Discontinuities within orientational fields configure topological defects [1]. They are often present in materials featuring orientational, nematic, order, and can be found in very diverse areas, which span a vast range of length scales [2]. Interestingly, in active nematic systems, topological defects imply not only strong orientational gradients but also well-defined flow and stress patterns in very localized regions [3]. Recent works have shown the influence of these defects in determining the dynamics of living systems, from cell monolayers [4–6] to simple organisms [7]. Here, we study how nematic monolayers composed of muscle cells self-organize and evolve under circular confinement, which enforces a topological charge S=+1. Under strong confinements, with size competing with the characteristic nematic correlation length, the formation of half-integer defects (s=±1/2) is hampered, resulting in spontaneous cellular arrangements with one single integer defect (s=+1) at the center of the islands, namely, rotating spirals and quasi-static asters [8]. These singularities imply distinct mechanical fields, which lead, eventually, to the localized expression of muscle-specific proteins and the formation of 3D nematic cellular protrusions [9].
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[4] G. Duclos, C. Erlenkamper, J.-F. Joanny, and P. Silberzan, Topological Defects in Confined Populations of Spindle-Shaped Cells, Nat Phys 13, 58 (2017).
[5] T. B. Saw, A. Doostmohammadi, V. Nier, L. Kocgozlu, S. Thampi, Y. Toyama, P. Marcq, C. T. Lim, J. M. Yeomans, and B. Ladoux, Topological Defects in Epithelia Govern Cell Death and Extrusion, Nature 544, 212 (2017).
[6] K. Kawaguchi, R. Kageyama, and M. Sano, Topological Defects Control Collective Dynamics in Neural Progenitor Cell Cultures, Nature 545, 327 (2017).
[7] Y. Maroudas-Sacks, L. Garion, L. Shani-Zerbib, A. Livshits, E. Braun, and K. Keren, Topological Defects in the Nematic Order of Actin Fibres as Organization Centres of Hydra Morphogenesis, Nat. Phys. (2020).
[8] C. Blanch-Mercader, P. Guillamat, A. Roux, and K. Kruse, Quantifying Material Properties of Cell Monolayers by Analyzing Integer Topological Defects, Phys. Rev. Lett. 126, 028101 (2021).
[9] P. Guillamat, C. Blanch-Mercader, K. Kruse, and A. Roux, Integer Topological Defects Organize Stresses Driving Tissue Morphogenesis, BioRxiv 2020.06.02.129262 (2020).