Sound waves are converted into electric signals by hair bundles (also known as stereocilia) in the cochlear hair cells. Stereocilia, a cluster of actin protrusions at the apical surface of hair cells, are organized into rows of graded height (Figure 1). Mechanical force induced by sound waves leads to stereocilia deflection towards the tallest row and subsequent activation of mechanoelectrical transducer channels at the tips of the shorter rows, thus achieving mechanoelectrical transduction (Gillespie and Muller, 2009). Such a staircase-like architecture is essential for auditory perception as manifested by severe deafness caused by genetic mutations that disrupt stereocilia morphology (Barr-Gillespie, 2015). However, little is known about how this planar asymmetry of stereocilia is achieved.
