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

Blastocyst implantation and successful establishment of pregnancy require d elicate interactions between the embryo and the maternal environment. Durin g preimplantation, maternal/embryo communication is mediated by the trophec toderm, In the late luteal phase, physiological changes occur in the endome trium to allow blastocyst implantation. The "window of implantation" repres ents the period of maximum uterine receptivity for implantation. In respons e to signals from the embryo, pregnancy-specific proteins are released in m aternal serum and a series of morphological, biochemical and immunological changes occur in the uterine environment. These systemic and local modifica tions can be considered to constitute "the maternal recognition of pregnanc y". The human hemochorial placenta arises primarily through proliferation, migration and invasion of the endometrium and its vasculature by the embryo nic trophoblast. The complex invasive processes accompanying implantation o f the embryo are controlled at the embryo-maternal interface by factors fro m decidualized endometrium and the trophoblast itself. An inflammatory reac tion and a proper maternal immune response allow survival and development o f the fete-placental unit. In this review, we focus on interactions between trophoblast and uterine tissues and on cellular mechanisms and molecular s ignals involved in the closely regulated process of implantation. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.