Figure 1. Expansion dynamics of millimeter-size cell monolayers.

(A) Footprint throughout 46 hr growth period of representative small (left) and large (right) circular tissues, with the tissue outlines drawn at 4 h increments. Initial diameters were 1.7 mm and 3.4 mm. (B) Small circles exhibit faster relative area, $A(t)/A_0$, increase than large circles, where A₀ and $A(t)$ are the areas of tissues at the beginning of the experiment and at time t, respectively. Purple points show the relative area increase, $A(t+t_0)/A(t_0)$, of small tissues from the time $t_0=30$ h when they reached the size of the large circles. (C) Average tissue density $\rho (t)=N(t)/A(t)$ has non-monotonic evolution in small tissues but monotonically increases in large tissues, where $N(t)$ is the number of cells in a tissue at time t. (D) Edge radial velocity v_r is largely independent of initial tissue size and cell density. We grouped initial cell densities as ${\rho }_1=[2350,3050]$ cells/mm², ${\displaystyle {\rho }_2=[1650,2350]}$ cells/mm², and ${\displaystyle {\rho }_3=[1300,1650]}$ cells/mm². (E) Experimental data on tissue shape and model fits. Assuming a constant migration speed v_n in direction normal to the edge, we can predict the area expansion dynamics of elliptical tissues with different aspect ratios. The model fits our data for all tissues with ${\displaystyle v_n\approx 29.5}$ µm/hr, yielding normalized ${\chi }^2$ values of 0.79, 0.13, and 0.06 for aspect ratios of 8, 4, and 1 respectively (${\chi }^2$< 1 indicates a good fit; see Materials and methods). In B, data are from n = 16 tissues across five independent experiments (small and large circles). In C, n = 11 across four experiments for small circles, and n = 9 across three experiments for large circles. In D, n = 16 across five independent experiments for small and large circles, $\rho ={\rho }_1$; n = 13 across three experiments for small circles, $\rho ={\rho }_2$; and n = 11 across three experiments for small circles, $\rho ={\rho }_3$. In E, n = 4 across two experiments for a/b = 1 and a/b = 4, and n = 5 across two experiments for a/b = 8. Shaded regions correspond to standard deviations.

Figure 1—figure supplement 1. Relative proliferation in small and large tissues.

Relative proliferation $N(t)/N(0)$ for small and large tissues. Purple points show the relative proliferation, $N(t+t_0)/N(t_0)$, of small tissues from the time t₀ when they reached the starting size of the large circles. Error bars for purple points are smaller than marker size. Data are from 16 tissues across five independent experiments for small and large tissues. .

Figure 1—figure supplement 2. Normal edge velocity v_n of elliptical tissues at the major and minor axes.

(A) Elliptical tissues spread with different normal velocities along their major and minor axes. Data are from elliptical tissues with the same initial area than small circular tissues. (B) Normal expansion velocity is roughly independent of the local radius of curvature r_c of the tissue edge for large radii of curvature. For radii of curvature smaller than ∼1 mm, the normal velocity decreases with decreasing r_c. This plot includes data both from circular tissues and from the major and minor axes of elliptical tissues, excluding the first 16 hr of expansion to eliminate any affects from initial front acceleration. See Materials and methods for calculation of r_c. .

Figure 1—video 1. Movie of expansion of representative small and large tissues in phase-contrast.

Download video file ^{(10.1MB, mp4)}

Figure 1—video 2. Finger-like protrusions emerge in the first 20 hr of tissue expansion.

Download video file ^{(1MB, mp4)}

Figure 1—video 3. Expansion of sample elliptical cell monolayers with varying aspect ratios.

Download video file ^{(8.3MB, mp4)}