RELATIONSHIP BETWEEN STAGES OF MAMMARY DEVELOPMENT AND SENSITIVITY TOGAMMA-RAY IRRADIATION IN MAMMARY TUMORIGENESIS IN RATS

Mature Wistar-MS rats were ovariectomized and treated with estradiol b enzoate and/or progesterone. Control animals were treated with olive o il. The rats were then exposed to gamma-rays and implanted with a pell et of diethylstilbestrol. The incidence of mammary tumors in rats trea ted with estradiol benzoate or with progesterone was significantly hig her than in rats in the non-treated control group, whereas, in vats tr eated with both estradiol benzoate and progesterone, the incidence was not significantly different from that in the controls. Histological e xamination of the mammary tumors showed 2 types of neoplasm: adenocarc inoma and fibroadenoma. Interestingly, over half of all the tumors in the rats treated with estradiol benzoate were adenocarcinomas, while f ibroadenomas were mainly induced in the rats treated with progesterone or with both estradiol benzoate and progesterone. The expression of e strogen and progesterone receptors in the tumor tissues showed some di fferences according to whether the groups were treated with estradiol benzoate or with progesterone. Morphologically, mammary glands at irra diation showed well-developed lobuloalveoli in both the estradiol-benz oate-treated rats and in those rats treated with both estradiol benzoa te and progesterone. This was consistent with the higher incorporation of [H-3]thymidine into the DNA in the mammary glands of rats in both of these groups. Our findings suggest that a more advanced development al stage of the mammary glands, dependent upon ovarian hormones, is re lated to a higher incidence of mammary tumors induced by irradiation. (C) 1995 Wiley-Liss, Inc.