Sperm intrusion into the implantation-stage blastocyst and its potential biological significance

The human embryo derives from fusion of oocyte and sperm, undergoes growth and differentiation, resulting in a blastocyst. To initiate implantation, the blastocyst hatches from the zona pellucida, allowing access from external inputs. Modelling of uterine sperm distribution indicates that 200-5000 sperm cells may reach the implantation-stage blastocyst following natural coitus. We show ultrastructural evidence of sperm cells intruding into trophectoderm cells of zona-free blastocysts obtained from the uterus of rhesus monkeys. Interaction between additional sperm and zona-free blastocyst could be an evolutionary feature yielding adaptive processes influencing the developmental fate of embryos. This process bears potential implications in pregnancy success, sperm competition and human health. The early human embryo develops into a blastocyst that hatches from its surrounding cover, the zona pellucida, prior to implantation. We show sperm intruding into zona-free blastocysts of rhesus monkeys. Interaction between additional sperm and zona-free blastocyst could be an evolutionary feature bearing implications in pregnancy success, sperm competition and human health.