Mitotic Activity in Hypophysis of Pregnant Rat after Injections of Estrogen

Abstract

Division of cells by mitosis increases in the hypophysis of young mature rats in the late estrous phase of the sexual cycle. 1 That this is due to the increase in the level of estrogen is shown by the fact that mitoses can be increased by injections of estrogenic hormones. 2 Thus, 25 μg of estradiol benzoate (Progyaon-B)∗ injected into 3-month-old ovariectomized rats 72 and 48 hours before death will result in an average mitotic activity of 22.09 mitoses per square mm of section. During pregnancy, however, even though there is a continued production of estrogenic hormone, mitoses are rare in the hypophysis after the 3rd day. 3 Just what level of estrogen occurs during pregnancy is not known, and it seems possible that either the level is not sufficiently high to cause an increased mitotic activity or that there is some substance that inhibits the mitosis-stimulating effect of the estrogen that is produced.

In order to test this point, a group of 22 pregnant animals approximately 3 months of age were injected with 25 μg of estradiol benzoate, 72 and 48 hours before death. In addition to these, 3 others that did not receive injections were killed on the 13th day of pregnancy to serve as controls. Of the pregnant animals that received injections of estradiol benzoate, 9 were killed on the 6th, 7th or 8th day of pregnancy and 13 on the 12th, 13th, 14th or 15th day. Of those killed early in pregnancy, 3 animals showed evidence that implantation had occurred but in 2 of these abortion had apparently occurred. Those killed later in pregnancy all had living fetuses.