Rat mammary tumors from carcinogen-induced nodules and their responsiveness to ovariectomy

The research objective was to determine whether DMBA-induced rat mammary tumors derived from transplanted mammary nodules differ in ovarian-responsiveness from tumors originating randomly from in situ mammary parenchyma. Nodule-derived tumors were from hyperplastic alveolar outgrowths in mammary fat pads, which were either first generation outgrowths (primary nodule outgrowths) or generations 3-6 of a tumorigenic nodule line (nodule outgrowth T18). Tumors were induced in primary outgrowths by secondary exposure to DMBA in vivo or in vitro. The nodule line was not re-exposed to DMBA because of its high oncogenicity (80-85%). Despite the histological similarity of the tumor derivatives, primary nodule outgrowths and the nodule outgrowth line developed a higher proportion of ovarian-independent to ovarian-dependent tumors (1:3 and 3:4 respectively) than did in situ mammary tissues (1:9). These results are consistent with the observation that nodule outgrowths were themselves ovarian-independent.