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Synoviocytes are the primary cell type found in the synovial membrane, which lines the cavities of diarthrodial joints. These joints, such as the knee, hip, and shoulder, are characterized by their extensive range of motion and are enclosed by a synovial capsule. The synovial membrane itself is a specialized connective tissue that produces synovial fluid, a viscous, lubricating substance essential for joint health.

There are two main types of synoviocytes: type A and type B. Type A synoviocytes are macrophage-like cells responsible for phagocytosis, meaning they engulf and remove cellular debris, worn-out cells, and other waste products from the synovial fluid. This function is crucial for maintaining the cleanliness and health of the joint. Type B synoviocytes are fibroblast-like cells that synthesize and secrete the major components of synovial fluid, primarily hyaluronic acid and lubricin. Hyaluronic acid provides viscosity and elasticity to the fluid, while lubricin reduces friction between the articular cartilage surfaces.

The collective function of synoviocytes is vital for the proper functioning of synovial joints. They maintain the homeostasis of the joint by regulating the composition and volume of synovial fluid, removing waste, and lubricating the articular surfaces. Dysregulation or damage to synoviocytes can contribute to various joint pathologies, including osteoarthritis and rheumatoid arthritis. In these conditions, synoviocytes can become hyperactive or undergo changes that lead to inflammation, excessive fluid production, and degradation of joint tissues.