Reconstitution of aster movement and cell division plane positioning in Xenopus egg extract

Microtubule (MT) asters are a self-organizing network of short, dynamic microtubules oriented radially outward from a microtubule organizing center (MTOC). To facilitate cell division, asters must grow and move through a crowded cytoplasm. Aster movement is thought to be driven by MT-length-dependent forces from cytoplasmic dynein opposed by hydrodynamic drag; however, it remains unclear how these forces propagate through the network of short, and short-lived, microtubules, resulting in aster movement. We reconstituted aster movement in cytoplasmic extract from Xenopus laevis frog eggs, and we imaged the growth and interaction of asters under slit-like confinement. Aster boundaries stopped growing when they interacted with neighboring asters. Boundaries between asters formed dynamic, polygonal tessellations that resembled 2D foams. MTOCs moved within asters relative to boundaries and centroids. This movement was partially inhibited by the CC1 fragment of dynactin, which inhibits dynein-dependent forces. We are modelling the dynamic mechanical properties of asters to probe whether known forces can explain the results. We conclude the short, dynamic nature of microtubules facilitates aster growth and movement through a crowded cytoplasm.