Synovial

Synovial is an adjective relating to the synovium, the soft tissue that lines the capsules of diarthrodial joints, tendon sheaths, and bursae. In medical use, the term describes structures and fluids associated with this lining.

In diarthrodial joints, the joint capsule consists of an outer fibrous layer and an inner synovial membrane. The synovial membrane seals the joint cavity and secretes synovial fluid, a viscous, lubricating liquid that fills the cavity. This fluid reduces friction between articular surfaces during movement, nourishes avascular articular cartilage, and helps distribute nutrients and waste.

The synovial membrane is composed of two main cell types: type A synoviocytes, macrophage-like cells, and type B synoviocytes, fibroblast-like cells. The tissue is well vascularized and contains subsynovial connective tissue.

Synovial fluid contains hyaluronic acid and lubricin, which contribute to its viscoelastic properties. It is formed by ultrafiltration of plasma and active secretion by synoviocytes, and its composition can change with disease or injury.

Pathology: Inflammation of the synovium is called synovitis and is a common feature of several joint diseases, including rheumatoid arthritis and gout. Chronic synovitis can lead to joint effusion and thickening of the membrane. Rarely, abnormal synovial proliferation or neoplasia can occur, such as pigmented villonodular synovitis or synovial sarcoma.