Identification of premalignant hyperplasia in methylcholanthrene-induced mammary tumorogenesis.

Mammary lobuloalveolar hyperplasia induced in rats by the administration of estradiol or progesterone did not evolve into advanced hyperplasia or cancer as was noted in animals that received these hormones and peroral instillations of methylcholanthrene. Although the early hyperplastic changes encountered in both groups were histologically indistinguishable, only cells from the lesions of the rats treated with methylcholanthrene and hormones exhibited significant numbers of aneuploid cells. This information indicates that, at least in the rat, not all instances of mammary hyperplasia possess the potential to develop into cancer. Chromosome analysis appears to represent a useful technique for the discrimination of these forms of hyperplasia. The significance of an appropriate inducer, i.e., methylcholanthrene, for the development of mammary carcinoma in this model system, as well as the well recognized proper hormonal milieu, is apparent.