Epithelial geometry reveals how positive tension feedback drives active cell intercalations during tissue convergence extension: Experiment

Morphogenesis often involves tissue-scale flows (plastic deformations) which require cell rearrangements (cell neighbor exchanges/intercalations). In a passive fluid, these rearrangements are driven by external forces. In contrast, in a living tissue, internally stresses generated by molecular motors can drive active intercalations. Using gastrulation of drosophila embryos as a model system, imaged, segmented, and tracked in toto, we show how tension inference can be used to distinguish between these two processes purely based on the local geometry of the cell array. Active intercalations are characterized by an increasing (relative) tension on the collapsing cell junction, while junction collapse in passive intercalations takes place under constant tension. Geometrically, junctional tensions are reflected in the angles between junctions at each vertex. This suggests a novel decomposition of tissue strain rate into a contribution reflecting changes in the junction angles reflecting changing tensions and an isogonal (angle preserving) contribution. This decomposition systematically links the tissue level dynamics to the cell/junction level dynamics. Moreover, the ensemble statistics of the tensions (i.e. angles) at each vertex allows us to robustly characterize the cell-scale mechanical state of the tissue. Our findings inform a minimal model based on positive tension feedback which reproduces the experimental observations and explains the coordination of subcellular stress generation underlying coherent tissue level flows.