Ultrastructure of the synovial membrane.

The normal synovial membrane as seen by electron microscopy is bordered by one to three layers of lining cells that include some with phagocytic function, others rich in endoplasmic reticulum for protein synthesis and additional cells with mixed properties. Lining cells with prominent Golgi apparatus probably produce the synovial hyaluronic acid. The synovial matrix contains collagen and mucopolysaccharide. It offers little barrier to diffusion of most materials. Superficial capillaries and venules are fenestrated and presumably allow rapid exchange of fluid and electrolytes. Deeper venules have thick endothelium that can separate at intracellular junctions and can phagocytize circulating particles. Other identified synovial structures are fibrocytes, macrophages, lymphatics, mast cells, non-myelinated nerves and fat.