Abstract
The mitotic spindle is a dynamic microtubule-based apparatus that ensures the faithful segregation of chromosomes by connecting chromosomes to spindle poles. How this pivotal connection is established and maintained during mitosis is currently debated. Here we combined large-scale serial electron tomography with live-cell imaging to uncover the spatial and dynamic organization of microtubules in the mitotic spindles in C. elegans. With this we quantified the position of microtubule minus and plus-ends as well as distinguished the different classes of microtubules, such as kinetochore, astral and spindle microtubules with their distinct properties. Although microtubules are nucleated from the centrosomes, we find only a few, if any, kinetochore microtubules directly connected to the spindle poles, suggesting an indirect pole to chromosome connection. We propose a model of kinetochore microtubule assembly and disassembly, in which microtubules undergo minus-end depolymerisation, resulting in a detachment from the centrosome. Our reconstructions and analyses of complete spindles expand our understanding of spindle architecture beyond the light microscopic limit.
