Proliferation of ductal outgrowths by carcinogen-induced rat mammary tumors in gland-free mammary fat pads.

The purpose of this study was to evaluate the gland-free mammary fat pad as a preferential site for transplantation of 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumors, since previous studies indicated their frequent failure to grow when transplanted in ectopic sites. Accordingly, samples from 14 primary mammary tumors (9 randomly selected and 5 ovarian-independent) were transplanted into the inguinal pairs of gland-free mammary fat pads of syngeneic female Lewis rats (9-12 wk of age). Although the percentage of takes was high (88%), the growth and morphogenetic capabilities of the transplants were strikingly different. Whereas transplants of ovarian-independent tumors consistently developed palpable carcinomas, transplants of all but 1 of the tumors selected at random (most of which were presumably ovarian-dependent) gave rise mainly to apparently normal ductal and hyperplastic outgrowths. The growth behavior of the transplants could not be correlated with the presence or absence in the tumors of mononuclear cells of the immune system. Unless the mammary fat pad selected against the growth of tumor cells, it seemed that the main alternative to tumor formation by the transplants was the proliferation of nontumorous outgrowths. These results draw attention to differences in the phenotypic stability of primary mammary tumors induced by DMBA and open up for further study the question of whether these differences reflect their cellular heterogeneity or their potential to undergo differentiation.