Proteoglycans of cartilage.

Proteoglycans are probably the most important nonfibrillar constituents of connective tissue although little is known about the proteoglycans of connective tissue other than cartilage. Hence this paper is mainly concerned with cartilage proteoglycans. Proteoglycans are found throughout connective tissue, but those of cartilage have certain features that distinguish them from proteoglycans of other connective tissues. Cartilage is an avascular tissue in which the cells are sparsely distributed in a stiff matrix. Although the content of water is high-about 70% in the cartilage of human femoral condyles (Maroudas et al., 1969)-it is precisely the presence of water in conjunction with proteoglycans and collagen that makes cartilage resilient and elastic. Cartilage may be regarded as a reinforced fibre network which in load-bearing joints has to withstand very high repetitive loads (Freeman and Kempson, 1973). As was first pointed out by Fessler (1960), a random macromolecular mesh (represented by proteoglycans in this instance) placed within a fibrous network (such as collagen) so that the macromolecules cannot move will impede the flow of interstitial water within the tissue when an external force is applied. Fluid pressure within cartilage rises immediately a load is applied, but as the water is driven out from the loaded area cartilage deforms only gradually because the proteoglycans entrapped in the collagen network impede the flow of interstitial water. Hence the compressive stiffness of cartilage over short intervals is directly correlated with the proteoglycan content measured as glycosaminoglycan (Kempson et al., 1970) (Fig. 1). Proteoglycans exert a swelling pressure that is constantly restrained by the collagen network in which they are entrapped. Maroudas (1975) has calculated the internal osmotic pressure of cartilage of human femoral heads to be about 3-4-3-6 atmospheres. Sorption isotherms also indicate that swelling pressure is mainly attributable to the proteoglycan component of cartilage (Mathews and Decker, 1977). The relative proportion of collagen to proteoglycan and other constituents varies in different types of connective tissue and largely determines the physical characteristics of the tissue. Collagen 67 accounts for about half the dry weight of fullthickness articular cartilage, but the amount decreases with depth from the articular surface (Maroudas et al., 1969; Muir et al., 1970). Conversely the proteoglycan content varies approximately inversely with the collagen content and there is a topographical variation in the overall composition of articular cartilage that appears to be characteristic of the individual (Maroudas et al., 1969; Muir et al., 1970; Kempson et al., 1973). The inverse relationship of proteoglycan and collagen contents is also clearly seen in human intervertebral discs (Adams et al., 1977).