Malignant Leydig cell tumor: clinical, histologic and electron microscopic features.

A 51-year-old man had a malignant Leydig cell tumor of the testis with elevation of serum estrone and androstenedione as well as urine chorionic gonadotropin. The fine structural features of the neoplasm resembled those of nontumorous Leydig cells as well as cells of Leydig cell adenomas, except for, nuclear pleomorphism, the absence of Reinke's crystals and the infrequency of microvilli. The abundance of smooth surfaced endoplasmic reticulum, well-developed mitochondria with tubular and vesicular cristae were regarded as representing ultrastructural expressions of enhanced secretory activity and consistent with the biochemical findings of increased hormonal production by tumor cells. Since Leydig cells have distinct ultrastructural features, ultrastructural investigation represents a valuable tool in establishing the Leydig cell origin of a tumor.