C. MISBAH – 08 JANVIER

Microorganisms, such as bacteria, algae, or spermatozoa, are able to propel themselves forward thanks to flagella or cilia activity. By contrast, other organisms employ pronounced changes of the membrane shape to achieve propulsion, a prototypical example being the Eutreptiella gymnastica. Cells of the immune system as well as dictyostelium amoebas, traditionally believed to crawl on a substratum, can also swim in a similar way. In this talk we will first focus on amoeboid swimming and show the types of elementary strokes that produce maximal
displacement and minimal dissipation. We present the effect of confinement, and show that generically the cells either navigate in a channel or accumulate at the walls. We will then address the general question on mammalian cells. We will show that cells may move more adequately thanks to cortex retrograde flow than due to shape changes. We confront the results with experiments on T-lymphocyte. Solving numerically and analytically the actomyosin kinetics on sphere coupled to fluid flow, we show that cells can undergo a spontaneous symmetry breaking of the actomoysin distribution that lead to propulsion. Supercritical and subcritical bifurcations from a non-motile to a motile state are found, and symmetry-breaking of actomyosin results in polarity driving cortex flow. We determine the propulsion speed in a fluid and discuss orders of magnitude that match real experiments. The system also exhibits a Hopf bifurcation corresponding to polarity oscillations arresting motility.