Synovial sarcoma: ultrastructural study and immunohistochemical analysis by a combined peroxidase-antiperoxidase/avidin-biotin-peroxidase complex procedure.

To analyze the purportedly epithelial features of synovial sarcoma, the antigenic profiles of 20 of these neoplasms (including 12 of the monophasic type) were studied by three different immunohistochemical techniques, and the results were correlated with ultrastructural observations in 10 cases. All of the biphasic tumors were immunoreactive for the epithelial markers epithelial membrane antigen (EMA) and cytokeratin (CK) and also had ultrastructural features of epithelial differentiation. In contrast, only one of five monophasic tumors had electron microscopic features suggestive of epithelial differentiation, but eight of the 12 were immunoreactive for CK or EMA by a procedure combining the peroxidase-antiperoxidase and avidin-biotin-peroxidase complex methods. It is concluded that synovial sarcoma, including the monophasic variant, is a mesenchymal tumor with epithelial features. Immunohistochemical studies are more sensitive than ultrastructural analysis for documenting epithelial differentiation.