Occurrence, transplantation, and histologic characteristics of nephroblastoma in the Nb hooded rat.

Ten kidney tumors conforming to nephroblastoma were encountered during a 10-year period in a closed colony of Nb hooded rats. Histologic examination of the primary neoplasms, and of a limited number of transplanted derivatives, indicates that spontaneous nephroblastoma in the rat is exclusively an epithelial neoplasm, consisting of dense aggregates of undifferentiated, basophilic blast cells set in a connective tissue stroma of benign character. This uniform entity should be clearly distinguished from renal mesenchymal tumor, a connective tissue neoplasm of heterogeneous form which is induced frequently in rats by certain chemical carcinogens.