Hyaline cartilage and perichondrium.

Hyaline cartilage and perichondrium.

(a): A section of hyaline cartilage shows chondrocytes located in matrix lacunae. Preparation for sectioning usually causes shrinkage of the matrix which may cause the chondrocytes to pull away from the matrix and become distorted. The upper part of the figure shows the more eosinophilic perichondrium, an example of dense connective tissue consisting largely of type I collagen. There is a gradual transition and differentiation of cells from the perichondrium to the cartilage, with elongated fibroblastic cells becoming larger and more rounded chondrocytes with irregular surfaces contacting the matrix secreted by the cells. X200. H&E.

(b): Diagram of the area of transition between the perichondrium and the hyaline cartilage. In living cartilage chondrocytes essentially fill their lacunae. Closely associated groups of two or four lacunae indicate isogenous groups, or clones of chondrocytes derived from the same cell. Staining differences are apparent between the matrix immediately around each lacuna, called the territorial matrix, and that more distant from lacunae, the interterritorial matrix. Collagen is more

abundant in the interterritorial parts of the matrix.

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 surrounding substances. Hyaline cartilage contains primarily type II collagen, although

small amounts of collagen types VI and IX are also present.

Cartilage proteoglycans contain chondroitin 4-sulfate, chondroitin 6-sulfate, and keratan sulfate, covalently linked to core proteins. Hundreds of these proteoglycans are bound noncovalently to long molecules of hyaluronic acid by link proteins, forming very large proteoglycan aggregates such as aggrecan that interact with collagen. Structurally, proteoglycans resemble bottle brushes, the protein core being the stem and the radiating

glycosaminoglycan (GAG) chains the bristles.

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 aggregates (Gr. isos, equal, + genos, family). Chondrocytes synthesize collagens and the other matrix molecules. As matrix is produced, cells in the aggregates are moved

apart and occupy separate lacunae.

Cartilage cells and the matrix often shrink during routine histologic preparation, resulting in both the irregular shape of the chondrocytes and their retraction from the matrix. In living tissue, and in properly prepared sections, the chondrocytes fill the lacunae completely.

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 diffuse through the perichondrium to reach the more deeply placed cartilage cells. Transport of water and solutes is promoted by the pumping action of intermittent cartilage compression and decompression. Because of the limits of diffusion, the maximum width of the cartilage is limited and cartilage usually is found as small, thin plates of tissue.

Chondrocyte function is hormone dependent. Synthesis of sulfated GAGs is accelerated by growth hormone, thyroxin, and testosterone and is slowed by cortisone, hydrocortisone, and estradiol. Cartilage growth depends mainly on the pituitary-derived growth hormone somatotropin. This hormone does not act on cartilage cells directly but promotes the endocrine release in the liver of insulin-like growth factor-1 (IGF-1), sometimes called somatomedin C. IGF-1 acts directly on cartilage cells, promoting their growth.

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