Morphology and functional roles of synoviocytes in the joint

The joint capsule exhibits a unique cellular lining in the luminal surface of the synovial membrane. The synovial intimal cells, termed synoviocytes, are believed to be responsible for the production of synovial fluid compone nts, for absorption from the joint cavity, and for blood/synovial fluid exc hanges, but their detailed structure and function as well as pathological c hanges remain unclear. Two types of synoviocytes, macrophagic cells (type A cells) and fibroblast-like cells (type B cells) have been identified. Type A synoviocytes are non-fixed cells that can phagocytose actively cell debr is and wastes in the joint cavity, and possess an antigen-presenting abilit y. These type a cells, derived from blood-borne mononuclear cells, can be c onsidered resident macrophages (tissue macrophages) like hepatic Kupffer ce lls. Type B synoviocytes are characterized by the rich existence of rough e ndoplasmic reticulum, and dendritic processes which form a regular network in the luminal surface of the synovial membrane. Their complex three-dimens ional architecture was first revealed by our recent scanning electron micro scopy of macerated samples. The type B cells, which are proper synoviocytes , are involved in production of specialized matrix constituents including h yaluronan, collagens and fibronectin for the intimal interstitium and synov ial fluid. The proliferative potentials of type B cells in loco are much hi gher than type B cells, although the transformation of subintimal fibroblas ts into type B cells can not be excluded. In some mammals, type B cells sho w features suggesting endocrine and sensory functions, but these are not re cognized in other species. The synoviocytes, which form a discontinuous cel l layer, develop both fragmented basement membranes around the cells and ju nctional apparatus such as desmosomes and gap junctions. For an exact under standing of the mechanism of arthritis, we need to establish the morphologi cal background of synoviocytes as well as their functions under normal cond itions.