Mitotic Spindle Assembly: Building the Bridge between Sister K-Fibers

The mitotic spindle performs the task of physically dividing the genetic material between the newly formed daughter cells. To achieve this, bundles of microtubules and associated proteins orchestrate forces that spatially organize and then separate the chromosomes. In the classic view of the spindle, the kinetochore microtubules (k-fibers) are tensed and, thus, straight, whereas interpolar bundles are curved and do not interact with k-fibers close to the spindle equator. The updated view of the spindle depicts k-fibers as curved and interacting with newly identified interpolar bundles, called bridging fibers, along their length. In this Opinion, we propose and discuss scenarios for the origin of this structure in the context of known spindle assembly mechanisms.