A thymic tumor with massive proliferation of myoid cells.

A case of thymic tumor, composed of both epithelial and myoid cells, was presented. The myoid cells occurred in clusters, and showed prominent proliferation between the nests of epithelial cells. The myoid cells had intracytoplasmic myofibrils, and there were foci of electron dense Z-lines along the course of these fibrils. Some of these cells possessed sarcomeres identical to those of striated-muscle cells. Histochemically, a significant amount of glycosaminoglycans was observed on the surface of the myoid cells.