Cell Division and Active Motility Drive Hexatic Order in Biological Tissues

During many fundamental physiological events, biological tissues undergo a transition from a solid-like state to a liquid-like state. Recent experimental observations further suggest that in 2D epithelial tissues, these solid-fluid transitions can happen via intermediate states akin to the intermediate hexatic phases observed during two-dimensional melting. The hexatic phase is characterized by a quasi-long-range orientational order without any translational order. While it has been shown that hexatic order in tissue models can be induced by active motility and thermal fluctuations, the role of cell division and apoptosis has not been explored, despite its importance in driving physiological events. In this work, we study the effect of cell division and apoptosis on global hexatic order within the framework of the self-propelled Voronoi model. Although cell division naively destroys order, and active propulsion facilitates deformations, their competition drives the transition from liquid to hexatic to liquid, as the active speed increases. The hexatic phase is promoted by the balance of defect generation and healing.