Image shows (actin polymerization at) free barbed ends and F-actin in the lamellipodium of a migrating primary murine embryonic fibroblast. Cell has been mildly permeabilized using saponin during a 2min incubation in cytoskeleton buffer containing X-rhodamine labeled G-actin. Intracellular X-rhodamine actin polymerized into filaments at the very leading edge of the lamellipodium, resulting in the narrow red band along the tip of the lamelipodium. Lamellipodial F-actin was labeled with Alexa488-phalloidin using glutaraldehyde fixation subsequently to the X-rhodamine actin incubation. The micrograph visualizes that the actin monomer incorporation into the lamellipodial actin filament structure is restricted to the front of the lamellipodium. This enables a directed extension of the branched actin network towards the plasmamembrane, which pushes the membrane forward thereby creating cell protrusion. Wide field epifluorescence image taken on using a 100X 1.49 NA lens, Semrock DA/FI/TR/Cy5-4x4M-B Sedat filter set, and an Orca II with 500 ms exposure.