synovium

The synovium, or synovial membrane, is a specialized connective tissue that lines the inner surface of synovial joints and also the linings of bursae and tendon sheaths. It forms a barrier between vascular tissue and the joint cavity and plays a central role in the production and maintenance of synovial fluid, which lubricates and nourishes joint tissues.

Anatomy and cellular structure: The membrane is two-layered. The inner lining, the intima, consists of a few cell layers of synoviocytes, including type A macrophage-like cells and type B fibroblast-like cells. The subintima is loose connective tissue containing blood vessels, lymphatics, nerves, and adipose tissue. The synovial membrane is highly vascularized and is capable of rapid cellular proliferation in response to injury or inflammation.

Function: The primary role of the synovium is to secrete synovial fluid, a viscous ultrafiltrate of plasma rich in hyaluronic acid and lubricating proteins such as lubricin. The fluid lubricates articular surfaces, reduces friction, and distributes nutrients to avascular cartilage and other joint structures. The synovium also participates in immune defense and waste removal within the joint.

Clinical relevance: The synovium is involved in numerous joint disorders. Synovitis denotes inflammation of the membrane and is a hallmark of conditions such as rheumatoid arthritis and infectious or reactive arthritis, where the intima may thicken and form pannus that invades cartilage and bone. Other disorders include pigmented villonodular synovitis and synovial chondromatosis. Imaging with ultrasound or MRI can reveal synovial thickening, effusion, or proliferation. Treatments target the underlying disease and may include NSAIDs, disease-modifying antirheumatic drugs, corticosteroid injections, or surgical synovectomy in selected cases.