(a) Scanning Electron Micrographs of the mid-basal coil of the cochlear sensory epithelium from Slc4a10+/+, Slc4a10+/trmb, Slc4a10trmb/trmb mice at 2, 6, 9 and 12 months of age. At 2 and 6 months of age, the number and appearance of the inner and outer hair cell stereocilia bundles are is as expected and similar across all three genotypes. At 9 months, there is some loss of outer hair cell bundles in the Slc4a10trmb/trmb mutant mice, which is not observed in the Slc4a10+/+ and Slc4a10+/trmb control mice. By 12 months of age, there is a substantial loss of outer hair cell bundles in the Slc4a10trmb/trmb mutant mice, which is not observed in the Slc4a10+/+ and Slc4a10+/trmb control mice. Magnification × 2,500. Scale bars, 10 μm (b) Outer hair cell bundle counts in the apical, mid and basal turns of the cochlear spiral in trombone mice. At 2 months of age Slc4a10+/+ (apex n=5, mid n=7, base n=5), Slc4a10+/trmb (apex n=3, mid n=6, base n=3), Slc4a10trmb/trmb (apex n=5, mid n=4, base n=3) mice all have similar numbers of OHC bundles. At 6 months of age Slc4a10+/+ (apex n=3, mid n=3, base n=3), Slc4a10+/trmb (apex n=4, mid n=3, base n=4), Slc4a10trmb/trmb (apex n=3, mid n=3, base n=3) mice all still have similar numbers of OHC bundles. However, by 9 months of age Slc4a10trmb/trmb (apex n=3, mid n=3, base n=3) mice have a reduced number of OHC bundles in all cochlear regions compared with Slc4a10+/+ (apex n=3, mid n=3, base n=3), Slc4a10+/trmb (apex n=2, mid n=3, base n=3) mice. At 12 months of age Slc4a10trmb/trmb (apex n=5, mid n=3, base n=6) mice show a further loss of OHC bundles in all cochlear regions compared with Slc4a10+/+ (apex n=2, mid n=3, base n=2), Slc4a10+/trmb (apex n=4, mid n=6, base n=3) mice. Although Slc4a10trmb/trmb mice show a progressive loss of OHC bundles throughout the cochlear spiral, no significant OHC bundle loss is observed in the Slc4a10+/+ or Slc4a10+/trmb mice up to 12 months of age. Slc4a10+/+ (black bars), Slc4a10+/trmb (grey bars), Slc4a10trmb/trmb (red bars). Data shown are mean ±s.e.m. and significance determined using a two-way ANOVA with Tukey's multiple comparisons test: **P<0.01, ***P<0.001, ****P<0.0001.