Granular cell myoblastoma

Two cases of granular cell myoblastoma, one multicentric, with tumors of the bronchus, tongue, and wrist were studied. Based on electron microscopic findings, it was concluded that granular cell myoblastoma arises from Schwann cells. In support of this conclusion was the demonstration of some tumor cells having morphological properties of Schwann cells, i.e., the limiting basement membrane and cytoplasmic filaments. Of greater importance was the observation of sequential cytoplasmic changes of Schwann cells in nerve fibers. The inception of these changes was characterized by focal cytoplasmic degradation leading to development of cytoplasmic vacuoles indistinguishable from those in the tumor cells. Some nerve fibers, because of these advance changes of Schwann cells, were recognized only by the neural growth pattern. As further evidence in support of our conclusion was the demonstration of nonmyelinated axons within the cytoplasm of tumor cells. A focal lysosomal defect of Schwann cells was suggested as an etiologic factor for granular cell myoblastoma.