An elastic shell model for animal cells

The mechanical properties of most animal cells are dominated by the actin cortex, a ~ 0.5 µm thick layer of actin filaments including a multitude of associated proteins, which is attached to the inner side of the cell membrane and encapsulates the cytoplasm. Cells can actively change their shape and volume, but osmotic pressure prohibits any substantial volume change in response to external forces. We have mechanically indented suspended spherical with a dual optical tweezers set-up to measure response. For small, slow indentations we find a linear elastic response. To relate this response to cell material properties we use finite-element modeling to model the cell as a weakly pressurized elastic shell. We can match experimental results for up to 5 % indentation by assuming a 0.5 µm thick cortex with Young’s modulus of 6 KPa. For realistic parameters, volume changes are indeed entirely negligible and deformation is accompanied by an increase in surface area. Assuming a simple Hertz model, in contrast, would result in a cell modulus of 12 Pa. This finding might explain the widely different values of cell stiffness reported in the literature.