Active forces shape the metaphase spindle through a mechanical

The metaphase spindle is a dynamic structure orchestrating chro-mosome segregation during cell division. Recently, soft matter approaches have shown that the spindle behaves as an active liq-uid crystal. Still, it remains unclear how active force generation contributes to its characteristic spindle-like shape. Here we com-bine theory and experiments to show that molecular motor-driven forces shape the structure through a barreling-type instability. We test our physical model by titrating dynein activity in Xenopus egg extract spindles and quantifying the shape and microtubule ori-entation. We conclude that spindles are shaped by the interplay between surface tension, nematic elasticity, and motor-driven active forces. Our study reveals how motor proteins can mold liquid crystalline droplets and has implications for the design of active soft materials.