The loss of Rac1 activity leads to impaired uropod formation in neutrophils. WT, Rac1^−/−, and Rac2^−/− neutrophils were prestimulated with fMLP and seeded on anti-CD18–coated slides for 30 minutes. Some cells were seeded on noncoated slides for 30 minutes as negative control. The cells were then fixed and stained with rhodamine-phalloidin and mounted with reagent containing DAPI for nucleus. (A) Representative pictures of fluorescent images of F-actin (in red) for each genotype from 3 independent experiments. The nucleus is visualized in blue. The arrows point out the leading edge. The arrowhead points out the width of the tail. Note the presence of narrow lamellipodia and tail in WT cells, whereas Rac1^−/− cells display a large lamelipodia and tail. Scale bar = 5 μm. (B) Quantification of body contraction by measuring the width of the cell (see “Materials and methods”). (C) Spreading area. (D) Cell polarity. The cells were divided into 2 equal transverse sections, and fluorescent intensity of F-actin was measured in arbitrary units. The results are expressed as fold-increased level of fluorescence of one pole of the cells compared with the opposite pole. Results show mean ± SEM.