Active mechanics in a model biological tissue

Daniel Matoz Fernandez
School of Life Sciences, University of Dundee


Mechanical signalling plays a key role in most biological processes in nature. For example, simple cell layers of tightly connected cells grow, divide and move, leading to a dynamic reorganisation of the entire tissue at macroscopy scale. This process is regulated by a complex set of chemical and mechanical signalling pathways. How the regulation of cell–cell interactions is transmitted to the tissue-level organisation is still a topic of active research.

Homogeneous cell sheets behave as a supercooled fluid at long time scales and as a glass at short time scales, showing large spatial fluctuations of the inter-cellular forces. These fluctuations cannot be pinpointed to a specific cell but extend over regions spanning several cells. They strongly resemble the fluctuations observed in supercooled colloidal and molecular liquids approaching the glass transition with evidence of dynamical heterogeneity, a hallmark of glassy dynamics that has been extensively studied in soft condensed matter physics. In spite of the many interesting similarities to soft glasses, cell sheets perceived as active materials constitute a new class of non-equilibrium system in which the interplay between activity, long range elasticity and cell interactions give rise to novel phases with unusual structural, dynamical and mechanical properties.

In this talk, we will explore with simple models how collective dynamics in two-dimensional cell sheets can show a rich non-equilibrium behaviour depending on the activity of the system. Moreover, I will show how reorganisation in dense active matter affect the general mechanics, in particular how activity introduce a crossover from linear flow to a shear-thinning regime with an increasing shear rate. To rationalize this nonlinear flow, I will derive a theoretical mean-field scenario that accounts for the interplay of mechanical and active noise in local stresses. These noises are, respectively, generated by the elastic response of the cell matrix to cell rearrangements and by the internal activity. In addition, to these microscopic scale findings, I will explore the possibility of extending these concepts to the macroscopic scale, such as in bacterial biofilm development and its structure formation.