Blastocyst-endometrial interaction: an appraisal of some old and new ideas.

The nature of the early interaction between blastocyst and endometrial epithelium is known to be highly specific within individual species. Despite this unique initial interaction, the trophoblast promptly establishes a common theme as invasive cells penetrate the endometrium and colonize its local blood vessels. In all animals with this type of implantation the blastocyst plays a more active role than the endometrial epithelium. The aggressive behaviour of the blastocyst may be induced by signals from the endometrium which has been primed with preimplantation ovarian steroids. Activation of the blastocyst may reflect triggering of synchronized paracrine loops, activated during the implantation window. Endometrial cytokines and eicosanoids are probably the primary signals that drive the interlinking paracrine loops and are essential for decidualization, trophoblast growth and invasion. Since the dominant feature of early implantation is rapid trophoblast migration, particularly in primates, the degree to which the blastocyst attaches to the apical surface of the luminal endometrial epithelium during implantation is uncertain. The thickness of the glycocalyx on the uterine luminal epithelium during the peri-implantation period varies considerably between species. Its role in blastocyst attachment, if any, and in trophoblast cell locomotion, requires further study. The molecular properties and functions of the uterine epithelial plasma membrane, and those of the interacting trophoblast at the site of implantation, have been largely neglected and require further extensive study.