Nature, Classification, and Nomenclature of Kidney Tumors Induced in the Rat by Dimethylnitrosamine2

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Abstract
Dimethylnitrosamine-induced kidney tumors of the rat occur as simple epithelial growths, sarcoma-like productions, or complexes of epithelial and sarcomatoid tissues. It is still undecided in the literature if they belong to a single histogenetic group, or if they originate from two or several cell lines. In our opinion, there are, on one hand, purely epithelial tumors seemingly derived from hyperplastic nephrons and, on the other hand, sarcomatoid tumors, which can be separated into three subgroups. Neoplasms of the first subgroup grow rapidly, occasionally invading neighboring perirenal structures. Their basic tissue is composed of large cells, resembles early renal blastema, and can differentiate into immature smooth or striated muscle or mesenchyme. It mimics early kidney organogenesis by clumping around tubules resembling collecting ducts, but fails to differentiate into primitive nephric tubules. Adult nephric components often present in it give no evidence of in situ genesis. Tumors of the second subgroup reach only moderate size and are chiefly characterized by centrally located, maturing periductal cell clumps that eventually transform into smooth muscle or undergo fibrosis. Dysplastic epithelial islands occasionally found in them seem to originate from preexisting tubules. Tumors of the third subgroup attain only minor size and are essentially composed of an outer zone of small spindle cells and a fibrosing center. All subgroups seem to proceed from the same line of interstitial cells. The generic name “stromal nephroma” is proposed for the whole group, with appropriate labeling of the subgroups.

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