Early embryonic signals: embryo-maternal interactions before implantation

During the interval between fertilization and the process of implantation, some form of embryonic signalling is necessary for maternal recognition of pregnancy to occur and to cause the hormonal changes to elicit the uterine transformation necessary for implantation. Moreover, somatic cells provide specific molecules which are involved in embryonic transport and development. In this review only the following potential signal substances, for which recent progress has been reported, will be described and discussed. The earliest characterized embryo-maternal interaction is represented by the embryo-derived platelet activating factor/early pregnancy factor system. Prostaglandin E2, produced by horse embryos, is involved in their selective transportation to the uterus leaving unfertilized ova in the oviduct. The autocrine and paracrine role of growth factors of embryonic and/or maternal origin are discussed, together with the various oviduct specific proteins which become associated with developing embryos in several species. It is concluded that increasing sensitivity of available techniques is revealing new mechanisms of embryo-maternal interaction, which, although far from completely elucidating this complex matter, will bring new insights to the delicate period which precedes implantation.