Estrogen signaling encourages blastocyst development and implantation potential

Purpose: Estrogen is well-known for preparinguterine receptivity. However, its roles in regulating embryo development and implantation are unclear. Our objective was to characterize estrogen receptor 1 (ESR1) in human and mouse embryos and determine the effect of estradiol (E₂) supplementation on pre- and peri-implantation blastocyst development. Methods: Mouse embryos, 8-cell through hatched blastocyst stages, and human embryonic days 5–7 blastocysts were stained for ESR1 and imaged using confocal microscopy. We then treated 8-cell mouse embryos with 8 nM E₂ during in vitro culture (IVC) and examined embryo morphokinetics, blastocyst development, and cell allocation into the inner cell mass (ICM) and trophectoderm (TE). Finally, we disrupted ESR1, using ICI 182,780, and evaluated peri-implantation development. Results: ESR1 exhibits nuclear localization in early blastocysts followed by aggregation, predominantly in the TE of hatching and hatched blastocysts, in human and mouse embryos. During IVC, most E₂ was absorbed by the mineral oil, and no effect on embryo development was found. When IVC was performed without an oil overlay, embryos treated with E₂ exhibited increased blastocyst development and ICM:TE ratio. Additionally, embryos treated with ICI 182,780 had significantly decreased trophoblast outgrowth during extended embryo culture. Conclusion: Similar ESR1 localization in mouse and human blastocysts suggests a conserved role in blastocyst development. These mechanisms may be underappreciated due to the use of mineral oil during conventional IVC. This work provides important context for how estrogenic toxicants may impact reproductive health and offers an avenue to further optimize human-assisted reproductive technology (ART) to treat infertility.