Testing the lamella hypothesis: the next steps on the agenda.

Introduction In the accompanying Speckle Microscopy Update (Vallotton and Small, 2009), the authors present a critique of the ‘lamella hypothesis’ – the proposal that the lamella extends to the leading edge of motile cells and overlaps with the lamellipodium (Ponti et al., 2004). In the 2004 paper, fluorescent speckle microscopy (FSM) combined with druginduced perturbations of actin-filament organization were used to substantiate the lamella hypothesis. Vallotton and Small now question the interpretation of this FSM data. They suggest that the lamella and lamellipodium should be considered as spatially distinct. As one of the corresponding authors of the 2004 article, I have been asked to provide a response to the critique by Vallotton and Small, and this is included below. References to figures point to the data presented in Speckle Microscopy Update I (Vallotton and Small, 2009), unless stated otherwise.