Integrins: expression, modulation, and signaling in fertilization, embryogenesis and implantation.

Normal morphogenesis and differentiation depend on the coordination of cell-cell and cell extracellular matrix (ECM) interactions. Integrins are a class of adhesion molecules that participate in the cell-cell and cell-substratum interactions and are present on essentially all human cells. All mammalian eggs express integrins at their surface, and the integrin alpha 6 beta 1 serves as a sperm receptor, mediating sperm-egg binding. In addition, certain integrin moieties appear to be regulated to within the cycling endometrium; the expression of beta 1 integrins in the early proliferative phase is restricted to the glandular epithelium, whereas stromal cells in the midsecretory phase also express beta 1 integrins. The expression of beta 1 integrins increases at the time of implantation and remains high in decidua during early pregnancy. A disruption of the integrin expression is associated with certain types of infertility in women. The apical surface of the mural trophectoderm does indeed possess functional integrins, and trophoblast interactions with ECM proteins depend largely on the integrin family of adhesion receptors. Thus, integrins play particularly important roles in fertilization and embryogenesis, including the process of implantation.