synovialmembran

The synovial membrane, or synovium, is a soft tissue layer that lines the inner surface of synovial joints, tendon sheaths, and bursae. It forms the innermost lining of the joint capsule and does not cover articular cartilage or the fibrous outer layer. The membrane consists of two components: the intima, a surface layer of synoviocytes, and the subintima, a loose connective tissue rich in blood vessels, lymphatics, and nerves.

The intima contains two main cell types: Type A macrophage-like synoviocytes that perform phagocytosis and immune functions, and Type B fibroblast-like synoviocytes that secrete components of the synovial fluid, including hyaluronic acid and lubricin. The membrane is highly vascularized and innervated, reflecting its roles in nutrition, lubrication, and sensory feedback for joints.

Functions: It produces and maintains synovial fluid, a viscous lubricant that fills the joint cavity. The fluid reduces friction between articular surfaces, nourishes avascular cartilage, and facilitates waste removal. The membrane also contributes to immune surveillance and tissue repair through the activity of resident synoviocytes and immune cells.

Clinical relevance: Inflammation of the synovial membrane—synovitis—is common in inflammatory arthritides such as rheumatoid arthritis, where the intima thickens and may form pannus that invades cartilage and bone. Chronic synovitis can lead to joint effusions, pain, and reduced mobility. Synovial pathology can contribute to osteoarthritis and other joint disorders, and synovial cysts or excessive fluid accumulation may arise in affected joints.