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 e vents 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 l ayer 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 t he uterus initiates implantation, and both soluble signals and association between molecules on apposed surfaces appear to be involved. A number of ce ll surface molecules have been implicated in the initial attachment between TE and LE. These include HSPG, Le-y and the H-type-l antigen, HB-EGF, trop hinin-tastinbystin complex, integrins, and extracellular matrix molecules s uch as osteopontin and laminin. Others, such as mucins, may need to be remo ved or modified to allow adhesion to proceed. Evidence for the role of thes e components is discussed.