A slender elongated projection from a cell. Filopodia extend typically from the leading edge of a motile cell, such as a fibroblast, or an elongating cell, such as the growing tip of a nerve fibre (see growth cone). Their finger-like shape is determined by bundles of actin microfilaments extending longitudinally from the leading edge of the filopodium to its base, where it joins the main body of the cell. Assembly or disassembly of these filament polymers causes extension or retraction of the leading edge. These processes are governed by chemical cues in the cell’s environment, such as signal molecules, which are detected by receptors in the plasma membrane of each filopodium. Hence, the cell can move forwards in a favourable direction or retreat from unfavourable surroundings. Compare lamellipodium.