Role of proteases in implantation

Implantation of the embryo into the endometrium is a critical step in the establishment of pregnancy and the failure of embryos to implant is a major limiting factor in the success of reproductive technologies. Furthermore, one or more of the molecules of importance at implantation could provide a suitable target for post-coital contraception. While there is considerable species variation in the extent to which the trophoblast invades the maternal endometrium and makes contact with the maternal blood supply, many of the molecular mechanisms are conserved among species. Three families of protease are involved in the matrix degradation required for implantation: the cysteine, serine and matrix metalloproteinases. Other proteases are required for the activation of regulatory molecules. Although trophoblast from all species appears to have a high invasive potential, this is limited by the presence of partner protease inhibitors, the presence of which provides restraint to this invasion. It is the balance between the proteases and their inhibitors at any focal point that determines the site and extent of trophoblast invasion. This review examines the literature regarding proteases and their inhibitors at early implantation sites across a range of species with very different forms of placentation and evaluates their common features and their dissimilarities.