Background: Previous studies have demonstrated that high levels of estradiol (E2) impair blastocyst implantation through effects on the endometrium; however, whether high E2 directly affects blastocysts is not well established. The present study sought to clarify the direct impacts of high E2 levels on blastocysts in vitro.
Methods: ICR virgin albino mice were used. Using an in-vitro 8-day blastocyst culture model, immunofluorescence staining for the estrogen receptor (ER), blastocyst outgrowth assays, differential staining and TUNEL assays of blastocysts, and embryo transfer, we investigated the main outcomes of exposure to different E2 concentrations (10-7 to 10-4 M) in vitro and in vivo.
Results: ERα and ERβ expression were detected in pre-implantation stage embryos. In vitro exposure of blastocysts to 10-4 M E2 for 24 h followed by 7 days culture in the absence of E2 caused severe inhibition of implantation and post-implantation development. The late adverse effects of E2 on post-implantation development still occurred at concentrations of 10-7 to 10-5 M. In addition, blastocyst proliferation was reduced and apoptotic cells were increased following exposure to 10-4 M E2. Using an in vivo embryo-transfer model, we also showed that treatment with high E2 resulted in fewer implantation sites (38% vs. 72% in control) and greater resorption of implanted blastocysts (81% vs. 38% in control).
Conclusion: Exposure to high E2 concentrations in vitro is deleterious to blastocyst implantation and early post-implantation development, mainly owing to direct impacts of E2 on implanting blastocysts. In clinical assisted reproductive technique (ART), high serum E2 concentrations not only affects the endometrium, but also affects blastocysts directly at the period of implantation.
Keywords: Blastocyst; Embryo; Estradiol; Implantation.
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