Embryonic implantation and leukocyte transendothelial migration: different processes with similar players?

A clear parallelism between the different steps in human embryo-endometrial apposition/adhesion/invasion and leukocyte-endothelium rolling/adhesion/extravasation can be established. During human implantation and leukocyte trafficking, a first wave of soluble mediators regulates the expression and functional activity of adhesion molecules such as L-selectin and integrins, which mediate both processes. Apical surfaces of human endometrial epithelium and endothelium are key elements for the initiation of molecular interactions to capture the blastocyst or leukocyte, respectively. Subsequently, the blastocyst and the leukocyte migrate through the epithelium and endothelium toward their final destination, the endometrial stroma, to initiate placentation or the inflammatory foci as part of the immune response. Similarities between the intermediate molecular mechanisms of these two physiologically unrelated processes are discussed.