Endometrial Pinopodes: Relevance for Human Blastocyst Implantation

Abstract

Implantation failure after assisted reproductive technology (ART) is still a major problem. Even after blastocyst transfer, implantation rates do not exceed 50%, most likely due to suboptimal endometrial preparation for implantation. Psychoyos formulated the theory of an implantation or nidation window, a transient period during which the endometrium is receptive and implantation can initiate. A lot of effort has been invested into a clear definition of endometrial receptivity, using both morphological markers and molecular markers, but the ethical constraints for use of human material mean that only indirect evidence has been provided by investigations of the human endometrium during the midluteal phase, whereas direct information on human blastocyst-endometrial interactions in vivo are missing. This chapter will concentrate on morphological markers of endometrial receptivity, as observed by scanning electron microscopy (SEM), and their relevance for blastocyst-endometrial interactions in vitro.