Figure 1.

(a) Experimental tracking of a T24 cell spreading on a fibronectin-coated substrate. At $t=0$, a transmission image is acquired, which allows determining A₀. For $t>0$, TIRF signal is acquired every 5 s, imaging the portion of the cell close to the substrate. White contours delineate the high fluorescence zone, excluding lower fluorescence peripheral zones (arrow heads). (b) TIRF evidence of transient protrusions of the lower fluorescence zones: TIRF images from $t=109$ s to $t=159$ s. The horizontal white line corresponds to the section shown on the kymograph (c). Inner contours delineate the high fluorescence zone as in a, outer contours delineate the lower fluorescence zone. (c) Kymograph of the lower fluorescence zones along a section of the cell. The central, high fluorescence zone is shown in black. The outer contour of the lower fluorescence zone is very dynamic and subject to sudden bursts of growth and retraction (arrow heads), visible on (a) at the relevant instant. The central, high fluorescence zone grows in a quasi-monotonic way, with a nearly constant growth rate until $t\approx 225$ s. (d) Dynamics of the high and lower fluorescence zones: movements of the barycenter of, respectively, the high fluorescence zone (black) and the lower fluorescence zone (gray) between two snapshots (5 s apart). The movements of the peripheral lower fluorescence zone have a threefold larger amplitude than the central high fluorescence zone. This corresponds to the existence of sudden bursts of growth and retraction of the peripheral zone that offset its barycenter.