Epithelial flow by controlled transformation of internal force-balance geometry

Shape changes of epithelia during animal development, such as convergent extension, are achieved through concerted mechanical activity of individual cells. While much is known about the corresponding large scale tissue flow and its genetic drivers, the question of cell-scale coordination remains open. We propose that this coordination can be understood in terms of mechanical interactions and instantaneous force balance within the tissue. Using whole embryo imaging data for Drosophila gastrulation, we exploit the relation between balance of local cortical tension forces and cell geometry. This unveils how local positive feedback on active tension and passive global deformations account for coordinated cell rearrangements. We develop a model that bridges the cell and tissue scale dynamics and predicts the dependence of total tissue extension on initial anisotropy and hexagonal order of the cell packing. Our study provides general insight into the encoding of global tissue shape in local cell-scale activity.