Increasing Levels of Estradiol Are Deleterious to Embryonic Implantation Because They Directly Affect the Embryo

Objective: To investigate whether the deleterious effect of E(2) on embryonic implantation is due to a direct effect on the endometrium, on the embryo, or both.

Design: Prospective, controlled in vitro study.

Setting: Tertiary infertility center.

Patient(s): Fertile patients in the luteal phase with histologically normal endometrium who were attending the infertility clinic as oocyte donors (n = 14).

Intervention(s): E(2) dose-response (0, 10(-8), 10(-7), 10(-6), 10(-5), and 10(-4) M) and time course (day 2 vs. day 5) experiments were performed in an in vitro embryo adhesion assay composed of human polarized endometrial epithelial cells obtained from fertile patients and mouse embryos.

Main Outcome Measure(s): Blastocyst formation rate and embryo adhesion rate.

Results: Monolayers of polarized endometrial epithelial cells expressed ERalpha at the mRNA level. The E(2) dose response of blastocysts with polarized endometrial epithelial cells (n = 235) demonstrated a progressive reduction in embryonic adhesion that was statistically significant at 10(-6) M. When polarized endometrial epithelial cells were treated alone with increasing doses of E(2) for 3 days and E(2) was then removed and blastocysts added (n = 410), embryonic adhesion was not significantly reduced, except at 10(-4) M. When 2-day mouse embryos (n = 609) were treated with increasing E(2) concentrations until day 5, the rate of blastocyst formation significantly decreased at a concentration >or= 10(-6) M, and embryonic adhesion decreased when blastocysts (n = 400) were obtained at a concentration >or= 10(-7) M. Time course experiments of embryos cultured for 2 days with polarized endometrial epithelial cells (n = 426) showed that the adhesion rate was higher at E(2) levels of 10(-7), 10(-6) and 10(-5) M compared with embryos cultured for 5 days (n = 495).

Conclusion(s): High E(2) levels are deleterious to embryo adhesion in vitro, mainly because they have a direct toxic effect on the embryo that may occur at the cleavage stage.