Cell migration and actin dynamics during early gastrulation depend on Nodal and Rac1 signaling. (A and B) Representative migration tracks over a 1-h period from embryos treated with DMSO carrier (A) and 50 µM Nodal receptor inhibitor SB-505124 (SB; B). Dorsal is to the right. Bars, 25 µm. (C and D) Quantification of migration persistence and instantaneous velocity shows that Nodal inhibition leads to significantly increased migration persistence and reduced migration velocity. DMSO, n = 74 cells; SB-505124, n = 48 cells. (E) Nodal inhibition increases lamellipodial lifetime. DMSO, n = 191 lamellipodia from 28 cells; SB-505124, n = 324 lamellipodia from 46 cells. (F–J) Nodal inhibition slows retrograde flow. Kymographs in G and I were generated along the red lines shown in F and H, respectively. Time is plotted horizontally, and the direction of membrane protrusion is oriented toward the top of the images. Bars: (F and H) 10 µm; (G and I) 5 µm. The rate of the retrograde flow is quantified in J. DMSO, n = 9 cells; SB-505124, n = 5 cells. (K and L) Representative migration tracks over a 1-h period from control embryos (K) and embryos expressing DN Rac1 (L). Bars, 25 µm. (M and N) Quantification of migration persistence and instantaneous velocity from control (Ctrl) embryos and embryos expressing DN Rac1. Loss of Rac1 activity significantly increases migration persistence and moderately reduces migration velocity. Control, n = 76 cells; DN Rac1, n = 98 cells. All error bars represent SEM. *, P < 0.05.