Not a hair out of place: Polarizing and orienting sensory hair cells.

Hair cells (HCs) are specialized sensory receptors in auditory, vestibular, and lateral-line organs that convert mechanical stimuli into neural signals. Auditory HCs transduce sound vibrations, vestibular HCs detect head movements for balance, and lateral-line HCs in aquatic vertebrates sense water currents, providing environmental awareness. A key feature of all HCs is their directional sensitivity, determined by the graded height architecture of their hair bundle. This arrangement ensures optimal HC activation when the hair bundle is deflected towards its tallest side. Within sensory organs, HCs and their hair bundles are precisely aligned within the epithelium plane, another key feature which produces coherent responses for accurate sensory representation. HC alignment is governed by planar cell polarity (PCP) cues relayed between neighboring cells. In some epithelia, such as the mammalian auditory epithelium, HCs are uniformly oriented. In other epithelia, PCP cues can be interpreted differently, and HCs exhibit a normal or reversed orientation creating a mirror-image HC organization. Several mechanisms generate directional sensors with proper alignment. During early development, the apical cytoskeleton breaks central symmetry and produces a staircase-like hair bundle. Over time, the asymmetrical apical cytoskeleton couples with distinct PCP mechanisms and signaling molecules at cell-cell junctions, orienting HCs properly within the sensory epithelium. This chapter highlights our current understanding of the intricate polarization processes that enable HCs to function as directional sensors, providing insights into their critical role in sensory perception and spatial orientation.