Abstract
Age-related hearing loss (presbyacusis) is the most common type of hearing impairment. One of the most consistent pathological changes seen in presbyacusis is the loss of spiral ganglion neurons (SGNs). Defining the cellular and molecular basis of SGN degeneration in the human inner ear is critical to gaining a better understanding of the pathophysiology of presbyacusis. However, information on age-related cellular and molecular alterations in the human spiral ganglion remains scant, owing to the very limited availably of human specimens suitable for high resolution morphological and molecular analysis. This study aimed at defining age-related alterations in the auditory nerve in human temporal bones and determining if immunostaining for myelin basic protein (MBP) can be used as an alternative approach to electron microscopy for evaluating myelin degeneration. For comparative purposes, we evaluated ultrastructural alternations and changes in MBP immunostaining in aging CBA/CaJ mice. We then examined 13 temporal bones from 10 human donors, including 4 adults aged 38–46 years (middle-aged group) and 6 adults aged 63–91 years (older group). Similar to the mouse, intense immunostaining of MBP was present throughout the auditory nerve of the middle-aged human donors. Significant declines in MBP immunoreactivity and losses of MBP+ auditory nerve fibers were observed in the spiral ganglia of both the older human and aged mouse ears. This study demonstrates that immunostaining for MBP in combination with confocal microscopy provides a sensitive, reliable, and efficient method for assessing alterations of myelin sheaths in the auditory nerve. The results also suggest that myelin degeneration may play a critical role in the SGN loss and the subsequent decline of the auditory nerve function in presbyacusis.
Highlights
Age-related hearing loss affects about half the population over 75 years of age [1]
A great majority of the somata of human type I spiral ganglion neurons (SGNs) are unmyelinated as demonstrated by several light and electron microscopic studies [18,19,20,21,22], suggesting that neural conduction may be slower in the human auditory nerve compared with other mammalian species
We examined age-related degeneration of the myelin sheath in the human auditory nerve using an immunohistochemical assay for myelin basic protein (MBP)
Summary
Age-related hearing loss (presbyacusis) affects about half the population over 75 years of age [1]. Primary degeneration of the auditory nerve has been demonstrated in animal models and humans through mechanisms not solely related to hair cell loss [6,7,8,9]. The remaining type II neurons (about 5%) are unmyelinated and innervate multiple outer hair cells but their function is still largely unknown in vivo [14,15,16]. Both the peripheral and central processes of type I neurons are enveloped in a thick myelin sheath that serves as an electrical insulating material. A great majority of the somata of human type I SGNs are unmyelinated as demonstrated by several light and electron microscopic studies [18,19,20,21,22], suggesting that neural conduction may be slower in the human auditory nerve compared with other mammalian species
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