Embryo implantation: New insights into possible targets for endometrial contraception

Agents which prevent or disrupt implantation are potential alternatives to currently available contraceptive methods. Despite several leads there are still many aspects of the implantation process which could be amenable to contraceptive attack. In order to exploit implantation as a site of contraception it will be necessary to understand the details of the window of uterine receptivity in the human, as well as the cellular and molecular processes which lead to implantation of the blastocyst. Although assisted reproductive technologies involving transfer of blastocysts at different stages of the cycle have thrown some light on the extent of the window of receptivity, such information does not elucidate the mechanisms involved. Furthermore this and other events occurring at implantation cannot easily be studied in the human, primarily because of ethical considerations. Three new approaches are providing information that a number of molecules hitherto not suspected are involved in implantation. These approaches are: (i) molecular probes to study gene expression; (ii) targeted disruption of genes in mice, and (iii) analysis of gene expression at implantation sites using analysis of mRNA by differential display. It will be necessary to define in more detail the species and tissue specificity of action of these candidate molecules and avenues to disrupt their action in human implantation, if they are to become targets of contraception.