Proliferating Active Nematic Collectively Aligns to Substrate

We study how nematic order develops in a system of elongated cells proliferating and migrating on a liquid crystal elastomer substrate. The anisotropic mechanical properties of the substrate act as an external field that guides the cells. The cells divide and grow into locally ordered domains that then interact with each other and the substrate. The system jams at high density, where both reorientations and proliferation stop. Using a continuum model of a proliferating active nematic, we examine the dependence of nematic order at jamming on the initial seeding density. Surprisingly, we find that smaller seeding densities lead to higher alignment, as seen in experiments [1]. For high seeding densities or fast growth, the system becomes jammed before it can fully align. Growth and alignment therefore compete. We then generalize the model to account for spatial gradients of cell density and orientation to study the spatiotemporal patterns that arise due to interaction with the asymmetric substrate. We see swirls due to cell-clustering at low-densities and bidirectional lanes of cells at high-densities aligned in the direction of the substrate. Our work has implications on the importance of birth-death processes and initial conditions on development of large-scale order in cell layers which are guided by external anisotropic fields (like hydra regeneration or ECM remodelling cells).

[1] Y. Luo et. al. Molecular-scale substrate anisotropy, crowding and division drive collective behaviours in cell monolayers J. R. Soc. Interface. 2023 20:20230160