Shaky hearing

Functional sensory hair cells in the inner ear have specialised microvilli — stereocilia — which are essential for hearing. The scanning electron micrograph at top shows the normal arrangement of stereocilia bundles on three outer hair cells in the inner ear of a three-day-old mouse. The stereocilia grow to form rows of graded height in a distinctive V-shaped bundle. Recent studies in mice have revealed a key role for an unconventional myosin, VIIA, in the organisation of these stereocilia bundles and the function of the cochlear hair cells. Myosin VIIA is encoded by the Myo7a gene, found at the shaker1 locus in mice. Mouse shaker1 mutants are well known for their inner ear defects, and mutations in the orthologous gene in humans also cause deafness. The micrograph at bottom left shows the disorganised stereocilia bundles on eight mutant outer hair cells from a Myo7a 816SB homozygote. The stereocilia form rows of graded height as usual, but the bundles become progressively more disorganised during the few days before and after birth and cochlear function is disrupted.