Lamellipodia architecture: actin filament turnover and the lateral flow of actin filaments during motility.

Consideration of the arrangement of actin filaments in the lamellipodia of crawling cells indicates that, in addition to a rearward flow of the actin cytoskeleton due to treadmilling, there is a lateral flow of filaments in both directions. The existence of such a lateral flow of actin filaments is supported by observation of the lateral movement of actin filament bundles in fibroblasts and by fluorescence photoactivation data. In addition to explaining the formation, dispersal and lateral movement of filament bundles, lateral filament flow could add a further velocity component to the centripetal flow of cytoplasm. As such, lateral filament flow may explain the discrepancies in the rates of the rearward flow of actin, cytoplasmic materials and dorsal particles.