Blebs are orientated by chemotactic gradients. (A) Orientation of blebs and F-actin–driven pseudopods in cells chemotaxing toward cyclic-AMP under 0.7% agarose. Blebs and pseudopods were identified by eye (144 blebs; 304 pseudopods). (B) Orientation of all projections with a maximum speed >1.5 µm/s, equating largely to blebs, as determined using modified QuimP10 software. Both blebs and pseudopods orientate with the chemotactic gradient, but the bimodal distribution of blebs is significantly different from the distribution of pseudopods, as shown by the following statistical tests: number of blebs formed from the front and rear halves of the cell is the same, binomial distribution test: P < 10−15; distribution of blebs and pseudopods is the same, circular Kuiper two-sample test: P < 0.001; unimodality of distribution, Hartigan’s dip test (Hartigan and Hartigan, 1985): P = 0.002 for blebs and P = 0.992 for pseudopods; preference of pseudopods to appear at the front of the cell compared with the blebs’ preference for the side, Fisher’s exact test: P < 10−15. Ax2 cells expressing the F-actin reporter ABD-GFP were used.
