The development of articular cartilage: II. The spatial and temporal patterns of glycosaminoglycans and small leucine-rich proteoglycans.

Articular cartilage is both morphologically and biochemically heterogeneous. Its susceptibility to degenerative diseases such as arthritis and its limited repair capacity has made cartilage the focus of intense study; surprisingly, little is known of its development. Using a panel of specific antibodies, we have documented the temporal and spatial patterns of the small leucine-rich proteoglycans fibomodulin, decorin and biglycan in the developing knee cartilage of the marsupial South American opposum (Monodelphis domestica) from parturition to adulthood. The major proteoglycan of cartilage, aggrecan, can be substituted with a variety of isomers of chondroitin sulphate (CS) and keratan sulphate (KS) glycosaminoglycans. Consequently, we have used monoclonal antibodies to determine the distribution of the chondroitinase generated epitopes of CS isomers (delta di-6S and delta di-4S oligosaccharide 'stubs'). Other monoclonal antibodies (3B3[-], 7D4) were used to investigate temporal changes in the expression of specific sulphation patterns within native chondroitin sulphate chains in addition to keratan sulphate chains (5D4). We found the distributions of the small proteoglycans (PGs) to be highly dynamic during development. Both fibromodulin and biglycan appeared to specifically label early articular cartilage as opposed to epiphyseal or growth plate cartilage. All 3 small PGs become preferentially distributed to the upper half of the adult articular cartilage depth. Similarly, delta di-6S, delta di-4S oligosaccharide 'stubs', KS and epitope 7D4 were variably distributed during development but all were again preferentially located to the upper depth of the mature tissue. The epitope recognised by antibody 3B3[-] was extensively distributed in the neonate, but became more restricted to hypertrophic chondrocytes by day 19. It was not detected in the adult tissue. These data suggest that in Monodelphis, proteoglycans are preferentially synthesised and elaborated in the upper half of the tissue depth and contrasts with the patterns observed in eutherian mammals. The data also pose questions as to the functional significance of these molecules within the tissues and to the idea that global patterns of matrix components exist in mammalian articular cartilages.