Physiology of embryo-endometrial cross talk

Implantation of the blastocyst stage embryo into the maternal endometrium is a critical determinant and a rate-limiting process for successful pregnancy. Embryo implantation requires synchronized changes in the endometrium before and after arrival of blastocyst into the uterine cavity. Extensive cross talks occur between the fetal and maternal compartments around the time of implantation which are reflected by morphologic, biochemical and molecular changes in the endometrial cells and the differentiating trophoblast cells. The embryo induced morphologic changes include occurrence of epithelial plaque reaction, stromal compaction and decidualization. Embryonic signals also alter the expression of a large number of transcription factors, growth factors and their receptors and integrins. Thus the embryo superimposes a unique signature on the receptive endometrium for successful implantation. Functionally, the embryo-endometrial cross talk is essential for endowing a “selector activity” to the receptive endometrium to ensure implantation of only a developmentally competent embryo. On selection, the decidua creates a conducive microenvironment for trophoblast invasion leading to placentation. Clinical evidences suggest that along with receptivity, a defective “selector” activity of the receptive uterus may be a cause of infertility and recurrent miscarriages. Defects in trophoblast invasion are associated with pregnancy complications like preeclampsia and intra-uterine growth retardation. It is envisaged that understanding of the embryo-endometrial dialogue leading to the “selector” activity, aids in development of appropriate therapeutic modalities for infertility related disorders and miscarriages. Conversely, it might also benefit the development of anti-implantation drugs for contraception.