Fig. 4.
The onset of force generation corresponds to a 90° cell body contact angle. (A) Principle of traction force measurements with side view. A cell is pulling on two parallel microplates, one stiff, and the other flexible with calibrated stiffness k. The force is retrieved from the flexible plate deflection δ, F = kδ. (B, B′, and C) Correlation between cell shape and force generation. (B) Time evolution of the cell traction force F (black), the cell body contact radius Rb (blue solid circles), and the central radius Rc (blue open circles). (B′) Zoom on short times showing that the onset of traction (F > 0) correlates with Rb = Rc, i.e., a 90° contact angle (cylindrical cell body shape). (Scale bar: 10 μm.) (C) Time tF at the onset of force generation as a function of the time t90 at which the cell body reaches 90° for various conditions [fibronectin coating with infinite stiffness (black circles), k = 12.5 nN/μm (blue solid circles), and k = 1.5 nN/μm (blue open circles); polylysine coating and infinite stiffness (red open circles)]. The data are symmetrically distributed around the line tF = t90, indicating that the change in cell shape from convex to concave and the onset of traction force generation were statistically simultaneous, independently from the rigidity of the plates the cells were pulling on, or from integrin engagement.