Epithelioid sarcoma. Ultrastructural and immunohistologic features suggesting a synovial origin.

A case of epithelioid sarcoma was studied light microscopically, ultrastructurally, and immunohistologically. The tumor was a deep antebrachial mass in a 57-year-old man, and led to antebrachial amputation. Light microscopically, the tumor consisted of nodules that often had central necrosis. The cells were uniform, with abundant eosinophilic or pale, translucent cytoplasm. Electron microscopic examination showed many filopodialike cytoplasmic surface extensions in many cells and occasional glandlike structures with specialized cell-to-cell junctions. The cytoplasm of some cells contained bundles of intermediate filaments that immunohistochemical examination showed to be of vimentin type. Some ultrastructural features resemble those of classic synovial sarcoma and the immunohistologic findings are identical with those of synovial cells, which may thus be the cell of origin of epithelioid sarcoma in this case.