Collective mechanics of microtubules and motors form hexagonal patterns

Microtubule asters, radial arrays of microtubules (MTs) nucleated by centrosomes, are fundamental for the spatial organisation and geometry of the cell. Previous studies have suggested that asters interacting with molecular motors can give rise to cellular patterns, such as those in the Drosophila syncytium. Now, Chaitanya Athale, Janet Chenevert and colleagues (Khetan et al., 2021) use artificially induced asters in oocytes of the ascidian Phallusia mammillata, together with computational modelling, to investigate the mechanical basis of such phenomena. Treating oocytes with the ribosomal S6 kinase inhibitor BI-D1870, they report the spontaneous formation of multiple cytoplasmic MT asters that give rise to tessellation patterns in the cytoplasm with a hexagonal geometry. This organisation is blocked upon inhibition of kinesin-5 with the drug monastrol. A minimal computational model in which kinesin-5 motor complexes stochastically interact with multiple MT asters reproduces these patterns. Importantly, the simulations reveal that hexagonal tessellation patterns scale with cell size when the aster packing density ɸ is ∼1.6. This suggests that the aster segregation patterns are related to the solution of the general circle packing problem, where the densest packing of circles in a two-dimensional plane is a hexagonal lattice. Taken together, this work points to an in vivo mechanical pattern-forming system established by interactions between microtubules and molecular motors.