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 components, for absorption from the joint cavity, and for blood/synovial fluid exchanges, but their detailed structure and function as well as pathological changes 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 debris and wastes in the joint cavity, and possess an antigen-presenting ability. These type A cells, derived from blood-borne mononuclear cells, can be considered resident macrophages (tissue macrophages) like hepatic Kupffer cells. Type B synoviocytes are characterized by the rich existence of rough endoplasmic reticulum, and dendritic processes which form a regular network in the luminal surface of the synovial membrane. Their complex three-dimensional architecture was first revealed by our recent scanning electron microscopy of macerated samples. The type B cells, which are proper synoviocytes, are involved in production of specialized matrix constituents including hyaluronan, collagens and fibronectin for the intimal interstitium and synovial fluid. The proliferative potentials of type B cells in loco are much higher than type A cells, although the transformation of subintimal fibroblasts into type B cells can not be excluded. In some mammals, type B cells show features suggesting endocrine and sensory functions, but these are not recognized in other species. The synoviocytes, which form a discontinuous cell layer, develop both fragmented basement membranes around the cells and junctional apparatus such as desmosomes and gap junctions. For an exact understanding of the mechanism of arthritis, we need to establish the morphological background of synoviocytes as well as their functions under normal conditions.

Three-dimensional ultrastructure of synoviocytes in the horse joint as revealed by the scanning electron microscope.

The synovial membrane displays a superficial cellular lining composed of two types of synoviocytes: "absorptive" macrophages (type A cells) and "secretory" fibroblast-like cells (type B cells). The types are intermingled and extend a variety of processes, rendering the cellular architecture of the synovial membrane difficult to visualize. Previous electron microscopic and histochemical studies failed to demonstrate the entire shape of synoviocytes, except our immunohistochemical study for protein gene product 9.5 in the horse joint. The present SEM study is the first to demonstrate the three-dimensional ultrastructure of synoviocytes as well as their distribution in the synovial membrane, using macerated samples from the horse carpal joints. The equine synovial membrane was largely covered by conspicuously developed synovial villi. Type A synoviocytes were closely similar to macrophages in regard to surface structure, and showed uneven distribution with the densest occurrence around the tips of the synovial villi. In the basal half of villi, type B synoviocytes, which were situated in close proximity to the synovial cavity, projected thick processes horizontally and intertwined to form a regular network of processes on the synovial surface. Those in the upper half of the villi were located in the abluminal layers and protruded an antenna-like process into the joint cavity with tips covered with long microvilli, in addition to forming the superficial plexus of processes. Type B cells were also provided with fine, membranous extensions that tended to cover the surface of synovial intima. The meshwork of horizontal processes, the antenna-like processes, and the membranous processes imply advantages in not only secretion but also sensation and regulation of the barrier function in the synovial membrane.