Tumors of the rat kidney induced by nitrosourea compounds.

Apart from other lesions, 51 renal neoplasms were induced in rats by administration of methylethy-, dimethyl- and phenylmethylnitrosourea. 3 tumors originated from the renal pelvis and were classified as transitional cell papilloma (1) or transitional cell carcinoma (2). 48 neoplasms were considered as "mesenchymal tumors". These growths were regarded as rat kidney specific tumors. They display a wide spectrum of histological structures: spindle cells which are capable to differentiate into smooth muscle cells, fibroblasts, angioblasts, fat cells, and even into cartilage thus forming the typical microscopical features which are completed by the tendency of mesenchymal cells to swirl around preexisting renal tubules in several closely investing layers. As the cell of origin is not yet established the term "mesenchymal tumor" seems to be the most suitable one. There is no basis for classifying these tumor as nephroblastomas and the two entities should be strictly separated.