Brn3c mice were bred as previously described [22]. Mice were genotyped and Brn3c null mutants were raised to a specific age (P0, N = 8; P7, N = 6; P8, N = 6; 6 months old, N = 2). The animals were deeply anesthetized with pentobarbital and transcardially perfused with 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4). Ears were postfixed in the same fixative for at least one day prior to dissection.