Cellular contacts in the synovial membrane of the cat and the rabbit: an ultrastructural study.

Regularly, four different types of cellular contacts are found in synovial folds and villi of the cobital joint of the cat (interdigitations, desmosomes, intermediate junctions, gap junctions). The same types of contact--with the exception of intermediate junctions--occur sporadically also in synovial fat folds of the knee joint of the rabbit. In both species, hemidesmosomes and discontinuous basement membranes are seen in the synovial lining layer. Cellular contacts predominate between A-cells and cells of the intermediate type, hemidesmosomes and incomplete basement membranes predominate in intermediate cells and B-cells. The latter are rare in A-cells. The importance of such contacts for mechanical, metabolic and electrical interactions of cellular elements in the synovial membrane is discussed. No unanimous concept as to their function can be advanced at present.