Polarized apical constriction possibly reflects tissue mechanics. (A) Images of ventral furrow cell outlines in a live embryo using Spider-GFP. Bar, 10 µm. (B) Schematic showing the method used to calculate the aspect ratio (a/b) and anisotropy (x/y) of a cell. (C) Mean cross-sectional area (red) and anisotropy (blue) over time for a single embryo. Error bars indicate SD (n = 42 cells). The broken line marks an anisotropy of 1 (isotropic). (D) Box and whisker plot of aspect ratios from Spider-GFP, Resille-GFP, and bicoid nanos torso-like (bnt) Resille-GFP embryos. Boxes extend from to 25th to 75th percentile, with a line at the median. Whiskers extend to the most extreme values. Each sample represents three embryos, with 123, 102, and 158 cells. (E and F) Apical area and anisotropy (E) or constriction rate and the time derivative of anisotropy (F) for an individual cell over time. Anisotropy increases both during (i) and between (ii) contraction pulses. The broken line marks a rate of 0.