Embryo–maternal interactions at the implantation site: a delicate equilibrium

Blastocyst implantation and successful establishment of pregnancy require delicate interactions between the embryo and the maternal environment. During preimplantation, maternal/embryo communication is mediated by the trophectoderm. In the late luteal phase, physiological changes occur in the endometrium to allow blastocyst implantation. The "window of implantation" represents the period of maximum uterine receptivity for implantation. In response to signals from the embryo, pregnancy-specific proteins are released in maternal serum and a series of morphological, biochemical and immunological changes occur in the uterine environment. These systemic and local modifications can be considered to constitute "the maternal recognition of pregnancy". The human hemochorial placenta arises primarily through proliferation, migration and invasion of the endometrium and its vasculature by the embryonic trophoblast. The complex invasive processes accompanying implantation of the embryo are controlled at the embryo–maternal interface by factors from decidualized endometrium and the trophoblast itself. An inflammatory reaction and a proper maternal immune response allow survival and development of the feto–placental unit. In this review, we focus on interactions between trophoblast and uterine tissues and on cellular mechanisms and molecular signals involved in the closely regulated process of implantation.