Active mechanics of nuclear envelope deformation and cytoskeletal force generation in closed mitosis

Cells undergoing closed mitosis provide an example of an active material confined within a deformable boundary: the mitotic spindle elongates inside an intact nuclear envelope that stretches and reshapes in response to spindle forces. Using live imaging, mechanical perturbation, and modeling, we show that nuclear envelope tension and cytoskeletal forces reorganize each other. Increasing envelope tension bends and slows the elongating spindle, while relieving tension or severing the spindle produces rapid relaxation of both structures. A Helfrich model of the nuclear envelope as a fluid membrane driven by active forces from the spindle predicts the observed shape transitions as the nucleus deforms. I will then discuss a second active-matter problem: the emergence of an ordered square array of microtubules in the fission-yeast spindle midzone. In our model, microtubule bundles self-organize into this geometry through distance-dependent active interactions that generate two preferred length scales, producing a stable lattice with spacing, symmetry, and packing that match experimental results. This work shows how active filament assemblies organize, deform their boundaries, and create robust mesoscale structures through force balance and nonequilibrium self-organization.