Cortical Flow-Driven Shapes of Non-Adherent Cells

Cells display a tremendous variety of shapes, from spheroidal, to spread, or dendritic. Cell shape is dynamic and can undergo large changes, which have functional roles in cell polarization and migration, cell division, or early stages of embryonic development.While the role of biochemical cues in regulating cell shapes is clearly recognized, ultimately the shape of a cell is determined by balance of forces. For eukaryotic cells, to a large extent the mechanical properties are controlled by the cytoskeleton and in particular the actin/myosin system, a dynamic meshwork of semiflexible polymers and the source of active stresses in the cell.A striking consequence of these active stresses are cytoskeletal flows at the scale of the entire cell. In the case of adherent cells on planar substrates, the impact of actin flows on cell shapes has been well-studied experimentally and has been reproduced by several models. In this context, the theory behind the shapes of non-adherent cells has been left aside.Recently, non-adherent polarized cells have been observed to have a pear-like, elongated shape [1]. Here, we present a a minimal model that describes the cell cortex as a thin layer of contractile active gel, and we show that the anisotropy of cortical stresses, controlled by cortical viscosity and filament ordering, can account for this morphology. The predicted shapes can be determined from the flow pattern only, and prove to be independent of the mechanism at the origin of the cortical flow. In the case of flows resulting from a contractile instability, we propose a phase diagram of 3-dimensional cell shapes that encompasses non-polarized spherical, elongated, as well as oblate shapes, all of which have been observed in experiment.[1] V. Ruprecht et al., Cell 160: 673-685 (2015).