Blockade of actin polymerization leads to extrusion of preexisting F-actin from the cell body. (a) U937 cells pretreated with CytoD or LatB were adhered to VCAM-1 under static conditions, exposed to fluid flow (2 dynes/cm² for 2 min), and perfusion fixed with PFA. F-actin was visualized with 488 phalloidin (green), and VLA-4 was visualized with Cy3-44H6 (red). Asterisks indicate cell bodies. Arrowheads indicate elongated upstream structures with or without an associated cell body. (b and c) U937 cells transfected with Lifeact-GFP were pretreated with CytoD (b) or LatB (c), suspended in a Mn²⁺-containing buffer, and adhered to a VCAM-1–coated surface in a flow chamber. Fluid flow was introduced (2 dynes/cm²), and sequential images were acquired. The relative time of each image is indicated. The dashed lines provide a spatial reference to the most upstream portion of the cell. See Videos 1 and 2. (d) Addition of LatB into the perfusion buffer. Lifeact-GFP–transfected U937 cells were adhered under static conditions and exposed to fluid flow (2 dynes/cm²). After 2 min, LatB was added, and sequential images were acquired. Differential interference contrast (DIC) and GFP images are shown (the relative time is indicated). See Video 3. Asterisks indicate cell bodies whose position is altered relative to the previous image. Note that occasional cells detach (star). Representative data are from three IEs. Arrows indicate the direction of fluid flow. Bars, 10 µm.