Proliferative Responses of Mouse Mammary Glands to 17{j-Estradiol and Progesterone and Modification by Mouse Mammary Tumor Virus 1

ABSTRACT-The responses of mammary glands of ovariectomized mice to 17 {j-estradiol and progesterone were measured by mitotic and [3H]thymidine-labeled indices in low-tumor C57BL mice and high-tumor BR6 mice. The BR6 mice were subdivided into those carrying murine mammary tumor virus (MuMTV) and those that had been freed from it by foster nursing. In all groups, continuous stimulation by the two hormones administered together resulted in a cell proliferation peak. The magnitude of response varied according to strain and the presence or absence of MuMTV. The maximal effect was seen in BR6 mice carrying the virus. Comparison of percentage of labeled mitoses curves showed that the duration of DNA synthesis was extended in BR6 mice with MuMTV. although not sufficiently to account for the observed differences in labeling indices. The responses to 17 {j-estradiol and progesterone administered separately also varied. Mice carrying the virus were more susceptible to stimulation by progesterone, whereas those without th