Cells migrate using the polymerization of actin filaments arranged in a network to push the membrane at their front edge, forming thin sheets of cytoplasm termed lamellipodia. In the textbook model, actin filaments in lamellipodia branch from their sides, forming dendritic-ljke arrays. Using the new technique of electron tomography we recently obtained the first 3D images of lamellipodia in cells vitreously frozen from the living state. Our findings reveal that actin filaments are linear and un-branched in lamellipodia and provide new details of the pushing machinery. The results show the importance of using appropriate methods of preparation and imaging in electron microscopy and provide a new basis for understanding the primary step in cell migration.

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