Overview Of Cartilage 164 Hyalin E Cartilag E 164 Elastic Cartilage 169 Fibrocartilage 169

HISTOGENESIS, GROWTH, AND REPAIR OF HYALINE CARTILAGE 170

v overview of cartilage v hyaline cartilage

Cartilage is a form of connective tissue composed of cells called chondrocytes and a highly specialized extracellular matrix

Cartilage is an avascular tissue that consists of chondrocytes and an extensive extracellular matrix. Prqduc£d_aiicL maintained by_jdiondmcyt£&,_.the cartilage matrix is solid andfirm but also somewhat pliable, which accounts for its resilience. The large ratio of glycosaminoglycans to type II collagen in the cartilage matrix permits^iffusion^ol^ub-stances between bIoocLvessels^injihe^t mounding connec-tlveQissu(Om^ reby maintaining the viability of the tissue. Also, the presence of Jarge amounts of hyaluronic acidjru:artjlage matqxjtiakesjQvejj aclaptecT to'bear^weight", especially at points of movement, as in synovial joints. Because it maintains this property even while growing, cartilage is a key tissue in most growing bones.

Three different kinds of cartilage are disnn^uTslTed~on the basis of characteristics of the matrix:

• Hyaline cartilage, characterized by matrix containing type 11 collagen fibers, proteoglycans, and hyaluronic acid

• Elastic cartilage, characterized by elastic fibers and elastic lamellae in addition to the matrix material of hyaline cartilage

• Fibrocartilage, characterized by abundant type I collagen fibers in addition to the matrix material of hyaline cartilage

Hyaline cartilage is distinguished by a homogeneous, amorphous matrix

The matrix of hyaline cartilage appears glassy in the living state, hence the name hyaline [Gr. hyalos, glassy]. Throughout the cartilage matrix are spaces called lacunae. Located within these lacunae are the chondrocytes. Hyaline cartilage is not a simple, inert, homogeneous substance, but a complex living tissue. It provides a low friction surface, participates in lubrication in synovial joints, and distributes applied forces to the underlying bone. Although its capacity for repair is limited, under normal circumstances it shows no evidence of abrasive wear over a lifetime. The macromolecules of hyaline cartilage matrix consist of collagen (predominantly type II fibrils), proteoglycans, noncollagenous proteins, and glycoproteins that give it its mechanical and biologic properties.

Hyaline cartilage matrix is produced by chondrocytes and contains three major classes of molecules

Three classes of molecules are present in hyaline cartilage matrix:

• Collagen molecules. Collagen is the major matrix protein. Four collagen types participate in the formation of relatively thin (20-nm-diameter), short matrix fibrils.

Type II collagen constitutes the bulk of the fibril; type XI collagen regulates the fibril size; and type IXcollagen facilitates fibril interaction with the matrix proteoglycan molecules. Type X collagen organizes the collagen fibrils into a three-dimensional hexagonal lattice. Because types II, IX, X, and XI are found in significant amounts only in the cartilage matrix, they are referred to as cartilage-specific collagen molecules. In addition, type VI collagen is also found in the matrix, mainly at the periphery of the chondrocytes, where it helps to attach these cells to the matrix framework. See Table 5.2 to review the different types of collagen.

• Proteoglycans. The ground substance of hyaline cartilage contains three kinds of glycosaminoglycans: hyaluronic acid, chondroitin sulfate, and keratan sulfate. As in loose connective tissue matrix, the chondroitin and keratan sulfates of cartilage matrix are isogenous group isogenous group

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