The molecular basis of embryo implantation in humans

The implantation of the human embryo is a double paradox, immunological and biological. The immunological paradox is that it consists of a heterologous graft in which the uterine immune system (via the cytokines) and the embryo's antigenicity (HLAG) collaborate to make possible both implantation and the maintenance of the pregnancy. The biological paradox arises because several different mechanisms must be successively implemented for these two epithelia to fuse and then for one to allow invasion by the other (that is, the for the endometrium to be decidualized by the trophoblast): preparation of the endometrium throughout the menstrual cycle under the influence of estrogens and then progesterone, with the involvement of growth factors (EGF, TGF and IGF), neoangiogenesis (estradiol, FGF and VEGF), recognition by the trophoblastic cells of the various components of the decidua and of the extracellular matrix (integrins and cadherin) and the progressive invasion of the decidua, to the depth of the spiral arteries (by the trophoblastic secretion of metalloproteases). A defective or excessive trophoblastic invasion can result in complications of pregnancy: early spontaneous miscarriage, preeclampsia and growth retardation of vascular origin in the case of defects, placenta accreta or percreta in the case of excess.