Composition of Articular Cartilage

The current state of knowledge on the structure and function of the macromolecules of articular cartilage is summarized. This highly hydrated tissue is composed of relatively few cells embedded in an extracellular matrix. The matrix consists of a fibrillar meshwork composed of mainly cross-linked type II collagen molecules, but with small amounts of type XI and IX collagen. The interstices of this meshwork are filled with the proteoglycan aggrecan bound, with link proteins, to chains of hyaluronan to form proteoglycan aggregates. The tissue also contains the small proteoglycans decorin, biglycan, and fibromodulin. The adhesion protein type VI collagen surrounds the chondrocytes and, in the extracellular matrix, binds to hyaluronan and other matrix molecules, probably to organize the matrix into higher ordered structures. The organization of these macromolecular structures with the water of the tissues endows articular cartilage with its characteristic mechanical properties.