Nodal signaling regulates Rac1 activity. (A–F) Visualization of Rac1 activity in embryos treated with DMSO (A–C) and SB-505124 (SB; D–F). A fluorescent Rac1 probe, RFP-PBD, was expressed in endodermal cells (A and D). Cells were colabeled with fluorescent dextran (B and E) as a volume marker. Rac1 activity was determined by generating ratiometric images between the RFP-PBD and dextran signals and was pseudocolored based on ratio value (C and F). Warmer colors indicate enrichment of PBD relative to dextran. Bars, 10 µm. (G) Quantification of the mean ratio of PBD to dextran indicates that Nodal inhibition reduces Rac1 activity. DMSO, n = 121 cells; SB-505124, n = 125 cells. All error bars represent SEM. *, P < 0.05. (H) Measurement of the size of cell regions where the PBD/dextran ratio is >1.0. Area of Rac1 activation is dramatically reduced upon Nodal inhibition.