Uterine Receptivity to Embryo Implantation

It is both flattering and humbling to be asked to address this topic, since quite a few people at this meeting could address it equally well or better. In fact, Koji wrote a review with almost exactly the same title 20 years ago (1). However, each of us would address the area somewhat differently. Over the years I have had the privilege of examining blastocysts and implantation sites of a large number of diverse species. It is consequently from the point of view of a comparative anatomist that I view uterine receptivity to embryo implantation. In considering uterine receptivity, we can divide the area into three sequential series of events: the initial preparation of the blastocyst for implantation, the receptivity to blastocyst adhesion and penetration of the uterine luminal epithelium, and accommodation and limitations to expansion of the implantation site.