Unique male mammary tumors developing in the inbred soft-furred field rats Millardia meltada.

Inbred lines of the soft-furred rat, Millardia meltada, were studied with special reference to spontaneous male mammary tumors. Adult males had the hyperplastic, pigmented inguinal mammary tissues and frequently developed bilateral mammary tumors. The tumors, no longer pigmented, were histologically well-differentiated adenocarcinomas associated with the myoepithelial cells and showed a variety of growth patterns depending on the stage of progression. They were transplantable to male, but not to female or castrated male nude mice. A significant number of androgen as well as estrogen binding sites were demonstrated in the nuclei of the transplanted tumor cells by in situ autoradiography. Neither virus particle nor mouse mammary tumor virus-specific antigen expression was detected in the tumor cells by electron microscopic and immunohistochemical studies. Normal females were generally free of pigment and tumor, although the androgen treatment induced marked pigmentation (deposit of nonfluorescent lipofuscin and hemosiderin) and hyperplasia of the female mammary epithelial cells, without a tumor. Possible relationships between pigmentation and tumor development were considered.