Synovial membrane

The synovium lines the capsule, intracapsular bone not covered by cartilage, intracapsular ligaments and tendons (e.g. the long head of biceps), and fat pads — but not the articular cartilage or the central parts of menisci and discs. It consists of an intima of 1–3 cell layers with no basement membrane, over a loose, vascular subintima. The intima contains two cell types: type A synoviocytes (macrophage-like; phagocytose debris) and type B synoviocytes (fibroblast-like; secrete hyaluronan and lubricin). Fenestrated capillaries in the subintima provide the plasma ultrafiltrate that forms the bulk of the synovial fluid.

The membrane may form folds (plicae) and villi; it is continuous with the lining of bursae and tendon sheaths that communicate with the joint (e.g. the suprapatellar bursa of the knee).