Successive processes of symmetry breaking are occurring during the sequence of early embryonic morphogenesis. These fascinating events raise fundamental questions that are still under debate and are challenging to investigate. In parallel, recent advances in fluorescent reporters, microscopy and image processing, make possible investigating the dynamics of large cell populations in a developing embryo. It led to the emergence of the in toto imaging concept and provides a unique opportunity to study embryonic development from the scale of the cellular mechanisms to the entire organism. In this context, the purpose of this proposal is to bring novel approaches and techniques of in toto imaging to dissect fundamental processes involved in anterior-posterior symmetry breaking and axis specification in early mammalian embryos. The combination, characterization and development of cutting-edge microscopy, optical manipulation and image processing techniques will enable us to investigate the collective movement of the visceral endoderm in early mouse embryo. This process is the first morphological event that breaks the anterior-posterior symmetry. This study should provide a detailed understanding of the mechanisms involved in the anterior-posterior polarity in mouse embryo. It should also illustrate the crucial role of individual cell behaviors in the emergence of large scale morphogenetic events. This interdisciplinary project will be conducted at Institut Jacques Monod (IJM, Paris). Willy Supatto, the principal investigator, will use his training and experience in optics, biophysics and developmental biology to start a new research activity within Jérôme Collignon’s team at IJM.