Figure 4. Rescue of mesoderm invagination in sna-defective embryos by mechanical pulses.

(a) Coordinated apical constrictions in Spider–GFP followed by mesoderm invagination in the WT, all of which are impaired in sna RNAi-injected embryos (n=6 on N=6 injected), are rescued (constrictions initiating at 96 s and mesoderm invagination at 456 s) after the initiation of magnetic stimulation of sna-dependent pulsations in sna RNAi-injected embryos (UML in red). Representative of n=8 of the N=13 embryos observed (full invagination (n=5), partial invagination (n=3), P<10⁻³, exact Fisher test, considering the N=12 snaRNAi-injected embryos of Figs 1e and 4a that do not invaginate). The shadow line is a vitelline membrane folding independent of the process. (b) Mechanical rescue and propagation along the mesoderm of apical stabilization of Myo-II and mesoderm folding in sna mutant embryos by local indent: sna-Myo-II-GFP embryo indented 3 min after the end of ventral cellularization, and 4 min before the movie initiation and rotated 90°. Representative of n=19 of the N=22 embryos observed (10 total invaginations, 9 partial invaginations, P<10⁻³, exact Fisher test considering that none of the sna mutants invaginate). Zoom on the mesoderm of the indented sna-Myo-II-GFP embryo. Representative of n=4 of the N=4 embryos observed in each case (P<10⁻³, exact Fisher test, with 3 total invaginations, 1 partial invagination). Observations are realized with a × 20 objective. Scale bar, 5 μm.