Interzonal microtubules are dynamic during spindle elongation.

The pattern and extent of microtubule assembly during spindle elongation has been examined in PtK1 cells by microinjection of biotin-tubulin and immunolocalization of biotin-tubulin-containing microtubules using antibodies to biotin. PtK1 cells were microinjected at 30 degrees C, incubated for various intervals to allow incorporation of biotin-tubulin into microtubules, then lysed, fixed and stained for biotin-tubulin and total tubulin. When mid- to late anaphase cells were examined at short times post-injection, using conventional fluorescence light microscopy, rapid incorporation of biotin-tubulin was detected throughout the interzonal region, between the separating chromosomes, and in the spindle asters. Using confocal fluorescence microscopy, the segments of biotin-labeled microtubules in the interzonal region were found to be continuous with the distal, or plus-ends, of unlabeled microtubules. When teleophase cells were examined, a marked decline in the extent of incorporation was apparent. Quantitative analysis of the total length of labeled polymer in the interzonal region of cells from mid-anaphase through telophase further reveals that the extent of incorporation was maximal during late anaphase, and decreased during telophase. The measured rate of interzonal microtubule growth remained relatively constant during this period. Our results provide direct evidence for plus-end elongation of interzonal microtubules during spindle elongation and further reveal that interzonal microtubules are highly dynamic during late anaphase spindle elongation. The implications of these results for the mechanism of anaphase B are discussed.