Controlled neighbor exchanges drive glassy behavior, 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 observed in foams, grains, and colloids. However, in contrast to non-living materials, the T1 transitions in biological tissues are rate-limited and cannot occur instantaneously due to the finite time required to remodel complex structures at cell-cell junctions. We study how this rate-limiting process affects the mechanics and collective behavior of cells by introducing this biological constraint in a vertex-based model as an intrinsic single-cell property. We report in the absence of this time constraint conventional motility-driven glass transition is observed characterized by a sharp increase in the intermittency of cell rearrangements. Remarkably, this glass transition disappears as T1 transitions are temporally limited. A unique consequence of limited rearrangements is also that the tissue develops spatially correlated streams of fast and slow cells, in which the fast cells organize into stream-like patterns with effective leader-follower interactions and optimally stable cell-cell contacts. We also compare the predictions with existing in-vivo experiments in Drosophila pupal development.