The synovial intimal cell layer comprises two morphological types of cell, A and B, with an intermediate type also postulated. Type A cells show features in common with other cells of the mononuclear phagocyte system, while type B cells appear similar to fibroblasts and are assumed to have synthetic activity. Muramidase is a marker of mononuclear phagocytic cells, and we have investigated the synovial membrane for the presence of this enzyme in cells by an immunogold labelling technique. Muramidase was localised within intracytoplasmic vacuoles in subintimal macrophages and type A synoviocytes. This finding provides further evidence that type A cells are closely related to macrophages.
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