[Epithelioid sarcoma--immunohistochemical and ultrastructural study].

Five cases of epithelioid sarcoma are reported, of which four were studied by immunohistochemistry and one by electron microscopy. Immunohistochemically, the results of cytokeratin showed that these four cases were positive to different degrees by polyclonal cytokeratin and three out of four revealed positive results with low molecular weight cytokeratin monoclonal antibody. Two cases were positive with carcinoembryonic antigen and the other two were negative. The immunohistochemical stain was helpful for differential diagnosis from other sarcomas, such as malignant fibrous histiocytoma and rhabdomyosarcoma. By electron microscopy, tumor cells could be divided into light and dark cells which possessed more or less cell organelles, intermediate microfilaments in the cytoplasm and numerous pinocytic vesicles lying along the cell membrane. The occasional presence of poorly developed desmosomes or the absence of tonofibril bundles and glandular structures can rule out the diagnosis of carcinoma. One of these cases was originally diagnosed as metastatic carcinoma to the skin by light microscopy, which was refuted by electron microscopy, and the keratin positive reaction by immunohistochemistry was helpful for the diagnosis of epithelioid sarcoma. Therefore, the authors suggest that immunohistochemical and electron microscopic studies be used for diagnosing difficult cases of epithelioid sarcoma. From our observation, we consider that the cells of epithelioid sarcoma may derive from undifferentiated mesenchymal cells possessing the potentials of differentiating into synovioblasts, histiocytes or fibroblasts.