Molecular interactions at the maternal-embryonic interface during the early phase of implantation

In mammals the embryo must implant in the uterus and develop a placenta to gain nutrition and facilitate gas exchange. In this article, the earliest events in this process are reviewed. The embryo can implant only when it has reached the blastocyst stage. The blastocyst is composed of an inner clump of cells, the inner cell mass, that gives rise to the fetus and an outer layer of trophectoderm (TE), the precursor of the placenta. Both blastocyst and uterus must differentiate in parallel to reach the appropriate state of maturity (activated blastocyst and receptive uterus) at which implantation can occur. Interaction between TE and the luminal epithelium (LE) lining the uterus initiates implantation, and both soluble signals and association between molecules on apposed surfaces appear to be involved. A number of cell surface molecules have been implicated in the initial attachment between TE and LE. These include HSPG, Le-y and the H-type-1 antigen, HB-EGF, trophinin-tastin-bystin complex, integrins, and extracellular matrix molecules such as osteopontin and laminin. Others, such as mucins, may need to be removed or modified to allow adhesion to proceed. Evidence for the role of these components is discussed.