Unraveling cytoplasmic streaming using a coarse-grained model of microtubule hydrodynamics

During the development of the fruit fly oocyte, flows with short-ranged correlations transition to a dramatic cell-spanning vortex, accompanied by coherent deformations in the microtubule cytoskeleton. Using a coarse-grained model for the hydrodynamics of ordered fibers, we show that sufficiently dense microtubule arrays, forced only by molecular motors transporting cargo, undergo a "swirling transition" that is fundamentally different than the buckling transition which leads to the flapping motion of isolated filaments. Our model produces streaming velocities consistent with in vivo measurements, and allows us to place bounds on the number density of kinesin-1 motors transporting cargo within the microtubule array.