Factors regulating interaction between trophoblast and human endometrium.

Implantation is a crucial step in human reproduction. Disturbances of this process are responsible for pregnancy failure after both in vivo and in vitro fertilization. The endometrium provides the implanting embryo with a unique substratum where the embryo communicates with biochemical signals, attaches itself, penetrates and grows without blood circulation. The highly proliferative phase of the cytotrophoblast, during early human embryogenesis, may be due to endogenous production of growth factors that may establish autocrine/short range paracrine stimulator loops which explain the tumor-like properties of these tissues. Endometrial BM penetration and stroma invasion may be due to the proteolytic capability of the human embryo. It is suggested that collagenase and the urokinase-like plasminogen activator are responsible for this activity. To clarify the molecular mechanisms involved in human embryo implantation several models are suggested: culture of blastocysts, culture of endometrial cells, and endometrial explant co-culture. Human blastocysts cultured with whole perfused human uteri make it possible to recognize some aspects of the entire implantation process and give us the possibility of improving the benefits provided by new technologies in reproductive medicine and reducing embryonic loss at an early stage.