Figure 1.
Auditory brainstem response (ABR) measurements of age-related hearing loss.
Panel A shows two representative ABR waveforms from the youngest monkey. Waves II and IV are marked. The top trace shows the response to click stimuli presented at 80 dB SPL, which was well above threshold, and the bottom trace shows the response in the same monkey to a 20 dB SPL click stimulus. No obvious response was noted at this stimulus intensity. Panels B-D show the data from each monkey organized by symbols and colorized to represent different age-groups. ABR thresholds (B) and age-related shift in threshold (C) as a function of stimulus and age of the monkey. The seven frequency pure tone average loss (PTAL; Panel D) increases as a function of age, r = 0.88, p<0.01.
Table 1.
Correlation matrix between age and ABR thresholds to clicks and tones.
Figure 2.
Histology of the rhesus monkey cochlea.
This specimen came from the 15 year old monkey. Whole mount (A), illustration (B) and semi-thin mid-modiolar section (C) of the rhesus monkey cochlea. Semi-thin section revealing inner and outer hair cells are shown in D. Arrows indicate stereocilia or soma of inner and outer hair cells. Semi-thin section revealing the stria vascularis are shown in E. Different levels of the base (B1,2), middle (M1,2) and apical (A1,2,3) turns are indicated in panel C. TM = tympanic membrane. Scale bar 1 mm (A), 10 µm (D,E).
Figure 3.
Effect of age and PTA on inner hair cells.
Age-normalized inner hair cell counts as a function of age (left) and PTA (right) in the lower base (top), upper base (middle), and lower apex (bottom). A few monkeys had decreased inner hair cells in the lower base and apex, but all were normal in the upper base.
Figure 4.
Effects of age and PTA on outer hair cells.
Age-normalized outer hair cell counts as a function of age (left) and PTA (right) in the lower base (B1), upper base (B2), lower apex (A1) and upper apex (A2). Many monkeys had decreased outer hair cells, particularly at the lower base and upper apex.
Figure 5.
Spiral Ganglion Cell (SGC) loss as a function of age.
Photomicrographs of SGCs in the base (left) and modiolus (right) of the 10 (A,B), 21.5 (C, D), and the 35 (E, F) year old monkeys. The packing density of SGCs decreases as a function age and region in the cochlea. Scale bar = 50 µm.
Figure 6.
Effects of age and PTA on spiral ganglion cells.
Age-normalized spiral ganglion cell counts as a function of age (left) and PTA (right) in the lower base (top), upper base (middle), and modiolus (bottom). There was a clear decrease in SGC as a function of both age and PTA.
Figure 7.
Relationship between the hair cells and spiral ganglion cells.
Age-normalized inner (A) and outer (B) hair cell counts as a function of age-normalized packing density of SGCs. The solid diagonal line represents the unity line. Points that fall above the unity indicate a greater loss of spiral ganglion cells compared to the hair cells.
Figure 8.
Effects of age and PTA on stria vascularis thickness.
Age-normalized stria vascularis thickness as a function of age (left) and PTA (right) in the lower base (top), upper base (middle), and lower apex (bottom). There was no statistically significant correlation across animals.
Figure 9.
Age-related increase of the number of significant pathological features in the cochlea of the rhesus monkey.
There was a statistically significant correlation, indicating that multiple histopathologies occur with natural aging. r = 0.90, p<0.001.