The Effect of In Vivo Estrogen-Treatment on Uterine Cell Proliferation in Ovariectomized Romanov Ewes

Embryonic death in sheep may occur in the pre-implantation period due to deficiencies in uterine function. Estrogens play an important role in preparing the uterus for implantation by enhancing histotroph and altering endometrial cell size and proliferation. We hypothesized that in vivo estradiol-17β (E2) treatment of ovariectomized ewes will result in enhanced uterine cell proliferation. Therefore, our objective was to determine the effects of E2 on cell proliferation in uterine compartments. After ovariectomy (at least 30 days), Romanov ewes (n = 15) received silastic implants containing 0 mg (controls; n = 7) or 100 mg of E2 (n = 8) at the axillary region. At tissue collection 24 h later, uterine cross-sections were fixed in formalin followed by immunohistochemical localization of Ki67 (a marker of proliferating cells; mouse monoclonal antibody from Vector Laboratories, Burlingame, CA). Tissue sections were then counterstained with fast red (Sigma, St. Louis, MO) to visualize cell nuclei. Images of luminal epithelium and endometrial stroma (5 areas each/tissue section) were analyzed to determine the labeling index (percentage of proliferating cells out of total cell number per selected area). Images of endometrial glands and myometrium were not analyzed because labeling index was very low. Ki67 was detected in cell nuclei in all uterine compartments including luminal epithelium, endometrial stroma and glands, and myometrium. Cell proliferation in the luminal epithelium was greater (P = 0.03) in E2-treated than control ewes (1.62 vs 0.26 ± 1.63%) and stromal cells tended to be greater (P = 0.08) in E2-treated than control ewes (0.41 vs 0.15 ± 0.1%). Thus, after 24 h of E2-treatment, cell proliferation in uterine luminal epithelium and stroma was enhanced compared to controls. These data indicate estrogens control uterine cell proliferation, and emphasize the importance of estrogens in regulation of uterine growth and function.