Nuclei are arranged in a common apical plane at the start of tissue extension. (A) 3D reconstruction of cell membranes (pink) and nuclei (blue) from live imaging data. Slanted view highlighting nuclear surfaces (left), side view showing close apposition of nuclei in the packed epithelium at the start of tissue extension (mid), and top view showing how efficiently nuclei are packed in the available cytoplasm (right) (related to Video 1). (B) Measurement of nucleus length pre- (−5 min) and mid- (+20 min) GBE. (C) Comparison of nucleus width at the midplane at pre- (−5 min) and mid- (+20 min) GBE; n = 558 for pre- and 332 for mid-GBE. (D) Comparison of nucleus volume before (pre = −5 min) and after (mid = +20 min) the onset of GBE. (E) The ratio of nucleus area to cell area at the nuclear midplane compared before (pre = −5 min) and after (mid = +20 min) the onset of GBE. For B, D, and E, n = 388 nuclei. (F) Probability of absolute position of nuclear midplane along the apical–basal axis at 0 and 20 min of GBE; n = 82 nuclei for 0 min and 47 nuclei for 20 min from k = 1 embryo. (G) Area of nuclei at their midplane pre- (−5 min) and mid- (+20 min) GBE; n = 388 nuclei. (H) Median SD of cell neighbor number from 5 min before to 20 min after the onset of GBE (left) and comparing SD of cell neighbor number at two time points, 5 min prior (pre) to the onset of GBE and 20 min mid-GBE initiation (right); n = 1,131 cells. 0 min indicates the onset of GBE and GBE is indicated by blue background. For B–G (except F), the data were collected from k = 5 embryos. The measured n values are regardless of timepoints where not indicated. For B, D, E, and G, statistical significance was calculated using Student’s t test. For C and H, statistical significance was calculated using the Mann–Whitney U-test. ns = not significant, ****P < 0.0001.