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Dynamics of soft objects, cells and tissus

par Delphine Débarre - publié le , mis à jour le

A wide range of biological systems present a complex behaviour involving passive or active mechanical response, motility, growth dynamics or biological interactions (such as cancer cell transmigration during metastasis). To understand these behaviours, our team focuses on a variety of systems from single, passive shells (colloidal shells or macroscopic examples from the vegetal world) and actively migrating or adhering cells, up to the scale of biological tissues and cell aggregates34, 35 Whether in the case of single objects interacting with some interface (such as a substrate) or collective dynamics of multiple objects, their mechanics involves two distinct aspects : their internal rheology, which is often the result of a combination of bulk properties and a specific behaviour of a thin layer at their outer surface (such as an elastic membrane or the actomyosin cortex of eukaryotic cells), and the transmission of force to the environment, which, in addition to action–reaction interactions, is often dominated by specific interactions between interface-bound molecules (as in the case of cell adhesion).
Experimentally, these systems can be studied by observing their dynamics under controlled conditions (e.g. under drying for shells, spores and wood, or in presence of an adhesive substrate for living cells), or alternatively by force-displacement measurements (e.g. using acoustic forces, AFM or TFM). From a theoretical point of view, the issues are mostly related to the interplay between shape and forces, and numerical approaches have to deal with the tracking of interfaces whose displacement is governed by complex, often nonlinear rheologies, both in the bulk and along their surface.

Rheology of the cytoskeleton : theory and experiments

Simulated growth of an actin comet around a bead. Cell motility : cancer cell in a 3D colla- gen matrix. 2D cell migration showing actin fibers and forces obtained using TFM (beads are used to obtain the displacement field).

The role of adhesion

Traction forces exerted by a cell on a sub- strate. Right-hand side, forces obtained by traction force microscopy using the AM method52 left-hand side, prediction of the forces from a rheological model of the adhe- sion and cytoskeleton

Cell spreading and migration

Biofilm formation

TFM measurement of the stress field devel- oped by an E. coli microcolony during its
early growth.

Bacterial cells mechanically interact with their envi- ronment, during the processes of adhesion, microcolony formation and biofilm development. In particular, the mechanical properties of an underlying substrate influ- ence the behavior of bacterial cells in contact with this surface, a parameter that still remains poorly under- stood, but could be a key in controlling surface con- tamination. Physical and biological approaches will be combined, using fluorescent mutants to visualize in situ the activation of genes of interest :