Primary thymic seminoma—a neoplasm ultrastructurally similar to testicular seminoma and distinct from epithelial thymoma

Epithelial thymoma and thymic seminoma, which are important to distinguish for therapeutic reasons, are often confused histologically. The purpose of this study is to compare the ultrastructural appearances of a primary thymic seminoma, a classical testicular seminoma, and an epithelial thymoma. The thymic seminoma shows many ultrastructural similarities to the classical testicular seminoma in that the tumor cells have bizarre nucleoli, relatively scanty organelles, abundant glycogen, and are rounded or polyhedral with few cell processes. Conversely, there are distinct differences between the thymic seminoma and epithelial thymoma; tumor cells of the latter have many elongated cytoplasmic processes, numerous cytoplasmic filaments and desmosomes, and much less complex nucleoli. Basement membrane material is prominent only in the epithelial thymoma. It is evident that the distinction between thymic seminoma and epithelial thymoma is readily recognizable by electron microscopic studies and terms such as seminomatous or pseudoseminomatous thymoma, which reflect doubt about thymic seminoma as an entity distinct from epithelial thymoma, are misleading.