Mechanical force locally damages, remodels, and stabilizes the lattice of spindle microtubules.

To segregate chromosomes at cell division, the spindle must maintain its structure under force. How it does so remains poorly understood. To address this question, we use microneedle manipulation to apply local force to spindle microtubule bundles, kinetochore fibers (k-fibers), inside mammalian cells. We show that local load directly fractures k-fibers and that newly created plus-ends often have arrested dynamics, resisting depolymerization. Force alone, without fracture, is sufficient for spindle microtubule stabilization, as revealed by laser ablating k-fibers under local needle force. Doublecortin, which binds a compacted microtubule lattice, is lost around the force application site, suggesting local force-induced structural remodeling. In turn, end-binding protein 1 (EB1), which recognizes guanosine triphosphate (GTP)-tubulin, is locally enriched at stabilization sites, both before and after force-induced fracture. Together, our findings support a model in which force-induced damage leads to local spindle microtubule lattice remodeling and stabilization, which we propose reinforces the spindle where it experiences critical loads.