Understanding cell contact constriction in epithelial morphogenesis through data driven reverse-time inference

Tissue elongation via convergent extension mediated by cell intercalation is a frequent mechanism in the development of metazoans. During Drosophila germband extension cell intercalation is achieved by acto-myosin driven junction shrinkage of neighboring cells, its resolution into a vertex of four neighboring cells, and subsequent junction formation in orthogonal direction. Despite tremendous progress in the understanding of the molecular underpinnings involved in this process called a T1 transition, a quantitatively tested mechanism that predicts the dynamics of individual cell-junctions is still not available. We here show how our mathematically novel approach of reverse time ensemble inference allows to infer the process of junction shrinkage in reverse time, starting from the endpoint of the dynamic, i.e. the 4-vertex. We apply our inference scheme to ~1000 T1 transitions and systematically rule out model classes of increasing complexity using all accessible ensemble statistics. We find visco-elastic like dynamics, infer the onset of each individual junction collapse and link the inferred junctional dynamics to the simultaneously recorded myosin dynamics.