Abstract

The shape of animal cells is primarily determined by the cellular cortex, a thin network of actin filaments and myosin motors that lies directly underneath the plasma membrane. The cortex enables the cell to resist external stresses, and generates the forces that allow cells to move and deform. We investigate how the mechanical properties of the cortex arise from the microscopic architecture of the network, and how controlled changes in these properties drive cell deformation. I will mostly focus on cortex mechanics during cytokinesis, a process directly driven by changes in cortex organisation. At anaphase onset, the cortex accumulates into an equatorial ring that drives furrow ingression. Although most studies of cytokinetic mechanics focus on force generation at the constriction ring, a contractile actomyosin cortex remains at the poles of dividing cells throughout cytokinesis. Using a combination of experiments and theory, we showed that the polar cortex makes cytokinesis an inherently unstable process, where any imbalance in contractile forces between the two poles can compromise the accurate positioning of the constriction ring. A theoretical model based on a competition between cortex turnover and contraction dynamics accurately accounts for the oscillations. Taken together, our findings reveal an inherent instability in the shape of the dividing cell, indicating that polar cortex contractility must be tightly controlled to ensure successful cytokinesis. When this control fails, the cortex displays oscillatory instabilities. Similar contractile instabilities might be involved in asymmetric division and in epithelial morphogenesis.