More than Two Populations of Microtubules Comprise the Dynamic Mitotic Spindle

The microtubules of the mitotic spindle mediate chromosome alignment to the metaphase plate, then sister chromatid segregation to the spindle poles in anaphase. Previous analyses of spindle microtubule kinetics utilizing fluorescence dissipation after photoactivation described two main populations, a slow and a fast turnover population, and these were ascribed to reflect kinetochore versus non-kinetochore microtubules, respectively. Here, we test this categorization by disrupting kinetochores through depletion of the Ndc80 complex. In the absence of functional kinetochores, microtubule dynamics still exhibit slow and fast turnover populations, though the proportion of each population and the timings of turnover are altered. Importantly, the data obtained following Hec1/Ndc80 depletion suggests other sub-populations, in addition to kinetochore microtubules, contribute to the slow turnover population. Further manipulation of spindle microtubules revealed a complex landscape. Dissection of the dynamics of microtubule populations provides a greater understanding of mitotic spindle kinetics and insight into roles in facilitating chromosome attachment, movement, and segregation during mitosis.