When size matters: the dynamic regulation of stereocilia lengths.

Stereocilia, the mechanosensitive protrusions in hair cells, are organized into rows of graded heights forming precisely uniform staircase patterns. The actin turnover process in stereocilia follows a treadmill model in which the rate of treadmilling is scaled to the stereocilium's length. Myosin XVa, which is present at the site of actin polymerization at concentrations proportional to the length of the actin filament bundles, plays a combined role with the treadmill machinery in regulating the steady state length of these actin protrusions, together with other myosins localized alongside the actin bundles.