Visualizing mitotic MT rearrangement has not been easy, because the process is so rapid. The new confocal videos of living cells expressing GFP-tubulin catch prophase in a way that imaging of fixed or microinjected cells never did. Rearrangement began in prophase with the depolymerization of cytosolic MTs and the formation of bundles and foci. Bundles were then pulled along other MTs extending from the centrosome. They continued to disassemble along the way, and eventually appeared to join with the forming spindle. A video of the entire process (which was completed within several minutes) is available. Astral MTs continued to scan the cytoplasm even as late as metaphase, drawing in errant MTs.
Dynein may regulate the timing of MT influx. Pulling was dependent on cytoplasmic dynein activity, indicating that MTs are both a track and cargo for this motor. Previously, nuclear envelope–associated dynein was shown to tear the envelope by pulling outward along microtubules. Possibly, the switch from dynein's interphase vesicular cargo to its mitotic cargoes are coordinately regulated, although the relevant signals to dynein are not yet known. ▪