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 blastocy st into the endometrium is a process which is very similar to tumour i nvasion of a host tissue; and (ii) that the cytotrophoblastic cells of a first trimester pregnancy retain almost all the properties of the t rophectodermal cells of the blastocyst and can thus be used as surroga tes to study the implantation process irt vitro. Our model considers t hat the trophectodermal cells, once they reach the endometrial basemen t 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 c ontact with the endometrial extracellular matrix (ECM), Integrins (alp ha 5 beta 1) anchor the embryo into the ECM and induce its secretion o f collagenases. These enzymes digest the ECM, allowing the embryo to b urrow into the endometrium. This process is under the paracrine contro l of endometrial cytokines and ECM glycoproteins.