Fig. 3. Tension landscape in the expanding serosa.

a Tissue laser ablations in the dorsal serosa at different reference stages using a two-photon laser ablation set-up. Images show the serosal tissue before (top) and after (bottom) laser ablation in Stage 1 and Stage 3 embryos expressing LifeAct-eGFP and EFA-nGFP. Ablations were oriented perpendicular to the anterior–posterior axis of the embryo and yellow ellipses show the extent of the cut. The colored lines highlight the displacement of the severed cell edges. Time stamps are mm:ss. Scale bar is 50 µm. Stage 1 N = 1, Stage 3 N = 1. b Tissue laser ablations in the dorsal and ventral regions of the serosa using a UV laser ablation set-up. Images show the serosal tissue before (top) and after (bottom) laser ablation in Stage 3 embryos expressing LifeAct-eGFP. Ablations were oriented perpendicular to the anterior–posterior axis of the embryo. The position of the cuts in a and b are indicated with red boxes in the embryo illustrations below c, d yellow ellipses show the extent of the cut. Annotations are as in a. Time stamps are mm:ss. Scale bar is 50 µm. Dorsal Stage 3 N = 1, Ventral Stage 3 N = 1. c Graph showing the recoil velocities after laser ablations using a two-photon laser in Stage 1, 3, and 4 embryos. Each dot represents one cut in one embryo inflicted in the dorsal serosal region indicated by the red boxes in the reference illustrations below the graph. The number of embryos (N) sampled at different stages were as follows: Stage 1 N = 11, Stage 3 N = 6, Stage 4 N = 5. d Graph showing the recoil velocities after laser ablations using a UV laser of serosal cells in Stage 3 embryos. Each dot represents one cut in one embryo inflicted in the dorsal or ventral serosal region indicated by the red boxes in the reference illustrations below the graph. The number of embryos (N) sampled were as follows: Dorsal N = 15, Ventral N = 10. Distributions were compared using Welch’s unpaired t test. e Segmented cartographic projections as in Figs. 1g and 2g with serosal cells color coded according to their tissue fluidity values measured by subtracting the local solid-to-fluid transition shape index threshold (blue curve in f) from the cell shape index for each segmented cell (see “Methods” section “Shape index analysis”). Positive values indicate fluid-like and negative values solid-like properties. f Scatter plots of cell shape alignment index (x-axis) and shape index (y-axis) values of individual cells in the maps shown in e. The cells are color coded according to their distance from the center of the serosa window. The blue line indicates theoretically predicted threshold value of shape index signifying solid-to-fluid structural transition. Points below the line indicate solid-like cells and points above the line fluid-like cells (see “Methods” section “Shape index analysis”).