A model for implantation of the human blastocyst and early placentation

We describe here a molecular model of blastocyst implantation which is based on two assumptions: (i) that implantation of the human blastocyst into the endometrium is a process which is very similar to tumour invasion of a host tissue; and (ii) that the cytotrophoblastic cells of a first trimester pregnancy retain almost all the properties of the trophectodermal cells of the blastocyst and can thus be used as surrogates to study the implantation process in vitro. Our model considers that the trophectodermal cells, once they reach the endometrial basement membrane, express integrins (alpha 6 beta 4) which anchor them into the basement membrane and induce their secretion of gelatinases. These proteases digest the basement membrane, allowing the embryo to make contact with the endometrial extracellular matrix (ECM). Integrins (alpha 5 beta 1) anchor the embryo into the ECM and induce its secretion of collagenases. These enzymes digest the ECM, allowing the embryo to burrow into the endometrium. This process is under the paracrine control of endometrial cytokines and ECM glycoproteins.