Integrin-mediated adhesion and signaling during blastocyst implantation

Blastocyst implantation in rodents and primates depends on adhesive interactions between trophoblast cells and the endometrial extracellular matrix. As the blastocyst contacts the uterine basal lamina, cells of the trophectoderm become adhesion-competent and conclude their phenotypic conversion from a polarized epithelium to invasive trophoblast cells that anchor the embryo in the uterine wall and eventually infiltrate the endometrium. Trophoblast cells become capable of adhesion to fibronectin as alpha(5)beta(1) integrins traffick to their apical surfaces. While integrin trafficking may be required for trophoblast adhesion to the endometrium, accumulating evidence indicates that, in response to contact with the extracellular matrix, additional molecules must be recruited to the apical surface before strong adhesion is attained. Based on the known interactions of integrins with cytoplasmic regulatory proteins, we propose that extracellular matrix-bound integrins mediate intracellular signaling cascades that strengthen their adhesive activity. We will review some of the known integrin signaling pathways that could regulate trophoblast adhesion and differentiation.