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The potential roles for embryotrophic ligands in preimplantation embryo development.

Identification of the role(s) extracellular ligands play in regulating the development of the mammalian preimplantation embryo is a controversial area. Unequivocal evidence for their role is complicated by the apparent overlapping actions of multiple ligands. The discovery that the embryo also releases its own repertoire of ligands and expresses their corresponding receptors has further constrained analysis of their roles. Conventional ligand ablation strategies have limited utility when the cell responding to multiple ligands also produces them. The application of methods for identifying signal transduction events that occur in the early embryo in response to ligands has allowed direct assessment of the actions of these putative trophic ligands. A range of ligands induce phosphatidylinositol-3-kinase mediated survival signalling, and this is required for normal embryo development. Survival signalling maintains apoptotic pathways in a latent state within normal somatic cells, and they may fulfill the same role in the early embryo. Survival signals can also mitigate the adverse response of embryos to genotoxic and non-genotoxic stressors. Currently, there is no unequivocal evidence for a direct role of these ligands in the induction of mitosis in the early embryo. Embryotrophic ligands, acting via their specific receptors, to activate a network of effectors to create pro-survival, anti-apoptotic settings within the preimplantation embryo and these are required for normal embryo survival.