Susceptibility of the mammary gland to carcinogenesis: I Differentiation of the mammary gland as determinant of tumor incidence and type of lesion.

The influence of age and mammary gland differentiation on the incidence of tumors induced by 7,12-dimethylbenz(a)anthracene (DMBA) was studied by correlating the development of the mammary glands of 20-180-day-old, virgin Sprague-Dawley rats with the number and type of tumors induced by DMBA administered at those various ages. The number of terminal end buds (TEBs), terminal ducts (TDs), and alveolar buds (ABs)/sq mm and their DNA-labeling indices (DNA-LI) were determined. Highest density of TEB occurred when the rats were 20 days old, decreasing thereafter. DNA-LI ranged between 25.2 and 29 in TEB of rats aged 30-55 days, which was coincident with the highest incidence of carcinomas. With aging, the number of TEBs and their DNA-LI decreased and the number of TDs and ABs increased, although with a low DNA-LI, which correlated with a lower incidence of carcinomas and higher incidence of benign lesions.