Active forces shape the metaphase spindle through a mechanical instability

Significance The mitotic spindle machinery is a bipolar structure that segregates sister chromatids during cell division. Spindle assembly relies on the self-organization of motor proteins and microtubule filaments—dynamic polymers that constitute the spindle building blocks. How motors ensure a bipolar spindle-like structure is not understood. Here we build a liquid crystal droplet description of the spindle that quantitatively accounts for pole focusing as a function of dynein activity. Our results show how active forces driven by molecular motors can shape liquid crystalline droplets.