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Buckling along boundaries of cell-size contrast as a mechanism for early vertebrate morphogenesis. Vincent Fleury1, Nicolas R. Chevalier1, Fabien Furfaro1 and Jean-Loup Duband2 1 Laboratoire Matière et Systèmes Complexes, Université Paris Diderot/CNRS UMR 7057 10 rue Alice Domont et Léonie Duquet 75013 Paris, France 2 Laboratoire de Biologie du Développement, Université Pierre et Marie Curie/CNRS UMR 7622, 9 Quai Saint-Bernard 75252 Paris, France We have filmed in vivo the early stages of chicken development, from the blastula stage until completion of neurulation, with cell resolution (Mag. 4X to 20X). Cell-resolved time-lapse microscopy shows that embryogenesis occurs by deterministic folding along prepatterned lines which are the boundaries between regions of differentiated cells. As differentiation is accompanied by a change in cell size and shape, a contrast of elasticity appears between differentiated areas, with a discontinuity along the boundaries. At the blastula stage the embryo exhibits concentric rings of cells, corresponding to 4 differentiations : presumptive neural tissue, presumptive flank tissue, presumptive Zona Pellucida, and Presumptive Zona Opaca. During gastrulation, the mesoderm involutes and pulls on the ectoderm. Mesoderm migration and pull was filmed by turning the embryo upside down. The stretch of the embryo provokes a folding (“buckling”) of the surface, which follows exactly the boundaries between the differentiated territories.These boundaries are dynamically advected and form large elongated “figure 8” patterns which fix the global animal morphology. With micromechanical experiments we are able to show that there exists indeed a contrast of elasticity between cellular territories. We also show that buckling occurs exactly at these lines, and that this buckling can be induced by anticipation, simply by relaxing the intrinsic stretch in the chicken egg (by cutting the vitelline membrane). This means that the animal shape is actually latent, and “coded” in the lines of elastic contrast, and that this elastic contrast is associated to differences in cell sizes. Quite surprisingly, it is observed that the concentric areas of differentiated cells exhibit cell sizes in a regular step-wise pattern approx. 5-10-15-20 micrometers, from the central part of the embryo (blastoderm) up to the Zona Pellucida and Zona Opaca (which will contribute to the digestive tract). This suggests that there exists a memory of the early cleavages conserved in the blastula, which serves as template for the folding modes. A simple mechanical analog formed of a rubber foil with a central stiffer area is presented which reproduces physically the principle of fold-locking at boundaries of elastic contrasts observed in neurulation. A similar mode locking of folds at lines of elastic contrast might be observed in other instances. This work presents a physical mechanism for the formation of a coherent animal with physically segregated physiological domains, based on a reference configuration which is a succession of rings, which are a memory of early stages of differentiation.