A mechano-chemical mechanism for symmetry breaking and patterning of the early chick embryo

The formation of the primitive streak in the chick embryo sets the body's bilateral symmetry and results from large-scale self-organized tissue flows driven by cell intercalation and ingressions. The onset of this directed motion requires a mechanism that orients the cell intercalations tangential to the embryonic-extraembryonic interface, which is still unknown. We discover a differential cell division rate between the embryonic and extraembryonic tissue, which generates mechanical anisotropy and, in turn, specifies the orientation of the intercalation. Furthermore, we show that a self-organizing Turing-patterning system in the extraembryonic tissue can induce a robust sickle-shaped mesendoderm precursor domain in the epiblast at the embryonic-extraembryonic interface. Coupling of the mechanism of mesendoderm formation, division-dependent mechanical anisotropy, and our mechano-chemical model for self-organized cell-flows allows us to reproduce and explain the development of the chick embryo from egg laying to the formation of the primitive streak as well as its resilience to experimental perturbations.