Reconstitution of basic mitotic spindles in cell-like confinement

Bipolar organization of the mitotic spindle is the result of forces generated by dynamic microtubules and associated proteins in interaction with chromosomes and the cell boundary. Biophysical experiments on isolated spindle components have provided important insights into the force-generating properties of different components, but a quantitative understanding of the force balance that results from their concerted action is lacking. Here we present an experimental platform based on water-in-oil emulsion droplets that allows for the bottom-up reconstitution of basic spindles. We find a typical metaphase organization, where two microtubule asters position symmetrically at moderate distance from the mid-zone, is readily obtained even in the absence of chromosomes. Consistent with simulations, we observe an intrinsic repulsive force between two asters that can be counterbalanced alternatively by cortical pulling forces, anti-parallel microtubule crosslinking, or adjustment of microtubule dynamics, emphasizing the robustness of the system. Adding motor proteins that slide anti-parallel microtubules apart drives the asters to maximum separation, as observed in cells during anaphase. Our platform offers a valuable complementary approach to experiments where essential mitotic components are typically removed, instead of added, one by one.