Fig. 1: The dynamics of mechanical properties in dorso-ventral cell populations during gastrulation.

a Physical cross-section of a fixed embryo during gastrulation (stage 6: VFF). DV populations depicted: in blue: dorsal ectoderm/amnioserosa; magenta: neuroectoderm; yellow: mesoderm. VM: ventral midline. Asterisks (*) indicate 8 central mesodermal cells. b BS maps of the mesoderm at the end of cellularisation (left) and during VFF (centre-left), and EMT (centre-right and right columns). c Quantification of the BS within the mesoderm (see Methods). Friedman statistic = 10, p = 0.0008. d BS maps of the neuroectoderm at the end of cellularisation (left) and during VFF (center and right columns), when it displaces ventrally. e Quantification of the BS within the neuroectoderm (see Methods). Friedman statistic = 10, p = 0.0008. f BS maps of the dorsal ectoderm at the end of cellularisation (left) and during squamous morphogenesis (center and right columns). g Quantification of the BS within the dorsal ectoderm (see Methods). F = 12.05, p = 0.0198. h Comparison of BS dynamics between the two subpopulations of the ectoderm: neuroectoderm (lateral; panels d, e) and dorsal ectoderm (dorsal; panels f, g). Statistical test: test for combined linear regression model, resulting in a single line: Brillouin Shift (MHz) = −0.33 × t + 0.28; slope comparison: p = 0.084; intercept comparison: p = 0.795. h BS is Brillouin shift. BS imaging (in GHz in top panels in b, d, f) in embryos with fluorescently labelled membranes (CaaX-eGFP: inverted grayscale, middle panels in b, d, f) and nuclei (H2Av-mRFP: inverted grayscale, bottom panels in b, d, f). D: Dorsal; V: Ventral. Quantifications in panels c, e and g: top panels: mean and SEM of the BS (in MHz) in each embryo; bottom panels: mean and SEM of the BS (in MHz) from embryos shown in the top panel (N = 5). SEM is standard error of the mean. BS was normalised to the onset of VFF. Yellow area in CaaX-eGFP and white-dashed encircled areas in BS maps indicate quantified areas used in panels (c), (e) and (g). In panels c, e, g the statistical test was: two-tailed RM (Friedman) one-way ANOVA followed by multiple comparisons -FDR corrected-. See main text for the exact p-values of multiple comparisons and Methods for details on each analysis; * is p < 0.05, ** is p < 0.01. Scale bars are 50 μm. Source data are provided as a Source Data file.