A delicate fluid-filled structure within each inner ear, called the cochlea, enables us to hear. Within its lining are cochlear hair cells. And on their surface are bundles of sound-sensitive stereocilia that are arranged into staircase-like rows of increasing height (pictured here very highly magnified, seen with an electron microscope). A molecule called myosin 15 regulates how these hair bundles develop and grow to their required height. Mutations in the myosin 15 gene can cause hereditary deafness in humans. Two different versions of myosin 15 are produced by stereocilia. A small version is produced in young stereocilia and then, once they mature, a larger version is produced. In mice that lack large myosin 15, stereocilia develop normally but then the first two rows deteriorate and the mice can’t detect sound. The bigger molecule is therefore crucial for maintaining healthy, ‘hearing’ stereocilia.
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