Figure 3.

Analysis of dynamic cell states. They are either spread (SS, SO) or moving (MS, MO), and either steady (SS, MS) or oscillatory (SO, MO). (A) Upper panels show experiments, lower panels simulations without noise. Horizontal scale bars represent 30 min, vertical scale bars 50 μm. The cut corner of the experimental MO state showed another cell. Simulations used $c_3$ = 0.0088 μm s⁻¹, SS, MS $B$ = 22 ng cm⁻², SO $B$ = 36 ng cm⁻², MO $B$ = 80 ng cm⁻². (B) Fraction of cells in the cell states in experiment (2878 h of trajectories) and in simulations sampled from experiments on a range of fibronectin concentrations (see section S1.2 for details). (C and D) Cell states of the noise-free model illustrated by their value of the friction coefficient $\kappa$ in the stationary state (C) and the cell velocity $v$ (D) for fibronectin concentration $B$. Only the state SS exists at low $B$. At the concentration denoted as branch point, moving states appear and coexist with the spread state. Oscillations start (dashed lines) at the fibronectin concentrations marked as Hopf bifurcations. All moving states exist as moving up and down (D). (E) Simulation without noise of the time course of edge velocity $v$ (solid line), retrograde flow $v_r$ (dashed line), cell length $L$, friction coefficient $\kappa$, and force on the edge membrane $F$ in state MO (blue front, orange back). (F) Measured time course of edge velocity and cell length in state MO (blue front, orange back). (G) The steady states MS (upper panel) and SS (lower panel) are excitable. They behave steadily with noise-free dynamics. Noise is switched on at the time marked by the red line. It causes behavior very similar to noisy oscillations (Fig. S5). Parameters of all simulations are listed in set 1 in Table S2.