Actin dynamics within endodermal cells change from early to late gastrulation. (A and B) Actin dynamics were analyzed by tracking lamellipodia through accumulations in GFP-UTRN fluorescence. Representative lamellipodia are highlighted in red in B from the cells in A. Bars, 25 µm. (C) Lamellipodial lifetime increases during late gastrulation. Early (70% epiboly), n = 523 lamellipodia from 45 cells; late (90% epiboly), n = 665 lamellipodia from 77 cells. (D) Orientation of lamellipodia formation with respect to the embryonic axes. V, ventral; A, animal; Vg, vegetal; D, dorsal. Lamellipodia formation is biased toward the dorsal direction during late gastrulation (P = 0.00163 by χ2 test). Early (70% epiboly), n = 45 cells; late (90% epiboly), n = 77 cells from two independent experiments. (E–I) Analysis of retrograde flow. Kymographs in F and H were generated along the red lines shown in E and G, respectively. Time is plotted horizontally, and the direction of membrane protrusion is oriented toward the top of the images. Red lines in F′ and H′ highlight retrograde-moving actin structures, which form streaks in the kymographs. The slope of these streaks was used to calculate the rate of retrograde flow (I), which decreases in late gastrulation. Early (70% epiboly), n = 12 cells; late (90% epiboly), n = 15 cells. Bars: (E–H) 10 µm; (F′ and H′) 5 µm. All error bars represent SEM. *, P < 0.05.
