Effect of collagen I on polarization and actin reorganization. Collagen I induced polarization and actin reorganization in SDF-1–pretreated Lin^– 129Sv but not in WASP-deficient cells. Lin^– cells from 129Sv and WASP-deficient mice (panels A-B) were either pretreated (panels B,D) or not (panels A,C) by SDF-1 for 10 minutes, allowed to adhere to collagen I–coated coverslips for 1 hour at 37°C, and processed for actin staining by rhodamine phalloidin as described in “Materials and methods.” Scale bar equals 10 μm.