Figure 1

(a) During cell spreading, a thin lamellipodial sheet extends from the cell body onto the substrate. (b) Total internal reflection fluorescence micrograph of a spreading cell. The bright region corresponds to the area adhered to the substrate. (c) Two overlayed snapshots of the leading membrane edge of a lamellipodium moving from right to left are shown in differential interference contrast. The edge position is marked with a white contour overlay.Reuse & Permissions

Figure 2

Adhesion area in isotropically spreading fibroblasts grows with a power law in time. Different but constant exponents $a_i$ in the various phases of spreading are evident in a double logarithmic plot. Exponents have been determined by fitting a piecewise linear function to the data; see Fig. 3. Adhesion areas $A_i$ at the transition points are indicated.Reuse & Permissions

Figure 3

Histograms of area growth exponents as obtained from the slopes of double logarithmic plots of adhesion area versus time, such as the ones shown in Fig. 2. We have analyzed 20 cells in total. The middle (continuous spreading) phase exhibits clustering corresponding to small (open bars) and large (solid bars) area growth during that phase.Reuse & Permissions

Figure 4

Normal velocity map of the particular membrane segment marked in Fig. 1b as a function of time. Note the two qualitatively different sections before and after time $t=40\mathrm{s}$ corresponding to a continuous and a periodically contractile spreading phase, respectively. The period of the latter is $T=17\pm 4\mathrm{s}$. The speeds of lateral waves of maximum contraction velocity are indicated. The encircled region marks a phase shift of a contraction.Reuse & Permissions

Figure 5

Fourier spectrum [see Eq. (2)] of the velocity map in Fig. 4 for the two different spreading phases below [(a), $N=120$] and above [(b) $N=200$] frame number $n=120$. The spectrum is spatially averaged over 70 points between position 2.0 and $3.8\mu \mathrm{m}$ along the contour. The transition between continuous and contractile spreading is characterized by a sharp shift in the position of the maximum of the Fourier spectrum. The boundary maximum in (a) corresponds to a mean velocity of $7.4\mu \mathrm{m}/\mathrm{m}\mathrm{i}\mathrm{n}$ in the continuous spreading phase. The peak at $s_{\mathrm{m}\mathrm{a}\mathrm{x}}=5$ in (b) corresponds to a repeat time $T=17\pm 4\mathrm{s}$ for the contractile spreading phase; see Eq. (3). The mean velocity $0.3\mu \mathrm{m}/\mathrm{m}\mathrm{i}\mathrm{n}$ is small. (c) depicts the peak position of the spectrum taken in a running time window with a width of $N=50$ frames, corresponding to the repeat time $T$, as a function of the first frame number of the window. Note that the peak position depends on the width of the time window used for Fourier transformation, since frequencies are measured with respect to that width. In (c) we used a smaller window width than in (b), in order to capture the sharpness of the transition.Reuse & Permissions