Junqueira's Basic Histology (14e) - Cartilage PDF

C H A P T E R HYALINE CARTILAGE 7 Cartilage 129 FIBROCARTILAGE 134 Matrix 131 CARTILAG...

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 ›â ºHYALINE 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 130 CHAPTER 7â … â …Cartilage FIGURE 7–1â ‡ 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. â ‡ 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  Hyaline Cartilage 131 FIGURE 7–2â ‡ 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 Cartilageâ ‡ â ‡ 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. 132 CHAPTER 7â … â …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 7–3â ‡ 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)  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 ›â ºELASTIC CARTILAGE Cartilageâ ‡ â ‡ 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 7–4â ‡ 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. 134 CHAPTER 7â … â …Cartilage canals, the auditory (Eustachian) tubes, the epiglottis, and the upper respiratory tract. Elastic cartilage in these locations FIGURE 7–5â ‡ 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. ›â ºFIBROCARTILAGE C 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 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. Chondrocytes of fibrocartilage occur singly and often in aligned 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- 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 connective tissue. ›â ºCARTILAGE 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.  Cartilage Formation, Growth, & Repair 135 FIGURE 7–6â ‡ Chondrogenesis. 7 C H A P T E R Cartilageâ ‡ â ‡ 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 7–7â ‡ 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) 136 CHAPTER 7â … â …Cartilage Cartilageâ ‡ 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. Cartilageâ ‡ 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  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. Cartilageâ ‡ â ‡ 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