Biochemistry of articular cartilage. Nature of proteoglycans and collagen of articular cartilage and their role in ageing and in osteoarthrosis.

The cartilage which covers the articular surfaces of bones of vertebrate animals has two main functions: the limitation of stress applied to the bone extremities and the provision of the smooth surfaces necessary for the joint to function as an efficient bearing (Kempson, Spivey, Freeman, and Swanson, 1969). To meet these functional demands the cartilage exhibits a combination of physical properties which are unique to the tissue. Boundary effects, resulting from the interaction of the cartilage surface with synovial fluid, provide thejoint with a bearing surface having a very low coefficient of friction. Articulation of a joint produces compressive stresses which arise from both muscle pull and gravity. In the absence of articular cartilage these stresses would be transmitted directly to the bones and would possibly fracture them. The viscoelastic nature of articular cartilage enables it to absorb the energy of these mechanical stresses. When an external stress is applied to the cartilage, the tissue undergoes a slowly progressive deformation. When the external stress is removed the cartilage can restore its original shape. The principal components of articular cartilage are the insoluble fibrous protein collagen and the soluble proteoglycans. A complex organization of collagen, proteoglycans, the fluid environment, and as yet uncharacterized components, endows the tissue with the capacity for reversible deformability, a property essential for its physiological function. The biochemist is primarily interested in the chemical composition of the tissue, the elucidation of the structure and organization of the constituent macromolecules, the nature of the synthetic and degradative systems in the tissue, and the influence of age and pathological state on these phenomena. Articular cartilage, as a tissue, has presented considerable difficulties to biochemical investigation. The tissue displays a complex morphology, as yet incompletely understood, and is very resistant to solubili-