In-situ cryoET of the metaphase human mitotic spindle

During eukaryotic cell division, a mitotic spindle composed of ~5000 microtubules and molecular motors assembles and is responsible for segregating chromosomes to the newly formed daughter cells. The development of biophysical models of how the collective behavior of these microtubules and motors shapes the spindle, generates forces, and ultimately segregates chromosomes has been severely limited by the resolution of traditional fluorescence microscopy, which cannot resolve individual microtubules and motors in the densely packed spindle. To overcome this, we employed in-situ cryo-electron tomography to image microtubules in the metaphase mitotic spindle in their native context. We segmented the trajectories and polarities of individual microtubules and found anti-parallel microtubules, which generate force when coupled with molecular motors. Taken together, our results provide a framework for utilizing in-situ cryoET as a tool to explain system-wide phenomena from microscopic interactions in active matter systems.