Effects of Neonatally-administered 17β-estradiol on Induction of Mammary Carcinomas by 7, 12-Dimethylbenz[a]anthracene in Female Rats

Various doses of 17β-estradiol (E 2 ) were administered subcutaneously to inbred female Sprague-Dawley (SD) rats once at birth. At 50 days after birth, rats in all the groups were given 10 mg of 7, 12-dimethylbenz[a]anthracene (DMBA). In the 1000 μg group, the incidence and number of mammary carcinomas were markedly low, while in the 10 μg group, a large number of mammary carcinomas was noted. Corpora lutea were observed in all rats in the control, 0.1, 1, 10 and 100 μg groups at 50 days old; however, no corpora lutea were observed in any rat in the 1000 mg group at age 50 days and at sacrifice. Observation of the whole-mount specimens showed a low number of terminal end buds (TEBs) in the 1000 μg group and a high number in the 10 μg group. It is suggested that neonatal administration of E 2 affects the gonadotropin-secreting system, resulting in a decrease of progesterone, which is thought to influence the progression of mammary carcinomas induced by DMBA. Moreover, neonatal administration of E 2 directly affects the mammary glands, and it is suggested that E 2 may promote differentiation of TEBs resulting in inhibitory effects on the initiation of mammary carcinomas.