Ooplasmic streaming: a self organized non-equilibrium mechanism for mixing

When Drosophila oocytes (AKA fruit-fly eggs) develop, they go through a stage where their internal contents are mixed. At first the mixing is slow with a lot of uncorrelated motion, but then it transitions to faster streaming where there are much longer range flow patterns. The above video microscopy was performed by a former physics major Anthony Bielecki, working in Bill Saxton's lab in MCD Biology. In it you can see long strings, that are actually microtubules. In this stage they are aligned, in slower streaming they are quite disordered. The microtubules appear to undulate in a chaotic way similar to seaweed in a tide-pool.

In collaboration with a physics graduate student, Matthew Brunner, Bill Saxton, Anthony Bielecki, and Corey Monteith, we have been elucidating the mechanism for this streaming.

The explanation that we came up with is that kinesin motors walk up the microtubules from the minus ends at the cortex (i.e. wall) of the cell, towards the plus end which is free. It appears that the minus ends are tethered to the cortex and their plus ends are free. As it walks up the microtubule, a walker will feel a drag due to the cytoplasmic viscosity and this will apply a force to the microtubule. This force will cause the microtubule to buckle, but in a curious way, because the forces applied by the walker will be tangent to the microtubule's direction and so this leads to wave like motion of the microtubule. This is a simulation of a microtubule next to the wall in a streaming velocity field.