The cuticular plate: a riddle, wrapped in a mystery, inside a hair cell.

The mechanosensitive hair cells of the inner ear are crucial to hearing and vestibular function. Each hair cell detects the mechanical stimuli associated with sound or head movement with a hair bundle at the apical surface of the cell, consisting of a precise array of actin-based stereocilia. Each stereocilium inserts as a rootlet into a dense filamentous actin mesh known as the cuticular plate. Disruption of the parallel actin bundles forming the stereocilia results in hearing impairments and balance defects. The cuticular plate is thought to be involved in holding the stereocilia in place. However, the precise role of the cuticular plate in hair bundle development, maintenance, and hearing remains unknown. Ultrastructural studies have revealed a complex cytoskeletal architecture, but a lack of knowledge of proteins that inhabit the cuticular plate and a dearth of mutations that perturb relevant proteins have hindered our understanding of the functions of the cuticular plate. Here, we discuss what is known about the structure and development of this unique and poorly-understood actin-rich organelle.