Fig. 1

Rapid periderm expansion is facilitated by oriented cell divisions. The zebrafish embryonic epidermis comprises the outer periderm and inner basal stem cell layer (A). We imaged the expansion of the periderm covering the posterior tail bud from 28 hpf onward (A′). (Scale bar, 100 µm.) (B) Stills from timelapse movies of embryos expressing occludinb-GFP at 0 and 10 h postimaging (hpi), showing the expansion of the periderm. (Scale bar, 100 µm.) (C) Higher magnification stills from timelapse of the periderm and the corresponding cells tracked show that the cells maintain their neighbors during imaging. (Scale bar, 10 µm.) (D) Quantification of the increase in length of the embryo and the number of mitoses observed during that time shows a high correlation with R2 = 0.828, suggesting that embryo elongation is facilitated by cell divisions. n = 7 embryos. (E) Quantification of the angle of cell divisions, defined as the angle between the mitotic spindle and the anterior–posterior axis of the embryo (Inset). The histogram shows that cell divisions are aligned along the A–P axis of the embryo. 0- being along the A–P axis. n = 7 embryos. (F) Periderm tissue expressing occludinb-GFP at 28 hpf and corresponding computed average stress tensor shows higher stress along the A–P axis compared to the D–V axis. (Scale bar, 100 µm.) (G) Ratio of stress anisotropy along A–P vs D–V for multiple embryos between 28 and 30 hpf shows that this is significantly higher than 1. One-sample t test P = 0.0064, n = 6 embryos