Morphological growth dynamics, mechanical stability, and active microtubule mechanics underlying spindle self-organization

Significance The spindle is a microtubule-based, self-organized structure essential for equal segregation of chromosomes during cell division. The spindle’s bipolar shape is robustly maintained for the segregation accuracy but is occasionally disrupted, yielding multipolar phenotypes. How microtubules favor nonbipolar self-organization despite high bipolar stability is not known. Here, we find that this stems from the bistable nature of microtubule self-organization within the cytoplasm, which paves a separate route of multipolar organization from a bipolar phenotype. We also find that these two phenotypes infrequently switch upon maturation and by perturbed force, indicating the structure’s mechanical plasticity underlying the bistability. Our study thus reveals the mechanics governing the robust yet stochastic nature of spindle morphogenesis, which has relevance to tumors and unhealthy oocytes.