The role of type IX collagen in osteoarthritis and rheumatoid arthritis.

Articular cartilage consists of a cellular and an extracellular compartment. The extracellular compartment is composed of collagen, proteoglycans, and noncollagenous matrix proteins. Collagen resists tensile forces and serves as an organizing skeleton that helps maintain the structural integrity of cartilage. Fourteen types of collagen have been identified. The cartilage-specific collagens are type II (the principal component), type IX, type X, and type XI. Type IX collagen is hypothesized to be the "glue" that holds together the type II collagen latticework of articular cartilage. Degradation of type IX collagen by proteolytic enzymes has been observed in the primary stages of osteoarthritis and rheumatoid arthritis. This degradation is thought to represent an "ungluing" of the collagen scaffold and has been proposed as the mechanism for the degenerative changes seen in osteoarthritic and rheumatoid cartilage.