Spontaneous and ethyl-nitrosourea-induced medullomyoblastomas in cerebellar vermis defect (CVD) mutant rats

A 26-week-old female cerebellar vermis defect (CVD) rat, a mutant with cerebellar vermis defect and cerebellar dysplasia, developed a brain tumor about 10 mm in diameter. Histopathologically, the tumor consisted of diffuse proliferation of small round to ovoid cells with hyperchromatic nuclei, occasionally containing round to strap-shaped myoblastic cells. Immunohistochemically, the small round cells expressed neuron-specific enolase and synaptophysin, indicating neuronal differentiation; myoblastic components reacted to desmin, myoglobin, and vimentin. Based on these findings, the case was diagnosed as a medullomyoblastoma (MMB). Furthermore, two cerebella tumors in CVD rats, which were induced by transplacental application of ethyl-nitrosourea, showed histopathology similar to the aforementioned case. MMB is a very rare tumor in humans and animals; thus, it is noteworthy that MMBs developed in CVD rats, involving the dysplastic cerebellum with abnormal migration of external granule cells.