Sperm in the implantation-stage blastocyst: Is it a check point of embryo quality?

Abstract The early human embryo derived from fusion of an oocyte with a single sperm undergoes growth and differentiation and results in an implantation-ready blastocyst. To initiate implantation, the blastocyst hatches from the zona pellucida, thus making it accessible to external inputs. Our modelling of sperm distribution through the uterus indicates that 200–5000 sperms following natural coitus during mid-luteal phase are in a position of reaching the implantation-stage blastocyst in the maternal uterus. We indeed have ultrastructural evidence of sperm cells intruding into the trophectoderm cells of uterine zona-free blastocysts obtained from rhesus monkeys. The question arises whether the negotiation between additional sperm and azonal blastocyst is a feature of evolution yielding adaptation processes influencing the developmental fate of an individual embryo or a neutral by-product in placental mammals. This process potentially bears implications in pregnancy success, sperm competition, and human health.