THE DEVELOPMENT OF ARTICULAR-CARTILAGE .1. THE SPATIAL AND TEMPORAL PATTERNS OF COLLAGEN TYPES

Articular cartilage is both morphologically and biochemically heteroge neous. Its susceptibility to degenerative diseases such as arthritis a nd its limited repair capacity have made cartilage the focus of intens e study; surprisingly, little is known of its development. Using a pan el of specific antibodies, we have documented the temporal and spatial patterns of collagen types I, II, III, VI and X in the developing kne e cartilage of the marsupial Monodelphis domestica from parturition to adulthood. Type I collagen was initially detected in the presumptive articular cartilage of the epiphyses in addition to the perichondrium. By 14 d postparturition, type I collagen was not detectable in the ep iphyseal cartilage apart from insertion sites of ligaments and tendons of the joint. Similarly, type III collagen was detected at insertion sites of the major ligaments and tendons and within the perichondrium/ periosteum but was never detected in the cartilage per se. Type II col lagen was predictably distributed throughout the cartilage matrix and was also detected in the perichondrium. Type VI collagen was widely di stributed throughout the cartilage matrix at parturition, but during d evelopment became restricted to a pericellular location particularly t owards the presumptive articular cartilage, i.e. the epiphysis. Intere stingly, generalised matrix immunopositivity was only retained in the hypertrophic cartilage of the secondary centre of ossification. After the formation of the secondary centre, type VI collagen became localis ed pericellularly in the deeper regions of the articular cartilage but was absent in the cartilage of the growth plate. Type X collagen show ed a novel distribution pattern. In addition to being synthesised by h ypertrophic chondrocytes, this collagen type was also expressed transi ently by some cells at the presumptive articular surface. Furthermore, these surface chondrocytes also stained histochemically for alkaline phosphatase, suggesting that they were terminally differentiated. The fate of these terminally differentiated cells is unknown.