Stable kinetochore-microtubule attachments restrict MTOC position and spindle elongation in acentrosomal oocytes

In mouse oocytes, acentriolar MTOCs functionally replace centrosomes and act as microtubule nucleation sites. Microtubules nucleated from MTOCs initially assemble into an unorganized ball-like structure, which then transforms into a bipolar spindle carrying MTOCs at its poles, a process called spindle bipolarization. In mouse oocytes, spindle bipolarization is promoted by kinetochores but the mechanism by which kinetochore-microtubule attachments contribute to spindle bipolarity remains unclear. This study demonstrates that the stability of kinetochore-microtubule attachment is essential for confining MTOC positions at the spindle poles and for limiting spindle elongation. MTOC sorting is gradual and continues even in the metaphase spindle. When stable kinetochore-microtubule attachments are disrupted, the spindle is unable to restrict MTOCs at its poles and fails to terminate its elongation. Stable kinetochore fibers are directly connected to MTOCs and to the spindle poles, and thus may serve as a measure that defines proper spindle length. These findings reinforce the hypothesis that kinetochores act as scaffolds for acentrosomal spindle bipolarity.