Embryos expressing RhoGEF2-CRY2/CIBN::GFPpm and the membrane marker GAP43::mCherry were mounted with the ventral epithelium facing the objective. To analyze apical dynamics at each activation cycle, the photo-activation time was limited to ~ 20 s and was followed by the acquisition of a 5 µm-sized apical image stack resulting in a total time resolution of 35 s. The basal surface of the anterior (left) half of the embryo was photo-activated. (C, D) (C) Diagram showing the extent of ratchet-like constrictions. The mean difference in apical area between adjacent peaks (relaxed cell surface) is indicated by a red dot and reflects the extent to which cells shrink in a single pulse. Cells in the non-activated region (NR) constricted on average more than 5 µm2, which is characteristic for ratchet-like constriction. Non-constricting cells in the activated region (AR, non-con.) displayed a mean difference in apical area per pulse of about 0, reflecting non-ratchet constrictions meaning that pulsing cells constricted but relaxed subsequently back to their initial size. Constricting cells in the activated region (AR, con.) show an intermediate behavior with less efficient constrictions than in the NR. Data points are significantly different as determined by ANOVA (F(2,539)=72.04, p=1.9e-28) and post-hoc Tukey test. (D) The pulsation period (T). All cells pulse with a period of about 90 s. There were no statistically significant differences between group means as determined by one-way ANOVA (F(2,540)=1.438, p=0.24). (C and D) Red dots indicate the mean values and the error bars represent the standard deviation. More than 500 pulses (nNR = 119, nAR, non-con. = 128, nAR, con = 295) of more than 100 cells were analyzed. Data information: In all panels, **** indicates p ≤ 0.0001, and n.s. indicates no statistically significant differences according to two-sample t-Test. 