Uterine Selection of Human Embryos at

Human embryos frequently harbor large-scale complex chromosomal errors that impede normaldevelopment.Affectedembryosmayfailtoimplantalthoughmanyfirstbreachtheendometrialepitheliumand embed in the decidualizing stroma before being rejected via mechanisms that are poorly understood.Here we show that developmentally impaired human embryos elicit an endoplasmic stress response inhuman decidual cells. A stress response was also evident upon in vivoexposure of mouse uteri to culturemediumconditionedbylow-qualityhumanembryos.Bycontrast,signalsemanatingfromdevelopmentallycompetent embryos activated a focused gene network enriched in metabolic enzymes and implantationfactors. We further show that trypsin, a serine protease released by pre-implantation embryos, elicits Ca