How we discovered fluorescent speckle microscopy

Fluorescent speckle microscopy (FSM) is a method for measuring the movements and dynamic assembly of macromolecular assemblies such as cytoskeletal filaments (e.g., microtubules and actin) or focal adhesions within large arrays in living cells or in preparations in vitro. The discovery of the method depended on recognizing the importance of unexpected fluorescence images of microtubules obtained by time-lapse recording of vertebrate epithelial cells in culture. In cells that were injected with fluorescent tubulin at ∼10% of the cytosol pool, microtubules typically appeared as smooth threads with a nearly constant fluorescence intensity. One day, when an unusually low concentration of fluorescent tubulin was injected into cells, the images from a sensitive cooled charge-coupled detector camera showed microtubules with an unusual “speckled” appearance—there were fluorescent dots with variable intensity and spacing along the microtubules. A first thought was that the speckles were an artifact. With further thought, we surmised that the speckles could be telling us something about stochastic association of tubulin dimers with the growing end of a microtubule. Numerous experiments confirmed the latter hypothesis. Subsequently the method we call FSM has proven to be very valuable. The speckles turned out not to be a meaningless artifact, but rather a serendipitous find.