Ultrastructure of epithelial plaque formation and stromal cells transformation by post- ovulatory chorionic gonadotropin treatment in the baboon (Papio anubis).

Background: To understand factors controlling endometrial responses to pregnancy, we have established a model using the baboon and examined the effects of infused human chorionic gonadotrophin (HCG) on the preparation of the luminal epithelium and stromal cell differentiation for the establishment of pregnancy. Methods: The ultrastructure of endometrium from normal day 10 post-ovulation animals, cycling females treated with either HCG or FSH (control), and a day 15 pregnant animal has been compared. Results: In the control endometrium, the luminal epithelium was smooth and regular, with underlying spindle shaped stromal cells. In pregnancy, the luminal epithelium underwent a plaque reaction, while stromal cells enlarged and developed filament-rich cell processes. Infusion of HCG produced changes similar to those seen in pregnancy, with generalized plaque formation and stromal decidualization, while in the animal treated with FSH there was no response. Conclusions: This study indicates that infusion of HCG into the uterus can duplicate many of the responses of the endometrium to pregnancy, although in this case the plaque reaction involved the whole of the luminal epithelium, rather than only the implantation site as in pregnancy.