Biochemical analysis of collagens at the ligament-bone interface reveals presence of cartilage-specific collagens.

The collagen fibrils of some ligaments attach to bone by passing through a zone that consists of nonmineralized and mineralized fibrocartilages. Very little, however, is known about the cells, the biochemical composition, the extracellular matrix organization, and function of these fibrocartilages. In this study, the collagens present in the fibrocartilages of the bovine medical collateral and anterior cruciate ligaments femoral attachments to bone were isolated, characterized, and their distribution at these sites was assessed by laser confocal microscopy. Types II, IX, and XI collagens were identified after pepsin digestion of the tissues in addition to the types I and V collagens. Immunoblotting using specific polyclonal antibodies confirmed the presence of types II and IX collagens at these sites. Immunofluorescence using confocal microscopy showed that type II collagen was prominent in the nonmineralized area and to a lesser extent in the mineralized zone of the insertion. Type IX collagen showed similar distribution as type II collagen. Type II collagen isolated from the ligament-bone interface contained half hydroxypyridinium cross-linking residues when compared to type II collagen isolated from articular cartilage of the same animals. These data indicate that the fibrocartilaginous zones at the ligament-bone interface are cartilaginous in nature. The cartilage collagens may play a role of anchoring the ligament to bone or the cartilage-like tissue may participate in the modulation of the mechanical stresses which are known to exist at the soft tissue-hard tissue interface.