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 embryos and endometrium and in vitro models. There is increasing evidence of a dialogue between embryo and endometrium that begins prior to implantation and evolves rapidly through the successive epithelial and stromal/vascular phases. This includes paracrine signals passing to endometrial tissue from the embryo, and vice versa. The production and timing of these signals by endometrium, and its ability to respond to signals from the blastocyst, are dependent on steroidal sensitization. A complex cascade of cell adhesion mechanisms and local tissue remodelling are required for the establishment of a stable haemochorial interface.
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