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Spontaneous shear flow in confined cellular nematics

Abstract : In embryonic development or tumor evolution, cells often migrate collectively within confining tracks defined by their microenvironment 1,2. In some of these situations, the displacements within a cell strand are antiparallel 3, giving rise to shear flows. However, the mechanisms underlying these spontaneous flows remain poorly understood. Here, we show that an ensemble of spindle-shaped cells plated in a well-defined stripe spontaneously develop a shear flow whose characteristics depend on the width of the stripe. On wide stripes, the cells self-organize in a nematic phase with a director at a well-defined angle with the stripe's direction, and develop a shear flow close to the stripe's edges. However, on stripes narrower than a critical width, the cells perfectly align with the stripe's direction and the net flow vanishes. A hydrodynamic active gel theory provides an understanding of these observations and identifies the transition between the non-flowing phase oriented along the stripe and the tilted phase exhibiting shear flow as a Fréedericksz transition driven by the activity of the cells. This physical theory is grounded in the active nature of the cells and based on symmetries and conservation laws, providing a generic mechanism to interpret in vivo antiparallel cell displacements. Collective cell migration is classically associated with adhesive cell-cell contacts that can ensure large velocity correlation lengths 3-12. However, cells lacking stable cell-cell adhesions such as fibroblasts have been shown to collectively orient in nematic phases 13-16 and move in "streams" in dense monolayers. Interestingly, such streams have been reported in vivo in embryonic development 1 and cancer 2. They are often accompanied by bidirectional flows of cells within the same strand. In particular, cancer cells migrating collectively in vivo in effective "channels" formed between collagen fibers have been Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:
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https://hal.archives-ouvertes.fr/hal-02325014
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G. Duclos, C. Blanch-Mercader, V. yashunsky, G. Salbreux, J.-F. Joanny, et al.. Spontaneous shear flow in confined cellular nematics. Nature Physics, Nature Publishing Group, 2018, 14 (7), pp.728-732. ⟨10.1038/s41567-018-0099-7⟩. ⟨hal-02325014⟩

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