Collagen of articular cartilage

Abstract

The extracellular framework and two-thirds of the dry mass of adult articular cartilage are polymeric collagen. Type II collagen is the principal molecular component in mammals, but collagens III, VI, IX, X, XI, XII and XIV all contribute to the mature matrix. In developing cartilage, the core fibrillar network is a cross-linked copolymer of collagens II, IX and XI. The functions of collagens IX and XI in this heteropolymer are not yet fully defined but, evidently, they are critically important since mutations in COLIX and COLXI genes result in chondrodysplasia phenotypes that feature precocious osteoarthritis. Collagens XII and XIV are thought also to be bound to fibril surfaces but not covalently attached. Collagen VI polymerizes into its own type of filamentous network that has multiple adhesion domains for cells and other matrix components. Collagen X is normally restricted to the thin layer of calcified cartilage that interfaces articular cartilage with bone.

Figure 1

The chondrocyte and extracellular matrix of articular cartilage showing the underlying collagen fibril meshwork (transmission electron microscopy).

Figure 2

The collagen II:IX:XI heterofibril. A molecular model of the collagen type IX fold and interaction site with a collagen II microfibril that can account for all known cross-linking sites between collagen II and IX molecules.