Clin Exp Otorhinolaryngol.  2010 Sep;3(3):126-135.

Comparison of Cochlear Morphology and Apoptosis in Mouse Models of Presbycusis

• Park SN
• Back SA
• Park KH
• Kim DK
• Park SY
• Oh JH
• Park YS
• Yeo SW

Abstract

OBJECTIVES
Morphological studies on presbycusis, or age-related hearing loss, have been performed in several different strains of mice that demonstrate hearing loss with auditory pathology. The C57BL/6 (C57) mouse is a known model of early onset presbycusis, while the CBA mouse is characterized by relatively late onset hearing loss. We performed this study to further understand how early onset hearing loss is related with the aging process of the cochlea.
METHODS
We compared C57 cochlear pathology and its accompanying apoptotic processes to those in CBA mice. Hearing thresholds and outer hair cell functions have been evaluated by auditory brainstem response (ABR) recordings and distortion product otoacoustic emission (DPOAE).
RESULTS
ABR recordings and DPOAE studies demonstrated high frequency hearing loss in C57 mice at P3mo of age. Cochlear morphologic studies of P1mo C57 and CBA mice did not show differences in the organ of Corti, spiral ganglion, or stria vascularis. However, from P3mo and onwards, a predominant early outer hair cell degeneration at the basal turn of the cochlea in C57 mice without definitive degeneration of spiral ganglion cells and stria vascularis/spiral ligament, compared with CBA mice, was observed. Additionally, apoptotic processes in the C57 mice also demonstrated an earlier progression.
CONCLUSION
These data suggest that the C57 mouse could be an excellent animal model for early onset 'sensory' presbycusis in their young age until P6mo. Further studies to investigate the intrinsic or extrinsic etiologic factors that lead to the early degeneration of organ of Corti, especially in the high frequency region, in C57 mice may provide a possible pathological mechanism of early onset hearing loss.

Keyword

Cochlea; Morphology; Hearing; Aging; Apoptosis

MeSH Terms

Aging
Animals
Apoptosis
Cochlea
Evoked Potentials, Auditory, Brain Stem
Hair
Hearing
Hearing Loss
Ligaments
Mice
Mice, Inbred CBA
Models, Animal
Organ of Corti
Presbycusis
Spiral Ganglion
Stria Vascularis

Figure

• 

  Fig. 1 Auditory brainstem response (ABR) threshold testing through P6mo for CBA and C57 mice. Raw data for the 32 kHz tone burst ABR show an elevated threshold and abnormal wave form in C57 mice from P3mo onwards (A). C57 mice demonstrate a significantly elevated hearing threshold at a 32 kHz tone burst ABR from P1mo onwards, but not for 8, 16 kHz, or clicks (B). C57 mice at P6mo show a clear defect in outer hair cell function from the mid-to-high frequency region, which has been observed as low distortion product otoacoustic emission (DPOAE) levels compared with the CBA mice (C). *P<0.05. NF: noise floor.

• 

  Fig. 2 Serial light microscopy changes of the organ of Corti from CBA (upper panels) versus C57 (lower panels) mice from base, mid, and apical regions of the cochlea. At P1mo (A), the morphology of the organ of Corti did not show differences between CBA and C57 strains. However, from P3mo (B), C57 mice showed degeneration of outer hair cells at the base of the cochlea. At P6mo (C), C57 mice consistently showed more severe damage in the region of the outer hair cells, compared with CBA. A rank-order grading method was used to rate the condition of supporting cells and the general shape of the organ of Corti. The average value for regional supporting cell condition in the base of C57 mice was significantly lower than that of CBA mice at P6mo. (D) Six mid-modiolar sections were counted and averaged to obtain a value from one animal. Scale bar 50 µm.

• 

  Fig. 3 Serial light microscopy changes of the spiral ganglion cells from CBA versus C57 mice from base, mid, and apical regions of the cochlea. C57 mice appear to have earlier and more severe degeneration of spiral ganglion cells (SGC) in the light micrographs (A-C). SGC densities showed a decreasing tendency in both mice strains, but no significant difference between them was evident (D). SGC density in C57 mice was higher than that of CBA mice until P3mo. At P6mo, the density of SGC was reversed, indicating earlier and more abrupt degeneration of SGC in C57 mice. Scale bar 50 µm.

• 

  Fig. 4 Morphology of the strain vascularis and spiral ligament of P6mo in CBA and C57 mice. C57 mice displayed a larger stria vascularis and spiral ligament until P6mo, compared with CBA mice (A). Quantification of the area showed significantly larger values in C57 mice (B). Scale bar 100 µm.

• 

  Fig. 5 Hair cells of P1mo CBA and C57 mice were intact from base to apex when stained with rhodamine-phalloidin (A). At P3mo, C57 mice showed occasional hair cell loss at the basal turn of the cochlea (B). At P6mo, multiple hair cell loss at the basal and mid turn of the cochlea was observed in C57 mice (C). CBA mice showed occasional hair cell loss at P6mo of age. *Representative sites of hair cell loss. Scale bar 10 µm.

• 

  Fig. 6 Apoptotic hair cell death at P6mo in the cochlea of CBA and C57 mice. Apoptotic cells were observed by either the TUNEL assay (A) or Western blot for cleaved caspase-3 (B). Strongly stained nuclei of the outer hair cells (arrows) which represent the more severe apoptosis were observed at the basal turn of the cochlea in C57 mice compared with CBA mice (A). Activated caspase-3 was more evident in C57 mice than in CBA mice at P6mo (B). OC: organ of Corti.

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