(a) Ventral epithelium of a Drosophila embryo expressing the optogenetic components and Gap43-Ch but not exposed to activating light before, during, and after the onset of ventral furrow formation. Single planes (top), YZ projections (side), and maximum intensity projections (bottom) are shown. Scale bar is 10 µm. Filled arrowheads indicates long-range bending of these cells. Time zero indicates start of filming. (b) Anisotropy scatter plots: Each dot represents a single cell within the solid white box in (a) at the corresponding time. The magnitude of anisotropy is plotted on the y-axis; the orientation of anisotropy, relative to the anterior-posterior axis of the embryo, is plotted on the x-axis. Dotted lines are provided to facilitate comparisons between plots. Insets show percentage of cells in each quadrant. Cells in the upper left quadrant exhibit highly aligned, anisotropic apical constriction. (c) Quantification of apical cell area for cells within solid white box in (a). Gray columns represent unactivated cells. Red columns represent optogenetically activated cells discussed in Figure 4. (d) Standard deviation of the apical cell areas shown in (c). Gray and red dots represent standard deviation of apical cell areas for each non-activated (gray) or activated (red, see Figure 4) embryo at time point indicated along the X axis. Black crossbars indicate the average standard deviation for a given time point. (e) Quantification of the bending of cells neighboring the ventral midline (white dotted box in a). X axis is the percent apical area change relative to 00:00 min; Y axis is the change in position of the centroid of the apical cell surface along the dorsal-ventral axis relative to 00:00 min. Data in b-d are from 463 cells from three embryos. Data in e are from 203 cells from three embryos.