Cell biology and soft matter physics both share similar orders of magnitude with two important differences: biological systems are clearly out of equilibrium and molecular specificity can be strongly relevant. We are using the standard tools of soft matter physics to provide a quantitative description of cellular systems, and with this analysis of biological systems, we are raising an interesting number of new and challenging physical questions. In collaboration with biologists, we mainly concentrate our efforts on understanding some of the physical features of cell morphology and dynamics and of the mechanics and growth of tissues.
Cells contain a large number of components, but if we focus on mechanical properties, only a few classes of these components are relevant, e.g. the cytoskeletal network, molecular motors, phospholipid membranes and a large class of adhesion molecules such as integrins or cadherins. We therefore study each of these components, keeping in mind the importance of the non-equilibrium behavior. In some cases, this requires the introduction of new physical concepts such as “active” membranes, “active” gels or “isothermal ratchet”, a model used to describe molecular motors by the Brownian motion of a particle switching between two different states.
To optimise our physical understanding, we systematically carry out quantitative comparison between theoretical and experimental data obtained by varying controlled parameters. For instance, we are contributing to the theoretical description of the actin cortex and its role during cell division and cytokinesis. We are also trying to understand what fixes the volume of a cell. As an example we show on Fig.1 the results of calculations of the shape of a sand dollar egg during cytokinesis.
We have now reached a reasonable physical understanding of single components and are consequently extending our analysis to the collective behavior of many components. We can already describe certain aspects of cell behavior such as pospholipidic nanotube pulling by molecular motors, cell motility, cell division and mechano-transduction. A large part of our work is now devoted to the effect of mechanical stresses on the growth of tissues. We have for example performed numerical simulations of tissues competing for space (Fig. 2).
We also keep close contact with the evolution of statistical physics. In particular we have derived a general relation between the linear response of a system to any external perturbation and its fluctuations. We have tested this relation on the mechanical properties of the hair bundle of ear cells. .