Active-gel Theory for Multicellular Migration in Tissues

Active materials are driven out of equilibrium by a constant consumption of energy at the microscopic level, which is converted into forces and motion. These include biological objects at different scales, ranging from active molecular motors to groups of animals. The physical theory that I will present is motivated by multicellular migration in tissues, which plays a key role in development, wound healing, and metastasis. We propose to describe it as permeation of an active, polar solvent in a viscoelastic environment. We make use of a thermodynamic framework that, similarly to continuum theories of liquid crystals, describes macroscopic physical properties and flows, relying on conservation laws and symmetries. We formulate an active-gel theory that provides a simple description of the dynamic reciprocity between migrating cells and their environment in terms of distinct relative forces and alignment mechanisms. We make new predictions regarding multicellular migration modes based on these mechanisms. Namely, cellular alignment to elastic strains is expected to drive phase-separation in the cellular concentration and orientation for sufficiently extensile collections of cells. Our theory can serve as basis for the analysis of multicellular migration modes and cancer-cell invasion from tumors into connective tissues.