Normal Anatomy and Histology

Kerckhofs, Annelies Siozopoulou, Vasiliki [UCL]

The synovium or synovial membrane is a distinct and rather complex structure that lines joints, tendons, fat pads, articular disks, and bursae. Its role is to maintain homeostasis. Its function and histomorphological aspect depend largely on the location and the particular joint. Synovium is found in synovial diarthrotic joints, where it namely forms the inner layer of the joint capsule. Histologically, the synovial membrane consists of two layers, a cellular intimal layer and a more fibrous subintimal layer. The intimal layer faces the articular space. Its main function is to produce and absorb the synovial fluid. The intima is composed of two types of cells, type A and type B synoviocytes. Type A cells serve as macrophages and are responsible for the phagocytosis of debris in the joint space. Type B synoviocytes are more dominantly present in the synovium and produce numerous extracellular matrix proteins, especially hyaluronic acid. The function of the subintimal layer, which is broad and contains blood vessels, nerves, and fibroblasts, is not yet fully understood. Synovium can be a site of development for a variety of benign, intermediate, and malignant tumors.