Spiraling galaxies of microtubules

Look inside a living cell as it prepares to divide and you will find in it arrays of stiff biopolymers -- microtubules --radiating outwards from mobile nucleating sites called centrosomes. By interacting with cell boundaries and motor-proteins, centrosomal arrays move and position genetic material in the cell. This motion takes place in the fluidic slurry -- cytoplasm -- that fills the cell. Given the complexity of real cells, understanding how centrosomes do their job is difficult. New experiments have created artificial cells enclosing artificial centrosomes that, like their wildtype counterparts, nucleate microtubule arrays and move. These experiments show microtubule arrays stably centered in its cell, arrays spinning like spiral galaxies, and rotating arrays switching back and forth like a washing machine. We recover and organize this complex dynamics in a fluid-structure model of growing microtubules pushing against the cell boundary, and against each other through the surrounding fluid. Analysis of a coarse-grained model shows that the system is controlled by a combination of microtubule density and cell size, and the collective organization of microtubule bending by hydrodynamics. Large-scale simulations show that rotation and oscillations arise from an intricate and surprising interplay between C-shaped and S-shaped microtubule bending modes.