Embryo implantation: A time for recalling and forwarding

The success of embryo implantation is a critical step towards further embryo development and preg-nancy outcome. The observations and investigations on embryo implantation have been over a century. A huge body of knowledge has been accumulated in anatomy, histology, ultrastructure and hormonal regulation; as well as recently in depth information about molecular signaling pathways got from studies of genomic wide gene screening and specific gene deletion. The knowledge from basic re-search has also substantially helped to initiate and improve the Artificial Reproductive Technology (ART) in clinical applications. Now we’ve known that the normal embryo implantation involves the em-bryo’s development into an implantation-competent blastocyst and the synchronized transformation of uteri into a receptive stage. The interdependent relationship between the blastocyst and uterus in-volves complicated hormonal regulation and local paracrine, juxtacrine interactions. In this paper, we review some important historical findings regarding uterine receptivity and blastocyst activation, as well as some less discussed topics such as embryo spacing, embryo orientation. Further understandings on de-tailed mechanisms during the process of embryo implantation will help cure women infertility as well as de-velop new generation of non-steroids contraceptives.