Active hexanematics: a paradigm for epithelial tissues

Tissue dynamics rules many biological processes such as embryogenesis, wound healing, or cancer metastasis. Even so, a framework that captures the order and symmetry of these phenomena has remained unclear. Active nematics has been typically used to model collective effects in cell monolayers, but recent research also shows that the six-fold anisotropy given by the polygonal shape of cells is relevant to understand the epithelial to mesenchymal transition, suggesting that hexatics plays a crucial role to describe the nature of these processes. Using cell-resolved numerical simulations and fluctuating hydrodynamics, we show that both nematic and hexatic order affect the collective behaviour of confluent epithelial layers, albeit at different length scales. While nematic order governs the system's dynamics at the large scale, thereby producing giant number-density fluctuations, hexatic order is relevant at smaller scales, where it suppresses density fluctuations and dictates the dynamics of topological defects. Motivated by these results, we propose a new paradigm for modelling epithelial layers based on the interplay between mechanical activity, nematic and hexatic order.