Title: Flow, Fluctuate and Freeze: Modeling Collective Cell Motion Using Soft Active Matter

Abstract: Active or living materials are made out of agents that can move on their own, such as birds, fish, or artificial systems such as robots or active colloids. Cells are a core active material, and the two-dimensional tissues known as epithelial cell sheets have a fundamental role in the developing embryo, and also in adult tissues including the gut and the cornea of the eye. Soft and active matter provides a theoretical and computational framework to understand the mechanics and dynamics of these tissues.
I will introduce the simplest useful class of models, active brownian particles (ABPs), which incorporate uncoordinated active crawling over a substrate and mechanical interactions. Using this model, I will show how the extended ‘swirly’ velocity fluctuations and the correlated motion of the edge seen in sheets on a substrate can be understood using a simple model that couples linear elasticity with disordered activity. We are able to quantitatively match experiments consisting of in-vitro epithelial cells.
Adding a different source of activity, cell division and apoptosis, to such a model leads to a novel ‘self-melting’ dense fluid state. Together, this model then allows us to explain what happens in a real biological system: The striking spiral flow pattern on the surface of the mouse cornea. It is in fact a steady-state flow pattern on a curved surface with a topological defect at the center where influx, division, extrusion and migration are finely balanced.