A Proliferating Nematic that Collecitvely Senses an Anisotropic Substrate

Motivated by recent experiments on growing fibroblasts, we have examined the development of nematic order in a colony of elongated cells proliferating on a nematic elastomer substrate. The elongated fibroblasts tend to align in the direction of the substrate but with two caveats - they only do so collectively, and they do it better when they start out fewer. Using a continuum model of a proliferating nematic liquid crystal, we show that jamming-induced arrest of reorientation and proliferation explains the dependence of order at jamming on the initial seeding density. We also identify a novel alignment mechanism that captures the collective nature of cellular reorientation. Through a grid-free coarse-graining of experimental data, we identify a strong correlation between cellular orientation and density gradients, demonstrating that cells preferentially align tangentially along the contours of high-density domains, i.e., perpendicular to the density gradient. Using this insight, we used a continuum model to show that cell proliferation in concert with anisotropic friction generates anisotropic flows that stretch high density domains along the direction of substrate order, resulting in collective alignment. Notably, this mechanism does not require flow-alignment of the nematic or deviatoric active stresses. Instead, our results show that activity enters through proliferation-induced flows, which may play an important role in the emergence of orientational order on the tissue scale.