The synovial membrane that lines diarthrodial joints is composed of a loose layer of cells (1-4 cells in depth) that overlie the surface of articular and periarticular subsynovial tissues including connective tissue, adipose tissue, tendon, bone, and articular cartilage. Using light microscopy, the synovial lining tissue appears discontinuous and lacks a clearly defined basement membrane, although the cells appear to be retained by a reticulum. Cells present below this membrane include mast cells and endothelial cells (up to 10% of the total cell population; refs. 1,2). Ultrastructural studies of the synovium have indicated that at least two major types of cells are present within the synovial membrane. Type A cells are monocytoid in appearance and contain abundant endoplasmic reticulum with fewer vesicles, vacuoles, and mitochondria. Type B cells are fibroblastoid in appearance and contain large granules, numerous filopodia, mitochondria, and intracellular vesicles. The percentage of Type A and B cells present within adult synovium varies between species (3-5). In addition, cells with an intermediate ultrastructural appearance and morphology may exist within adult synovium (6).