Visualization of the living cytoskeleton by video-enhanced microscopy and digital image processing.

Two steps led to our present-day view of the cytoskeleton as a highly dynamic structure that is actively involved in force generation for various kinds of cell motility and, as a result, is itself often actively moving. The first step was the introduction of video microscopy, especially of the Allen Video Enhanced Contrast-Differential Interference Contrast Microscopy (AVEC-DIC), which allows the visualization of cellular structures in the light microscope that are up to 10 times smaller than the limit of resolution. This enables one to see images of unfixed, unstained, native or purified microtubules and actin bundles, and their interaction with membrane-bound organelles. The second step was the discovery of a system exceptionally well-suited to study microtubule and organelle movements, namely, the extruded axoplasm of the squid giant axon. From this axon the cytoplasm can be extruded free from surrounding plasma membrane, and individual microtubules and organelles can be separated from the bulk axoplasm. The study of these native microtubules by AVEC-DIC microscopy yielded a great number of quite unexpected details of the dynamic behaviour of both the microtubules themselves and the motility associated with them.