Spermatocytic seminoma in the rat.

During the necropsy of a 2-yr-old untreated control rat from a carcinogenicity study, a tan brown nodular mass was observed within the tunica albuginea of one testis. Both testes were processed for light and transmission electron microscopy. Histologically, the testicular mass was composed of round to polyhedral cells with distinct cell boundaries, amphophilic cytoplasm, fine nuclear chromatin, and a delicate fibrovascular supporting stroma. Ultrastructurally, the tumor cells displayed features associated with spermatocytic differentiation. Based on the light microscopic and the ultrastructural features, the tumor was considered a spermatocytic seminoma of the testis, a rare tumor in the rat.