Ultrastructural localization of hyaluronic acid in the synovium of the goat knee joint.

In the Japanese miniature (Shiba) goat, the synovial membrane contains synoviocytes referred to as type A (macrophage-like cells) and type B cells (fibroblast-like cells) in the intimal layer. Small capillaries and blood vessels of varying sizes were located in the extracellular matrix in the synovial subintima. The type A cells in the synovium possessed numerous vesicles, vacuoles and lysosomes as well as pinocytotic vesicles. These ultrastructural features indicating phagocytosis showed distinct positive reactions following hyaluronan staining. On the other hand, in the type B cells, hyaluronic acids were present in the surface coat of the plasma membrane and its periphery. Additionally, perivascular connective tissue of the small capillaries and blood vessels and interfibrous matrix contained hyaluronan. The results suggest that hyaluronic acid, in the synovial tissue, is synthesized on the plasma membrane of type B cells, and taken up by type A cells. Moreover, hyaluronan is involved in cellular functions in the synovial connective tissue.