Developmental distribution of collagen type XII in cartilage: Association with articular cartilage and the growth plate

Collagen type XII is a member of the fibril-associated collagens and is cha racterized by a short triple-helical domain with three extended noncollagen ous NC3 domains. Previous studies suggested that collagen XII is a componen t of cartilage but little is known about its spatial-temporal distribution. This study uses a polyclonal antibody to the purified NC3 domain to invest igate its developmental distribution in rat forelimb. Collagen XII was pres ent at the joint interzone on embryonic day 16 (E16d) and restricted to the presumptive articular cartilage by E18d. Labeling of the articular surface intensified as development progressed postnatally (day I [1d] to 28d) and extended approximately six cell diameters deep. In juvenile rats, collagen XII antibodies also labeled the longitudinal and transverse septa of stacke d chondrocytes in the growth plate. However, collagen XII was not associate d at any developmental stage with the cartilaginous secondary ossification center and was only weakly expressed in epiphyseal cartilage. Ultrastructur al localization of the NC3 domain epitope showed labeling of the surface of collagen II fibrils, both in tissue and in isolated fibrils. The results p resented provide further evidence that articular cartilage differs substant ially from the underlying epiphyseal cartilage and that different chondrocy tic developmental fates are reflected in the composition of their extracell ular matrix starting early in development. In addition, collagen XII was di stributed in areas of cartilage with more organized fibril orientation and may have a role in promoting alignment or stabilizing such an organization, thereby creating a matrix capable of withstanding load-bearing forces.