Unravelling the structure of the lamellipodium

Summary Pushing at the cell front is the business of lamellipodia and understanding how lamellipodia function requires knowledge of their structural organization. Analysis of extracted, critical-point-dried cells by electron microscopy has led to a current dogma that the lamellipodium pushes as a branched array of actin filaments, with a branching angle of 70 degrees , defined by the Arp2/3 complex. Comparison of different preparative methods indicates that the critical-point-drying-replica technique introduces distortions into actin networks, such that crossing filaments may appear branched. After negative staining and from preliminary studies by cryo-electron tomography, no clear evidence could be found for actin filament branching in lamellipodia. From recent observations of a sub-class of actin speckles in lamellipodia that exhibit a dynamic behaviour similar to speckles in the lamella region behind, it has been proposed that the lamellipodium surfs on top of the lamella. Negative stain electron microscopy and cryo-electron microscopy of fixed cells, which reveal the entire complement of filaments in lamellipodia show, however, that there is no separate, second array of filaments beneath the lamellipodium network. From present data, we conclude that the lamellipodium is a distinct protrusive entity composed of a network of primarily unbranched actin filaments. Cryo-electron tomography of snap-frozen intact cells will be required to finally clarify the three-dimensional arrangement of actin filaments in lamellipodia in vivo.