So-called extraskeletal Ewing's sarcoma. Report of a case with ultrastructural analysis.

The light and the electron microscopic features of an extraskeletal round-cell tumor resembling Ewing's sarcoma are described. Ultrastructural observation revealed features similar to Ewing's sarcoma of bone. Variable agrees of nuclear complexity are described. These soft tissue tumors are probably composed of undifferentiated mesenchymal cells; there is no ultrastructural evidence to indicate the cell of origin.