Epithelio-reticular cell thymoma with lymphocytic "emperipolesis." An ultrastructural study.

The ultrastructure of an epithelio-reticular cell thymoma associated with myasthenia gravis is described. The neoplastic cells of an epithelio-reticular nature were dominant in the neoplasm, being closely associated with lymphocytes and scattered non-neoplastic macrophages. The thymic epithelio-reticular cell showed abundant glycogen and some lipid droplets inside the cytoplasm, which was vacuolated and had many elongated processes attached to desmosomes and a very prominent nucleolus. The appearance was similar to that of the immature embryonic epithelial cell of the thymus seen in mammals. Very close contacts existed between the thymic lymphocytes and the epithelio-reticular cells, the appearance being suggestive of the phenomenon of "emperipolesis." Cell death occurred secondarily through nuclear pyknosis and chromatolysis. Lymphocytic debris appeared inside the epithelio-reticular cells and, to some extent, in the cytoplasm of the mesenchymal marcrophages not directly involved in the phenomenon of "emperipolesis."