Dynamic features of microtubules as visualized by dark-field microscopy.

We have reviewed recent progress in the dynamic features of microtubules in vitro as visualized by dark-field light microscopy using high intensity illumination. Observations of individual microtubules in real-time have made it possible to analyze the unique characteristics of microtubules exactly. The following three topics have been discussed: (1) treadmilling, i.e., the simultaneous assembly of tubulin at one end and disassembly at the other end on a single microtubule at a steady state. (2) Dynamic instability, i.e., the very unusual phenomenon in which two populations of microtubules coexist: those in one population elongating while those in the other shortening in the absence of MAPs. Both ends of the microtubules exist either in the growing or the shortening phase, and alternate between the two phases in a stochastic manner. (3) Morphogenesis of liposomes by microtubule growth. Tubulin is encapsulated into model membrane vesicles, liposomes. Polymerization of the encapsulated tubulin causes a change in shape of the spherical liposomes to form bipolar or multipolar vesicles, suggesting that microtubules have an active function in the morphogenesis of membranous organelles and cells.