Spontaneous nephroblastoma with striated muscle differentiation in an F344 rat.

An eleven-month old male F344/DuCrj (F344) rat was found dead and had right kidney mass at necropsy. Histopathologically, the mass was composed of nests of neoplastic stellate cells. At the center of the nests, neoplastic epithelial cells formed a tubular structure. In the fibrous connective tissue surrounding the nests, neoplastic cells with striations demonstrable by phosphotungstic acid hematoxylin were observed. Immunohistochemically, neoplastic stellate cells were partially positive for Wilms Tumor 1 and vimentin, and neoplastic cells with striations were partially positive for desmin. We diagnosed this tumor as a nephroblastoma with striated muscle differentiation. To our knowledge, this is the first case of nephroblastoma with apparent striated muscle differentiation in an F344 rat.