Spindle-cell epithelial thymoma. Fine-structural and tumor lymphocyte observations.

A fine-structural study of a spindle-cell epithelial thymoma in a patient with pemphigus and autoimmune hemolytic anemia is presented and compared with the few previously described. Because light-microscopic features suggested hemangiopericytoma, critical fine-structural comparisons between spindle-cell epithelial thymoma and hemangiopericytoma are detailed. Based upon groups of tonofilaments with desmosomal insertions, abundant well-formed desmosomes, negligible numbers of pinocytotic vesicles, and an absence of myofilaments and dense bodies, an epithelial origin for this tumor is proposed. Langerhans' cell granules, a new observation in thymoma, were found in cells of probably histiocytic origin. Tumor lymphocyte studies revealed that more than 95% of cells formed E rosettes, 36% formed EAC rosettes, yet none contained surface immunoglobulin. The significance of these observations is discussed.