Microanatomy of normal areolar synovium in diarthrodial joints
Normal areolar synovium contains synovial lining (intimal) cells that are two to three cells deep. This synovium lines all intracapsular structures except the contact areas of articular cartilage. The synovial lining cells reside in a matrix rich in type I collagen and proteoglycans.
There are two main types of synovial lining (intimal) cells that can only be differentiated by electron microscopy. Type A cells are macrophage-like and have primarily a phagocytic function. They are derived primarily from bone marrow via circulating monocytes. Type B cells are more abundant than type A cells. They are fibroblast-like and derived from division of tissue resident fibroblasts within the synovium. They produce hyaluronic acid (hyaluronan), which causes the increased viscosity of synovial fluid.
Other cells found in the synovium include antigen-presenting cells (dendritic cells) and mast cells. The synovium does not have a limiting basement membrane but does have fenestrated capillaries below the synovial lining cells. The deeper synovial tissue matrix contains minor collagens, hyaluronic acid, small arterioles/venules, lymphatic vessels, and nerve fibers derived from the capsule and periarticular tissues.