Lifelong dynamic maintenance of stereocilia bundles in mammalian auditory hair cells.

Our sense of hearing is initiated when sound-induced vibrations deflect specialized mechanosensory projections, stereocilia, emanating from the apical surface of the inner ear hair cells. Each hair cell has dozens of stereocilia organized into rows of increasing heights, forming a "hair" bundle. This "staircase" architecture of the stereocilia bundle is common for all vertebrate hair cells and essential for normal mechanosensitivity. Yet, the molecular mechanisms underlying its formation and lifelong maintenance in non-regenerating mammalian auditory hair cells represent a fascinating but yet puzzling problem for cell biology. Recent data demonstrating that stereocilia dimensions are controlled by the ionic current through mechanosensitive channels at the tips of stereocilia may help in solving this puzzle. The current chapter describes potential molecular mechanisms of stereocilia bundle formation and maintenance, as well as the mechanisms that optimize the mechanical properties of the hair bundle for effective mechanosensation.