Fig. 3: Actomyosin relaxation at the PS triggers the disharmonizing of cell cycle and pre-mitotic rapid apical movements.

a Nucleus representation. b-b’ Z-projection of a transverse section from a E7 embryo, stained for Lamin B1 (green) (b), with zooms on nuclei from the anterior, posterior and PS regions (b’). Scale bars: 25 µm (b) and 10 µm. c-c” Violin plots representing nuclei aspect ratio at E6.25 (c), E6.5 (c’) and E7 (c”). Inner doted lines indicate third quartile, median, and first quartile. Normality was assessed using a Shapiro–Wilk test followed by a two-tailed Kruskall-Wallis test with Dunnet post hoc. E6.25: n = 4 embryos, E6.5: n = 4 embryos, E7: n = 4 embryos, with 20 nuclei per region. d Z-projection of transverse sections from E6.5 embryos cultured for 2 hours with vehicle or ROCK inhibitor at 40 µM, stained for nuclei (DAPI, cyan), basement membrane (collagen IV, yellow), mitosis (Phh3, magenta) and F-actin (phalloidin, grey). Arrowheads indicate non-apical mitoses. Scale bars: 25 µm. Split violin plots representing the mitotic index (e) and non-apical mitotic index (f), in embryos cultured with control vehicle or 40 µM ROCK inhibitor. Normality was assessed using a Shapiro–Wilk test followed by a two-tailed Mann-Whitney. Control: n = 26 slides from 12 embryos; 40 µM ROCK inhibitor: n = 14 slides from 7 embryos. g Violin plots representing nuclei aspect ratio in embryos cultured with vehicle or 40 µM ROCK inhibitor. Normality was assessed using a Shapiro–Wilk test followed by a two-tailed Mann-Whitney or unpaired t-test. Control: n = 11 embryos; 40 µM ROCK inhibitor: n = 5 embryos. h Scatterplot representing the percentage of non-apical mitosis in the region of the epiblast and PS in various simulations: control, relaxation of INM (aN/bN) springs, relaxation of all springs and relaxation of apical adhesion springs. For each condition, 100 simulations with 19 PS cells surrounded by 40 control cells. Error bars: SEM. i Table recapitulating the settings of epiblast and PS cells in control and test simulations. For the simulations, only the modified parameters are indicated. A-P Anterior - Posterior, Ap-Bas Apical – Basal, PS primitive streak, INM interkinetic nuclei migration, CC cell cycle, B detachment of basal adhesions from the basal line, aN apical-Nuclei spring, bN basal-Nuclei spring, aa apical adhesion spring.