Size matters, but how biological processes are scaled to match the size of their target—for organisms, organs, cells, and organelles—is largely a mystery. Now, Rzadzinska et al. (page 887) find that the turnover rates of neighboring actin structures depend on their respective sizes, thus renewing the entire superstructure synchronously.
The structures are stereocilia—the mechanosensitive actin pillars sticking out of sensory hair cells of the inner ear. Adjacent parallel rows of stereocilia of graded height form a staircase-like pattern. Long regarded as the epitome of a stable cytoskeleton ensemble, the current authors recently showed that stereocilia bundles are in fact continuously turning over. This may be necessary to maintain the stereocilia's function over a lifetime.
They now find that the actin paracrystal that makes up the core of each stereocilium rebuilds itself via polymerization at the tip, treadmilling downwards, and dismantling at the base,...
