Spontaneous formation of chaotic protrusions in a polymerizing active gel layer

Abstract

The actin cortex is a thin layer of actin filaments and myosin motors beneath the outer membrane of animal cells. It determines the cells' mechanical properties and forms important morphological structures. Physical descriptions of the cortex as a contractile active gel suggest that these structures can result from dynamic instabilities. However, in these analyses the cortex is described as a twodimensional layer. Here, we show that the dynamics of the cortex is qualitatively different when gel fluxes in the direction perpendicular to the membrane are taken into account. In particular, an isotropic cortex is then stable for arbitrarily large active stresses. If lateral contractility exceeds vertical contractility, the system can either from protrusions with an apparently chaotic dynamics or a periodic static pattern of protrusions.

Pinching the cortex of live cells reveals thickness instabilities caused by myosin II motors

“…However, none of these studies accounts for our observation of active submicrometer fluctuations of cortical thickness caused by myosin II activity. We thus turn to an extension of the minimal description in ( 24 ) that accounts for stress anisotropies ( 28 ). Here, the cortex is treated as an active viscous gel, with a constant influx of material at the membrane (representing polymerization) and homogeneous disassembly.…”

“…The latter has two contributions: an effective hydrostatic pressure and the contractile stress generated by active processes in the gel. Both components depend on the gel density, and we write (27,28)…”

Phase separation on surfaces in presence of matter exchange

“…First, formally the assumption of a homogeneous bulk is not justified. Second, although the patterns may look qualitatively similar to patterns obtained in the full description that also accounts for the bulk, important features may be missed [22,24].…”

Pinching the cortex of live cells reveals thickness instabilities caused by Myosin II motors

“…But none of these studies accounts for our observation of active sub-micron fluctuations of cortical thickness caused by Myosin II activity. We thus turn to an extension of the minimal description in (24) that accounts for stress anisotropies 25 . Here, the cortex is treated as an active viscous gel, with a constant influx of material at the membrane (representing polymerization) and homogenous disassembly.…”