Articular cartilage matrix metabolism

In aging adult articular cartilage, changes occur in water content, collagen content, proteoglycan composition and matrix component turnover. Water may account for as much as 80% of the weight of the articular cartilage. The dry weight of organic matrix and cells in adult articular cartilage is 30% of the wet weight. The water content is highest near the surface and lowest in the zone of calcified cartilage. Collagen (Type II) accounts for 50% of the dry weight of articular cartilage. Glycosaminoglycans range from 14022% of the dry weight. The concentration of proteoglycans varies inversely with the collagen concentration. The glycosaminoglycans concentration also varies with age. Keratin sulphate content increases with age while chondroitin sulphase decreases with age. Lipids and DNA account for 1.3 to 1.8 per cent of the dry weight. Non-collagen protein accounts for 10 to 20% of the dry weight. Chondrocytes are metabolically active in replacing their surrounding matrix. Synthesis of collagen and proteoglycans by chondrocytes is well documented. The ability of cartilage cels to degrade matrix components is less well understood. Approximately one-half of the matrix of glycosaminoglycans is replaced each year while only one-half of the collagen matrix is replaced every ten years. Little is known at this point about the degradation of collagen in ormal articular cargilage. Proteolytic enzymes may be formed by chondrocytes or brought in contact with cartilage and other joint structures by polymorphonuclear leukocytes or synovial cells. Proteinase inhibitors might permit excessive action by proteolytic enzymes synthesized in normal or physiological amounts. Prostaglandins and prostaglandin synthetase inhibitors play an important role in pathologic states. Pathways of matrix dissolution in rheumatoid and osteoarthritis are elaborated.