The cell biological basis of human implantation

Understanding the cellular basis of implantation and placental development depends on a combination of the limited morphological evidence in the human with data from other primates, separate studies of pre-implantation embryo s and endometrium and in vitro models. There is increasing evidence of a di alogue between embryo and endometrium that begins prior to implantation and evolves rapidly through the successive epithelial and stromal/vascular pha ses. This includes paracrine signals passing to endometrial tissue from the embryo, and vice versa. The production and timing of these signals by endo metrium, and its ability to respond to signals from the blastocyst, are dep endent on steroidal sensitization. A complex cascade of cell adhesion mecha nisms and local tissue remodelling are required for the establishment of a stable haemochorial interface.