Contractile Film Model for Polymorphism in Adherent Cells

The optimal shapes attained by contractile cells on elastic substrates are determined by the crosstalk between intracellular forces and extracellular forces of adhesion. We model an adherent stationary cell as a contractile film bounded by an elastic cortex and connected to the substrate via elastic links. When the adhesion sites are continuously distributed, optimal cell shape is constrained by the adhesion geometry, with a spread area sensitively dependent on the substrate stiffness and contractile tension. For discrete adhesion sites, equilibrium cell shape is convex at weak contractility, while developing local concavities at intermediate values of contractility. Increasing contractility beyond a critical value, controlled by substrate stiffness, cell contour undergoes a discontinuous transition to a star-shaped configuration with cusps and protrusions, accompanied by a region of bistability and hysteresis.