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Taxol-induced anaphase reversal: evidence that elongating microtubules can exert a pushing force in living cells.

The effects of taxol on mitosis in Haemanthus endosperm were studied. Immuno-gold staining was used to visualize microtubules; observations on microtubule arrangements were correlated with studies in vivo. Mitosis is slowed down, but not arrested, by taxol over a wide range of concentrations. Taxol promotes the formation of abundant new microtubules and lateral association within and between microtubule arrays (spindle fibers). This leads to a pronounced reorganization of the spindle, especially at the polar regions. Chromosome arms may be pushed toward the equator in metaphase. Anaphase chromosomes, with their kinetochores still pointing to the poles, move backward before resuming their poleward migration. During anaphase, the interzone is depleted of microtubules and trailing chromosome arms are stretched and often torn apart by rapidly elongating polar microtubules. Fragments are transported away from the poles, apparently "riding" on the tips of microtubules. This provides evidence of "pushing" by elongating microtubules. The desynchronization of anaphase, often observed as one of the first effects of taxol, indicates that the anchorage of different kinetochore fibers varies. The data draw attention to modifications of spindle structure due to increased microtubule lateral associations and to the role of this process in spindle integrity and chromosome movement.