Anisotropic apical constriction results from a-p epithelial tension. (A and B) Apical myosin II (green) and subapical F-actin used to visualize cell outlines (white) in fixed armRNAi embryos. (A) A tear similar to those observed in live embryos. Red arrows indicate cells that have constricted isotropically. (B) A later-staged embryo where myosin II has rounded up into rings. (C) Disruption of AJs results in loss of cell–cell adhesion. Time-lapse images of cell outlines (Spider-GFP) during a tear in an armRNAi embryo. Red cell–cell contacts are lost and different cells come into contact (blue). (D) Isotropic apical constriction occurs upon tearing. Time-lapse images of cell outlines (Spider-GFP) in an E-CadherinRNAi embryo. Blue dots indicate initially anisotropic cells that constrict isotropically upon loss of epithelial integrity. Apical area (E) and anisotropy (F) were quantified for the cells labeled in D. Dotted lines indicates the time of the tear. Bars: (A and B) 10 µm; (C) 5 µm; (D) 5 µm.