Actin dynamics in a Rac1(Q61L)-expressing PtK1 cell. Fast retrograde flow occurs in the lamellipodium and slow retrograde flow in the lamellum. The cell was microinjected with X-rhodamine-conjugated actin and observed by spinning disk confocal microscopy. Fluorescent speckle time-lapse series were acquired on a spinning disk confocal microscope with a 100/1.4 NA Plan Apo objective lens. Images were taken every 5 s and are played back at 15 frames/s. For still images, see Fig. 5B of associated manuscript (J Cell Biol, 161:845-851, 2003).