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Fluorescent speckle microscopy (FSM) is a technology used to analyze the dynamics of macromolecular assemblies in vivo and in vitro. Speckle formation by random association of fluorophores with a macromolecular structure was originally discovered for microtubules. Since then FSM has been expanded to study other cytoskeleton and cytoskeleton-binding proteins. Specialized software has been developed to convert the stochastic speckle image signal into spatiotemporal maps of polymer transport and turnover in living cells. These maps serve as a unique quantitative readout of the dynamic steady state of the cytoskeleton and its responses to molecular and genetic interventions, allowing a systematic study of the mechanisms of cytoskeleton regulation and its effect on cell function. Here, we explain the principles of FSM imaging and signal analysis, outline the biological questions and corresponding methodological advances that have led to the current state of FSM, and give a glimpse of new FSM modalities under development.
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Movie M1: Fluorescent Speckle Microscopy (FSM) time-lapse series of a newt lung epithelial cell migrating at the border of an epithelial monolayer. The cell was microinjected with a small amount of X-rhodamine labeled actin. Frame rate of raw movie: 10 sec/frame. Frame rate of replay: 5 frames/sec. Download movie file (AVI) Movie M2: Animated kinetic map of time variable turnover of the F-actin network in movie M1. A steady state representation of the same data is displayed in Figure 2d. Each frame of the animation rates of F-actin assembly and disassembly is computed as the moving integration of classified speckle appearances and disappearances over 5 frames (50 sec). Download movie file (AVI) Movie M3: Fluorescent Speckle Microscopy (FSM) time-lapse series of a newt lung epithelial cell migrating at the border of an epithelial monolayer. The cell was microinjected with a small amount of X-rhodamine labeled actin. Speckle flow mapping by multi-frame correlation analysis of this movie is illustrated in Figure 4. Single-speckle trajectory analysis is illustrated in Figure 5. Frame rate of raw movie: 10 sec/frame. Frame rate of replay: 5 frames/sec Download movie file (AVI) Movie M4: Fluorescent Speckle Microscopy (FSM) time-lapse series of microtubules in the meiotic spindle of Xenopus laevis egg extracts. This movie indicates the antiparallel flux of the two overlapping microtubule scaffolds. Due to space constraints, the result of single-speckle flow analysis of these flux fields is not shown in the paper. We refer to Figure 6 in Reference 49 for an illustration of the solution to this tracking problem by graph-based speckle assignment of consecutive frame triplets. Frame rate of raw movie: 10 sec/frame. Frame rate of replay: 5 frames/sec. Download movie file (AVI) Movie M5: Evolution of the transition between lamellipodium and lamellum networks over time in movie M1, as tracked by the kinetic (turnover) and kinematic (flow) differences between the F-actin regions. The boundary of the first frame is shown in Figure 9a. Download movie file (AVI)