Fig. 2. Progression of cochlear degeneration in the WT C57BL/6 mice during aging.

a–c Histological analysis of spiral ganglion neurons (SGNs) (a), spiral ligaments (SL) (b), and stria vascularis (SV) (c) at the apical, middle, and basal turns in the 2-, 5-, and 12-month-old mice (n = 6 in each group). SGN density (a), type IV fibrocyte density in the SL (b), and SV thickness (c) were quantified. Green arrows indicate neuronal soma (a), fibrocytes (b), and SV width (c). Areas of type IV fibrocytes are indicated by dashed green triangles. The data represent the mean ± SEM. *P < 0.05, **P < 0.01. Two-way ANOVA followed by Tukey’s multiple comparison test was applied. d Images of the surface preparation of the hair cells at 2 and 12 months. Missing outer hair cells (OHCs) are indicated with white arrowheads. Most OHCs in the basal turns at 12 months were lost, as indicated in the area circumscribed by the dashed line. IHCs, inner hair cells. Missing OHCs were quantitatively analyzed by evaluating 90 OHCs at each turn (n = 5 in each group). The data represent the mean ± SEM. **P < 0.01. Unpaired two-tailed Student’s t-test was applied. Scale bars correspond to 50 μm (a–c) and 40 μm (d).