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In all three forms, cartilage is avascular and is nourished by the diffusion of nutrients from capillaries in adjacent connective tissue (perichondrium) or by synovial fluid from joint cavities. In some instances, blood vessels traverse cartilage to nourish other tissues, but these vessels do not supply nutrients to the cartilage. As might be expected of cells in an avascular tissue, chondrocytes exhibit low metabolic activity. Cartilage has no lymphatic vessels or nerves.
The perichondrium (Figures 7–2 and 7–4) is a sheath of dense connective tissue that surrounds cartilage in most places, forming an interface between the cartilage and the tissue supported by the cartilage. The perichondrium harbors the vascular supply for the avascular cartilage and also contains nerves and lymphatic vessels. Articular cartilage, which covers the surfaces of the bones of movable joints, is devoid of perichondrium and is sustained by the diffusion of oxygen and nutrients from the synovial fluid.
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HYALINE CARTILAGE Hyaline cartilage (Figure 7–2) is the most common and best studied of the three forms. Fresh hyaline cartilage is bluish-white and translucent. In the embryo, it serves as a temporary skeleton until it is gradually replaced by bone. In adult mammals, hyaline cartilage is located in the articular surfaces of the movable joints, in the walls of larger respiratory passages (nose, larynx, trachea, bronchi), in the ventral ends of ribs, where they articulate with the sternum, and in the epiphyseal plate, where it is responsible for the longitudinal growth of bone (see Chapter 8: Bone). Matrix Forty percent of the dry weight of hyaline cartilage consists of collagen embedded in a firm, hydrated gel of proteoglycans and structural glycoproteins. In routine histology preparations, the collagen is indiscernible for two reasons: the collagen is in the form of fibrils, which have submicroscopic dimensions; and the refractive index of the fibrils is almost the same as that of the ground substance in which they are embedded. Hyaline cartilage contains primarily type II collagen (Figure 7–1). However, small amounts of collagen types IX, X, XI, and others are frequently present. Cartilage proteoglycans contain chondroitin 4-sulfate, chondroitin 6-sulfate, and keratan sulfate, covalently linked to core proteins. Up to 200 of these proteoglycans are noncovalently associated with long molecules of hyaluronic acid, forming proteoglycan aggregates that interact with collagen (Figure 7–3). The aggregates can be up to 4 m in length. Structurally, proteoglycans resemble bottlebrushes, the protein core being the stem and the radiating glycosaminoglycan chains the bristles.
The high content of solvation water bound to the negative charges of glycosaminoglycans acts as a shock absorber or biomechanical spring; this is of great functional importance, especially in articular cartilages (see Chapter 8: Bone). In addition to type II collagen and proteoglycan, an important component of cartilage matrix is the structural glycoprotein chondronectin, a macromolecule that binds specifically to glycosaminoglycans and collagen type II, mediating the adherence of chondrocytes to the extracellular matrix. The cartilage matrix surrounding each chondrocyte is rich in glycosaminoglycan and poor in collagen. This peripheral zone, called the territorial, or capsular, matrix, stains differently from the rest of the matrix (Figures 7–2 and 7–4). Perichondrium Except in the articular cartilage of joints, all hyaline cartilage is covered by a layer of dense connective tissue, the perichondrium, which is essential for the growth and maintenance of cartilage (Figures 7–2 and 7–4). It is rich in collagen type I fibers and contains numerous fibroblasts. Although cells in the inner layer of the perichondrium resemble fibroblasts, they are chondroblasts and easily differentiate into chondrocytes. Chondrocytes At the periphery of hyaline cartilage, young chondrocytes have an elliptic shape, with the long axis parallel to the surface. Farther in, they are round and may appear in groups of up to eight cells originating from mitotic divisions of a single chondrocyte. These groups are called isogenous (Gr. isos, equal, + genos, family). Cartilage cells and the matrix shrink during routine histological preparation, resulting in both the irregular shape of the chondrocytes and their retraction from the capsule. In living tissue, and in properly prepared sections, the chondrocytes fill the lacunae completely (Figure 7–5).
Chondrocytes synthesize collagens and the other matrix molecules. Because cartilage is devoid of blood capillaries, chondrocytes respire under low oxygen tension. Hyaline cartilage cells metabolize glucose mainly by anaerobic glycolysis to produce lactic acid as the end product. Nutrients from the blood cross the perichondrium to reach more deeply placed cartilage cells. Mechanisms include diffusion and transport of water and solute promoted by the pumping action of intermittent cartilage compression and decompression. Because of this, the maximum width of the cartilage is limited. Chondrocyte function depends on a proper hormonal balance. The synthesis of sulfated glycosaminoglycans is accelerated by growth hormone, thyroxin, and testosterone and is slowed by cortisone, hydrocortisone, and estradiol. Cartilage growth depends mainly on the hypophyseal growth hormone somatotropin. This hormone does not act directly on cartilage cells but promotes the synthesis of somatomedin C in the liver. Somatomedin C acts directly on cartilage cells, promoting their growth. MEDICAL APPLICATION Cartilage cells can give rise to benign (chondroma) or malignant (chondrosarcoma) tumors. Histogenesis Cartilage derives from the mesenchyme (Figure 7–6). The first modification observed is the rounding up of the mesenchymal cells, which retract their extensions, multiply rapidly, and form mesenchymal condensations of chondroblasts. The cells formed by this direct differentiation of mesenchymal cells, now called chondroblasts, have a ribosome-rich basophilic cytoplasm. Synthesis and deposition of the matrix then begin to separate the chondroblasts from one another. During development, the differentiation of cartilage takes place from the center outward; therefore, the more central cells have the characteristics of chondrocytes, whereas the peripheral cells are typical chondroblasts. The superficial mesenchyme develops into the perichondrium.
