Pathology of metastasizing tumors in nitrosomethyl urea-induced rat mammary carcinoma.

The metastatic capacity of rat mammary tumors induced with N-nitrosomethyl urea was tested in BUF/N inbred female rats by successive transplantation. After the first and second passage, tumor cells appeared diffusely distributed throughout the bone marrow and spleen, confirming results reported by others; no other metastases were observed. After six successive transplantations, large, well-defined tumor nodules were observed in the liver, spleen, lung, and the peritoneal surface of the intestines in 40% of the injected animals. The morphology of the primary and metastatic tumors was compared by light microscopy. The tumors appeared to be adenocarcinomas with differing degrees of differentiation. No morphological differences could be observed between the primary and the metastatic tumors.