Sbf function is required for apical constriction. (A) YZ cross sections (schematic, top) and XY still images (bottom) of cell outlines revealed with Spider:GFP during ventral furrow formation. (B) Still images of embryos expressing Spider:GFP in Sbf shRNA background during ventral furrow formation. (C) Quantification of cell area change over time in control, Sbf shRNA, and Rab35 shRNA backgrounds. (D) Quantification of the absolute rate of area change in control, Sbf shRNA, and Rab35 shRNA embryos. (C and D) n = 3 (control), 4 (Sbf shRNA), and 3 (Rab35 shRNA) embryos. (E–H) Absolute rate of cell area change in control and Sbf shRNA embryos during 0–2 min (E), 2–4 min (F), 4–6 min (G), and 6–8 min (H). (E–H) n = 152 (control), 196 (Sbf shRNA), and 216 (Rab35 shRNA) cells. (I–K) Examples of control (I), Sbf shRNA (J), and Rab35 shRNA (K) cell centroid trajectories movement traced over 8 min during ventral furrow formation. Time is color-coded (see color bar at the bottom). Scale bars represent 5 μm. Error bars indicate standard errors. Statistical significance was calculated using a Student’s t test. ***, P < 0.0005. In A and B, embryos are oriented with anterior up and posterior down.