Growth The growth of cartilage is attributable to two processes: interstitial growth, resulting from the mitotic division of preexisting chondrocytes, and appositional growth, resulting from the differentiation of perichondrial cells. In both cases, the synthesis of matrix contributes to the growth of the cartilage. Interstitial growth is the less important of the two processes. It occurs only during the early phases of cartilage formation, when it increases tissue mass by expanding the cartilage matrix from within. Interstitial growth also occurs in the epiphyseal plates of long bones and within articular cartilage. In the epiphyseal plates, interstitial growth is important in increasing the length of long bones and in providing a cartilage model for endochondral bone formation (see Chapter 8: Bone). In articular cartilage, as the cells and matrix near the articulating surface are gradually worn away, the cartilage must be replaced from within, since there is no perichondrium there to add cells by apposition. In cartilage found elsewhere in the body, interstitial growth becomes less pronounced, as the matrix becomes increasingly rigid from the cross-linking of matrix molecules. Cartilage then grows in girth only by apposition. Chondroblasts of the perichondrium proliferate and become chondrocytes once they have surrounded themselves with cartilaginous matrix and are incorporated into the existing cartilage (Figures 7–2 and 7–4). Degenerative Changes MEDICAL APPLICATION In contrast to other tissues, hyaline cartilage is more susceptible to degenerative aging processes. Calcification of the matrix, preceded by an increase in the size and volume of the chondrocytes and followed by their death, is a common process in some cartilage. Asbestiform degeneration, frequent in aged cartilage, is due to the formation of localized aggregates of thick, abnormal collagen fibrils. Poor Regeneration of Cartilage Tissue Except in young children, damaged cartilage regenerates with difficulty and often incompletely, by activity of the perichondrium, which invades the injured area and generates new cartilage. In extensively damaged areas—and occasionally in small areas—the perichondrium produces a scar of dense connective tissue instead of forming new cartilage. |
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ELASTIC CARTILAGE Elastic cartilage is found in the auricle of the ear, the walls of the external auditory canals, the auditory (eustachian) tubes, the epiglottis, and the cuneiform cartilage in the larynx. Elastic cartilage is essentially identical to hyaline cartilage except that it contains an abundant network of fine elastic fibers in addition to collagen type II fibrils. Fresh elastic cartilage has a yellowish color owing to the presence of elastin in the elastic fibers (Figure 7–7).
Elastic cartilage is frequently found to be gradually continuous with hyaline cartilage. Like hyaline cartilage, elastic cartilage possesses a perichondrium. |
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FIBROCARTILAGE Fibrocartilage is a tissue intermediate between dense connective tissue and hyaline cartilage. It is found in intervertebral disks, in attachments of certain ligaments to the cartilaginous surface of bones, and in the symphysis pubis. Fibrocartilage is always associated with dense connective tissue, and the border areas between these two tissues are not clear-cut, showing a gradual transition. Fibrocartilage contains chondrocytes, either singly or in isogenous groups, usually arranged in long rows separated by coarse collagen type I fibers (Figure 7–8). Because it is rich in collagen type I, the fibrocartilage matrix is acidophilic.
In fibrocartilage, the numerous collagen fibers either form irregular bundles between the groups of chondrocytes or are aligned in a parallel arrangement along the columns of chondrocytes (Figure 7–8). This orientation depends on the stresses acting on fibrocartilage, since the collagen bundles take up a direction parallel to those stresses. There is no identifiable perichondrium in fibrocartilage. |
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INTERVERTEBRAL DISKS Each intervertebral disk is situated between two vertebrae and is held to them by means of ligaments. The disks have two components: the fibrous annulus fibrosus and the nucleus pulposus. The intervertebral disk acts as a lubricated cushion that prevents adjacent vertebrae from being eroded by abrasive forces during movement of the spinal column. The nucleus pulposus serves as a shock absorber to cushion the impact between vertebrae. The annulus fibrosus has an external layer of dense connective tissue, but it is mainly composed of overlapping laminae of fibrocartilage in which collagen bundles are orthogonally arranged in adjacent layers. The multiple lamellae, with the 90° registration of type I collagen fibers in adjacent layers, provide the disk with unusual resilience that enables it to withstand the pressures generated by impinging vertebrae. The nucleus pulposus is situated in the center of the annulus fibrosus. It is derived from the embryonic notochord and consists of a few rounded cells embedded in a viscous matrix rich in hyaluronic acid and type II collagen fibrils. In children, the nucleus pulposus is large, but it gradually becomes smaller with age and is partially replaced by fibrocartilage. Herniation of the Intervertebral Disk MEDICAL APPLICATION Rupture of the annulus fibrosus, which most frequently occurs in the posterior region where there are fewer collagen bundles, results in expulsion of the nucleus pulposus and a concomitant flattening of the disk. As a consequence, the disk frequently dislocates or slips from its position between the vertebrae. If it moves toward the spinal cord, it can compress the nerves and result in severe pain and neurological disturbances. The pain accompanying a slipped disk may be perceived in areas innervated by the compressed nerve fibers—usually the lower lumbar region. |
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