The Role of the Estrogen Receptor in Uterine Epithelial Proliferation and Cytodifferentiation in Neonatal Mice*

We have examined the relationship between localization of estrogen receptor (ER) and selected cell responses induced by estrogen in the neonatal CD-1 mouse uterus. The following simultaneous staining techniques were used to determine whether only uterine epithelial cells with ER are capable of showing proliferative and secretory activities after estrogen treatment: 1) ER localization and [3H]thymidine incorporation using immunohistochemistry and autoradiography; and 2) immunohistochemical double staining of ER and an estrogen-induced secretory protein lactoferrin (LF). Uterine tissues from two strains (CD-1 and BALB/c) mice on day 4 of age showed detectable ER by immunostaining in both epithelial and stromal cells. Day 4 CD-1 mice received a single injection of diethylstilbestrol (DES; 20 micrograms/kg BW). The percentage of ER-immunostained uterine epithelial cells and the intensity of the staining rapidly increased from 6-36 h, indicating that estrogen stimulates the expression and detectability of ER during the course of hormone treatment. Twenty-eight and 81% of epithelial cells showed positive ER immunostaining 6 and 24 h, respectively, after the DES treatment. The intensity of ER-immunostained stromal cells, however, was slightly decreased by the treatment. DES administration elicited epithelial cell proliferation. Labeling indices of epithelial cells reached a maximum (approximately 44%) 12 h after an injection of DES, and a second small peak was recognized 24 h after the treatment. Labeling indices of positively ER-immunostained epithelial cells and those of negatively stained epithelial cells showed almost the same pattern for 6-18 h after treatment. The respective maximum values were 45% and 43%. When day 4 mice were given three daily injections of DES in saline and killed 12 and 24 h after the last injection, significant amounts of LF were recognized in the apical cytoplasm of the epithelial cells. The epithelial cells that showed LF immunostaining always exhibited ER immunostaining in the nuclei. These results suggest that exogenous estrogen elicits increased expression of ER, DNA synthesis, and cell proliferation in neonatal uterine epithelial cells associated with low levels of ER, while there was a direct relationship with the presence of ER in epithelial cells and the induction of LF.