capillary Nucleus Golgi Nucleus Vesicles – CO2 + H2O H+ + CH3 Ruffled Section of border circumferential sealing zone Bone matrix Microenvironment of low c b pH and concentrated MMPs Osteoclasts are large multinucleated cells that are derived by the metalloproteases and other hydrolytic enzymes. Acidification of fusion in bone of several blood-derived monocytes. (a) Photo of the sealed space promotes dissolution of hydroxyapatite from bone showing two osteoclasts (Ocl) digesting and resorbing bone bone and stimulates activity of the protein hydrolases, producing matrix (B) in relatively large resorption cavities (or Howship lacu- localized matrix resorption. The breakdown products of collagen nae) on the matrix surface. An osteocyte (Oc) in its smaller lacuna is fibers and other polypeptides are endocytosed by the osteoclast also shown. (X400; H&E) and further degraded in lysosomes, while Ca2+ and other ions are (b) Diagram showing an osteoclast’s circumferential sealing zone released directly and taken up by the blood. where integrins tightly bind the cell to the bone matrix. The sealing (c) SEM showing an active osteoclast cultured on a flat substrate of zone surrounds a ruffled border of microvilli and other cytoplas- bone. A trench is formed on the bone surface by the slowly migrat- mic projections close to this matrix. The sealed space between the ing osteoclast. (X5000) cell and the matrix is acidified to ~pH 4.5 by proton pumps in the (Figure 8–6c, used with permission from Alan Boyde, Centre for ruffled part of the cell membrane and receives secreted matrix Oral Growth and Development, University of London.) 08_Mescher_ch08_p138-160.indd 144 26/04/18 10:00 am  Types of Bone 145 At the microscopic level both compact and cancellous FIGURE 8–7 Compact and cancellous bone. bones typically show two types of organization: mature lamel- C H A P T E R lar bone, with matrix existing as discrete sheets, and woven bone, newly formed with randomly arranged components. Lamellar Bone Most bone in adults, compact or cancellous, is organized as lamellar bone, characterized by multiple layers or lamel- 8 lae of calcified matrix, each 3-7 μm thick. The lamellae are Bone Types of Bone organized as parallel sheets or concentrically around a cen- tral canal. In each lamella, type I collagen fibers are aligned, with the pitch of the fibers’ orientation shifted orthogonally Compact Cancellous (by about 90 degrees) in successive lamellae (Figure 8–1a). bone bone This highly ordered organization of collagen within lamellar bone causes birefringence with polarizing light microscopy; the alternating bright and dark layers are due to the changing orientation of collagen fibers in the lamellae (Figure 8–8). Like the orientation of wood fibers in plywood the highly Macroscopic photo of a thick section of bone showing the corti- ordered, alternating organization of collagen fibers in lamellae cal compact bone and the lattice of trabeculae in cancellous adds greatly to the strength of lamellar bone. bone at the bone’s interior. The small trabeculae that make up highly porous cancellous bone serve as supportive struts, collec- An osteon (or Haversian system) refers to the com- tively providing considerable strength, without greatly increas- plex of concentric lamellae, typically 100-250 μm in diam- ing the bone’s weight. The compact bone is normally covered eter, surrounding a central canal that contains small blood externally with periosteum and all trabecular surfaces of the vessels, nerves, and endosteum (Figures 8–1 and 8–9). cancellous bone are covered with endosteum. (X10) Between successive lamellae are lacunae, each with one osteocyte, all interconnected by the canaliculi containing the cells’ dendritic processes (Figure 8–9). Processes of adjacent In long bones, the bulbous ends—called epiphyses cells are in contact via gap junctions, and all cells of an osteon (Gr. epiphysis, an excrescence)—are composed of cancellous receive nu