synoviocytes

Synoviocytes are specialized cells that line the inner surface of the synovial membrane (the intima) of diarthrodial joints, tendon sheaths, and bursae. They contribute to the production and maintenance of synovial fluid, which lubricates and nourishes articular cartilage. The synovial lining is dominated by two main cell types that intermingle within the intima.

Type A synoviocytes are macrophage-like and function in phagocytosis and immune surveillance within the joint. Type B synoviocytes are fibroblast-like and are the principal producers of hyaluronic acid, lubricin (PRG4), proteoglycans, and other extracellular matrix components that contribute to the viscosity and lubrication of the synovial fluid. Together, these cells regulate joint homeostasis, respond to mechanical stimuli, and participate in the exchange of nutrients between the synovial fluid and cartilage. They also secrete a range of cytokines, chemokines, and other mediators that coordinate local inflammation and tissue remodeling.

In health, synoviocytes help maintain a balanced joint environment. In inflammatory or destructive joint diseases, they can become activated and contribute to pathology. For example, in rheumatoid arthritis the synovial lining undergoes hyperplasia and forms pannus, with fibroblast-like synoviocytes producing matrix metalloproteinases and inflammatory mediators that contribute to cartilage and bone damage. Understanding synoviocyte biology informs approaches to manage joint inflammation and degeneration.

Originally, type A synoviocytes are derived from the monocyte lineage, reflecting their macrophage-like properties, while type B synoviocytes arise from mesenchymal or fibroblast-like precursors.