Controlled neighbor exchanges drive intermittency and cell streaming in epithelial tissues

Cell neighbor exchanges are integral to tissue rearrangements in biology, including development and repair. Often these processes occur via topological T1 transitions analogous to those in foams, grains and colloids. However, in contrast to non-living materials the T1 transitions in tissues are rate-limited and cannot occur instantaneously due to finite time required to remodel complex structure at cell-cell junctions. Here we introduce this important biological constraint in a vertex-based model as an intrinsic single-cell property and study how this rate-limiting process affects the mechanics and collective behavior of cells in a tissue. We report in the absence of this time constraint, the tissue undergoes a motility-driven glass transition characterized by a sharp increase in the intermittency of cell-cell rearrangements. Remarkably, this glass transition disappears as T1 transitions are temporally limited. As a unique consequence of limited rearrangements, we also find that the tissue develops dynamically heterogeneous pockets of fast and slow cells, in which the fast cells organize into long streams with leader-follower interactions, maintaining optimally stable cell-cell contacts. We compare our predictions with existing in-vivo experiments on Drosophila pupal development.