Super-resolution imaging using structured illumination reveals that Sdk proteins form string-like structures at vertices. Clonal analysis showed that two cells, rather than three cells, contributing Sdk are sufficient for tAJ localisation. Here, we survey a wide range of Drosophila epithelia and establish that Sdk is a resident protein at tricellular AJs (tAJs), the first of its kind. In a previous study, we discovered that in Drosophila embryos, the adhesion molecule Sidekick (Sdk), well-known in invertebrates and vertebrates for its role in the visual system, localises at tricellular vertices at the level of AJs. In particular, resident proteins at tricellular vertices were identified only at occluding junctions, with none known at adherens junctions (AJs). Compared to bicellular contacts, however, much less is known. In epithelia, tricellular vertices are emerging as important sites for the regulation of epithelial integrity and function. epithelia | morphogenesis | vertex models | Drosophila These findings suggest that convergent extension is associated with a transition to more fluid-like tissue behavior, which may help accommodate tissue-shape changes during rapid developmental events. We show that changes in cell shape and alignment over time in the Drosophila germband predict the onset of rapid cell rearrangement in both wild-type and snail twist mutant embryos, where our theoretical prediction is further improved when we also account for cell packing disorder. From theoretical considerations and vertex model simulations, we predict that in anisotropic tissues, two experimentally accessible metrics of cell patterns-the cell shape index and a cell alignment index-are required to determine whether an aniso-tropic tissue is in a solid-like or fluid-like state. We show that, in contrast to isotropic tissues, cell shape alone is not sufficient to predict the onset of rapid cell rearrangement. We study this question in the converging and extending Drosophila germband epithelium, which displays planar-polarized myosin II and experiences anisotropic forces from neighboring tissues. However, the mechanisms that allow or prevent tissue reorganization, especially in the presence of strongly aniso-tropic forces, remain unclear. This includes epithelial tissues, which often narrow and elongate in convergent extension movements due to anisotropies in external forces or in internal cell-generated forces. Within developing embryos, tissues flow and reorganize dramatically on timescales as short as minutes. Our work suggests that molecular mechanisms that act as a brake on T1 transitions could stiffen global tissue mechanics and enhance rosette formation during morphogenesis. Moreover, we find that increasing the T1 delay time increases the percentage of higher-fold coordinated vertices and rosettes, and decreases the overall number of successful T1s, contributing to a more elastic-like - and less fluid-like - tissue response. We extend this analysis to tissues that become anisotropic under convergent extension, finding similar results. We show that the molecular-scale T1 delay timescale dominates over the cell shape-scale collective response timescale when the T1 delay time is the larger of the two. We study how variations in T1 delay time affect tissue mechanics, by quantifying the relaxation time of tissues in the presence of T1 delays and comparing that to the cell-shape based timescale that characterizes fluidity in the absence of any T1 delays. In this work, we incorporate this idea by augmenting vertex models to require a fixed, finite time for T1 transitions, which we call the ″T1 delay time″. Such processes could take a long time compared to other timescales in the tissue. But unlike foams, cells must execute a sequence of molecular processes, such as endocytosis of adhesion molecules, to complete a T1 transition. The simplest rearrangement in confluent cellular monolayers involves neighbor exchanges among four cells, called a T1 transition, in analogy to foams. Large-scale tissue deformation during biological processes such as morphogenesis requires cellular rearrangements.
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