Filopodia are motile structures, being able to extend, retract and move laterally as they sense their environment. Lateral movement is particularly important in allowing the structure to sense stimuli prior to its adhesion with another cell or substrate.
The direction of cell movement in a migrating cell is primarily controlled by actin filament assembly at the leading edge. In the diagram above, increased actin polymerization at the side of the filopodium (blue arrow) pushes the membrane forward and to the left (panel #1). The forces produced by actin polymerization against the actin bundles of a filopodium can lead to lateral movement (white arrow, panels #2 & 3). Under the microscope, filopodia appear to cross-over one another as distinct units when they are separated within the three-dimensional framework of the cell (panels #4-6). When a laterally moving filopodium encounters another filopodium within the same three-dimensional space, the two filopodium can fuse to become one; this frequently increases the width and length of the resulting filopodium (panels #7 & 8).
Lateral movement may be a consequence of filopodial protrusions being tilted with respect to the retrograde flow of actin . This movement is inhibited once filopodia adhere to cells or substrates , however it is important to note that newly emerged filopodia seldom adhere to the substratum at their tips and are instead more likely to adhere at their bases .
Lateral movement and merging of filopodia may also stimulate further elongation of filopodia, suggesting that this may be an alternative method for promoting filopodial maturation and growth cone advancement on less adherent substrates .