Brief Communication: Frequency of Mammary Cell Division in Relation to Age: Its Significance in the Induction of Mammary Tumors by Carcinogen in Rats2

Abstract

³H-thymidine incorporation into the DNA of mammary glands in virgin rats over a 2-hour period increased from 30 days to 50 days of age; it peaked at 50 days and declined abruptly after 70 days, in good correlation with well-known changes in the incidence of mammary tumors induced by polycyclic aromatic hydrocarbons. The degree of mammary development and the serum prolactin level (a potent stimulator of mammary tumor growth) were significantly lower at 30 days than after 50 days of age, but there were few differences in these measures between 50 and 110 days. Fifty-day-old rats given grafts of 3 pituitaries at 30 days of age, in whom the incidence of carcinogen-induced mammary tumors was inhibited, showed significantly lower ³H-thymidine incorporation than did normal rats of the same age, despite similar serum prolactin levels and greater mammary development. These results strongly suggested that mammary tumor induction by a carcinogen depends principally on the frequency of mammary cell division at the time a carcinogen acts on the gland.