Variable fates of filopodia. (A and B) Development of actin bundles in the lamella from lateral folding of filopodia into the base of the lamellipodium (transitions marked with arrows). Cells were transfected with mCherry-actin (red) and EGFP-fascin (green; see Video 3). Times are given in minutes and seconds. (C) Folding into, kinking (top arrow), and withdrawal (bottom arrow) of filopodia into lamella. The same cell as in B is shown. See Video 3. (D) Transition of radially oriented filopodia into stress fiber bundles. The filopodia marked 1–4 at time 5:40 remain essentially stationary as the cell front advances and finally appear as bundles in the lamella (30:00). Arrowheads mark equivalent positions on the filopodia through the sequence. At the position marked, the filopodia become kinked and separate from the lamellipodium/filopodium boundary, except filopodium 2, which continues extending and contributes a further bundle to the lamella. Filopodium 5 (5:40) fuses with two other filopodia; the resulting filopodium subsequently bends in two places and flows into the lamella (26:20; see Video 1). Bars, 5 μm.