Relaxation kinetics controls cell transport in crowded active monolayers

Research indicates that the speed at which soft cells in crowded, active layers relax plays a crucial role in their transport. Prior simulations suggested this movement was due to their deformability. However, a dense layer exhibits the interplay of two factors: the energy cost of a deformation and the relaxation rate. A new study dissects these elements in an active phase-field model of a near-confluent monolayer, where the relaxation rate and free energy can be altered independently.

Crowding does not physically push cells back, but rather slows them through the relaxation of their deformations against neighboring cells. This process negates most of their propulsion. By slowing this relaxation in a subset of cells, while maintaining the same energy expenditure and propulsion, the transport of these cells is more than doubled.

Meanwhile, the remaining cells see their transport speed increase by half. Conversely, soft cells lose their advantage, and at high concentrations, their transport even reverses. This effect disappears when their relaxation rate matches that of normal cells. Regardless of composition, the overall mix behaves like a uniform layer, with transport influenced by the geometric mean of interfacial tension.

The study concludes that passive relaxation kinetics is a significant variable in transport, implying that the fluidity of heterogeneous tissues may depend on the rate at which cells regain their shape, rather than solely on their stiffness.