Malignant synovioma: electron microscopical findings in three patients and review of the literature.

Ultrastructural findings in three malignant synoviomas are described. Two typical "biphasic" tumours contained "epithelial" cells possessing filopodia or microvilli, specialised cell attachment and a basal lamina, and smaller "stromal" cells showing transitions to fibroblasts. In one case, the microvilli included fibrils resembling those in epithelial cells of the intestine and renal tubules. The third tumour was mainly spindle-celled with little epithelial differentiation and no clear division into cell types, but intracytoplasmic microfibrils were conspicuous, forming ovoid masses. The tumour cells differ a good deal from normal human synovial cells but some of the features of the neoplasm are found in inflamed human synovium and in normal synovial membranes of other species. The cytoplasmic fibrils in the third case are similar to those reported by others in epithelioid sarcoma, a tumour that may be of related origin to synovioma; however, the phenomenon may be merely degenerative.