Fig. 2. ABR threshold measurements and pathological structures in Six1^+/- adult mouse ears. (A) Average threshold±s.e.m. for wild-type and Six1^+/- ears in backgrounds 129/Sv and C57BL/6J (129/Sv, n=6 wild type, 13 Six1^+/-; C57BL/6J, n=6 wild type, 10 Six1^+/-). Averages used 90 dB SPL for thresholds beyond the upper limit of the sound system. The hearing loss was variable among mice and ears. Of the 13 Six1^+/- mice tested, eight mice had severe hearing loss in both ears (threshold shifted by 50 dB between 15 and 32 kHz), two mice had mild hearing loss in both ears (threshold shifted by 20 dB), whereas three mice had normal hearing in the right ears and >70 dB loss in the left ears. Broken lines, C57BL/6J strain; unbroken lines, 129/Sv strain. (B-G) Histological analysis in wild type and Six1^+/- ears. (B) Wild-type ear showing the stapes seated in the oval window (not visible because apposed by the stapes footplate, sf), and one arch of the stapes (s). The stapedial artery (a) is normally positioned. Part of the long process of the malleus (lp) is also present in this section. nVII, the VIIth cranial nerve. (C,D) Six1^+/- ears showing that the footplates of the stapes are seated in the oval window. However, the VIIth nerve passed abnormally close to the oval window and the stapes or filled up the middle ear space near the oval window (arrows). Part of the stapes arches and the long process of the malleus are present in these sections. (E) Wild-type superior region of the middle ear space showing the junction of the malleus (ma) and incus (in). tm, tympanic membrane. (F,G) Six1^+/- superior region of the middle ears showing the junction of the malleus and incus. The middle ear space was small and partially filled with loose connective tissue (arrow). The tympanic membrane was also abnormal (arrowheads). ABR threshold from these ears demonstrated a severe hearing loss. Scale bars: 100 µm.