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 lubricating 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. An exception is articular cartilage, which, in many individuals, breaks down with age (Folder 7.1). The macromolecules of hyaline cartilage matrix consist of collagen (predominantly type II fibrils and other  cartilage-specific collagen molecules), proteoglycan aggregates containing GAGs, and multiadhesive glycoproteins (noncollagenous proteins). Figure 7.2 illustrates the relative distribution of the various components that constitute cartilage matrix.
Hyaline cartilage matrix is produced by chondrocytes and contains three major classes of molecules.
Three classes of molecules exist in hyaline cartilage matrix.  Collagen molecules. Collagen is the major matrix pro tein. Four types of collagen participate in the formation of three-dimensional meshwork of the relatively thin (20-nm diameter) and short matrix fibrils. Type II collagen cells 3–5% multiadhesive glycoproteins 5% proteoglycans (aggrecan) 9% collagens 15% intercellular water 60–80% 5% III, VI, X, XII, XIV 15% IX, XI 80% II FIGURE 7.2 • Molecular composition of hyaline cartilage. Cartilage contains 60–80% of the wet weight of intercellular water, which is bound by proteoglycan aggregates. About 15% of the total weight is attributed to collagen molecules, of which type II collagen is the most abundant. Chondrocytes occupy only 3–5% of the total cartilage mass.
Osteoarthritis, a degenerative joint disease, is one of the most common types of joint diseases. The pathogenesis of osteoarthritis is unknown, but it is related to aging and injury of articular cartilage. Most individuals show some evidence of this disease by age 65. The disease is charac-terized by chronic joint pain with various degrees of joint deformity and destruction of the articular cartilage. Os-teoarthritis commonly affects weight-bearing joints: hips, knees, lower lumbar vertebra, and joints of the hand and foot. There is a decrease in proteoglycan content, which re-sults in reduction in intercellular water content in the carti-lage matrix. Chondrocytes also play an important role in the pathogenesis of osteoarthritis. By producing interleukin-1 (IL-1) and tumor necrosis factor (TNF-), the production of metalloproteinases is stimulated, whereas synthesis of type II collagen and proteoglycans by the chondrocyte is in-hibited. In the early stages of the disease, the superficial layer of the articular cartilage is disrupted. Eventually, de-struction of the cartilage extends to the bone, where the ex-posed subchondral bone becomes a new articular surface. These changes result in progressive reduction of mobility and increased pain with joint movement. Osteoarthritis has no cure, and treatment focuses on relieving pain and stiff-ness to allow a greater range of joint movement. Os-teoarthritis may stabilize with age, but more often it slowly progresses with eventual long-term disability.
 FOLDER 7.1