Live cell and in vitro mitotic spindles as arrested liquid crystal tactoids.

Microtubule self-organization is a fundamentally important phenomenon from both physics and biological point of view. One interesting and important structure is the mitotic spindle, a football-shaped structure used to align and ultimately separate the chromosomes during cell division. Recent studies have claimed that the meiotic spindle is internally organized like a liquid crystal tactoid with many, short microtubules that are fluid-like and can coalesce like droplets. Using live cell experiments, we find that the regions of mitotic spindle near the chromosomes are more fluidized, but other regions are arrested. Using photoactivation, we can measure the dynamics of different regions of the spindle and the relative concentration of microtubule-associated proteins and motors as a function of the microtubule concentration. In addition, we use in vitro reconstitution experiments of microtubule polymerization in the presence of a microtubule crosslinker, MAP65, and depletion agents that can form spindle-like structures. Using FRAP experiments, we found that these spindle-like assemblies are not fluid, but rather arrested, unlike liquid crystals. Using this combination of live cell and in vitro reconstitution, we are uncovering the physical organizations of the mitotic spindle.