Cytokines and implantation.

There is strong evidence that cytokines and growth factors play an important role as local mediators of the actions of steroids on the endometrium to prepare it for implantation. These factors have also been shown to act in both an autocrine and paracrine manner to regulate the development of preimplantation embryos in several species. Attempts to define the function of each cytokine have involved receptor localization, establishment of the mode of control by steroid hormones, and functional assays in vivo and in vitro. However, because of the complex and redundant nature of cytokine networks, defining which of this plethora of factors plays a critical role in implantation has proved difficult. Although the development of preimplantation embryos can be influenced directly by cytokines, the in vitro culture of embryos from several species in defined media indicates that exogenous cytokines are not essential for development to the blastocyst stage. Nonetheless, supplementation of media with growth factors may prove valuable in overcoming the detrimental effects of embryo culture in vitro, which is widely used in assisted reproduction techniques in humans and domestic species. The creation of mouse strains in which specific genes for growth factors or adhesion molecules are deleted has also proved important in defining factors essential in implantation, as well as those that play a less significant role. Mice unable to express leukaemia inhibitory factor in the endometrium fail to support implantation, indicating a critical role for this protein in producing a receptive endometrium. Conversely, mouse embryos of the CF-1 strain, which lack the receptor for epidermal growth factor, fail to attach, indicating that this receptor is necessary for producing an implantation competent embryo. It is likely that abnormal expression of such receptors or their ligands in the endometrium underlies some forms of human infertility. Examining the actions of these factors in the endometrium will allow dissection of the molecular basis of embryo attachment and implantation.