Comparison of Age-Related Pigmentary Changes in the Auditory and Vestibular Systems Within Mouse and Human Temporal Bones

Abstract

BackgroundMelanin pigmentation is present within the auditory and vestibular systems of the mammalian inner ear and may play a role in maintaining auditory and vestibular function. Melanocytes within the stria vascularis (SV) are necessary for the generation of the endocochlear potential (EP) and decreased EP has been linked to age-related hearing loss. Melanocytes and pigment-containing “dark cells” are present within the vestibular system, but have a less well-defined role. African–American individuals have increased pigmentation within the SV and vestibular system, which is hypothesized to be related to lower rates of age-related hearing loss and vestibular dysfunction. It remains unclear if increased pigmentation confers lifelong protection against hearing loss and vestibular dysfunction.MethodsMouse temporal bones were collected from juvenile (3–4 week) and aged (20–32 months) CBA/CaJ mice. Pediatric and adult human temporal bones from Caucasian or African–American individuals were examined from the Johns Hopkins Temporal Bone Collection. Information regarding Fitzpatrick skin type were unavailable, and self-identified race/ethnicity was used as a proxy. Images were taken using light microscopy at 20× magnification. ImageJ software (v1.53) was used to measure pigment within the SV and vestibular system.ResultsIn mouse temporal bones pigmentation within the SV increased with age, but pigmentation within the vestibular system did not increase with age. In human temporal bones pigmentation within the SV increased with age and pigmentation within the vestibular system increased within the wall of the utricle, but not other regions of the vestibular system. African–American individuals had higher amounts of pigment within the SV and vestibular system, among both pediatric and adult populations.ConclusionStria vascularis pigmentation increases with age in mouse and human temporal bones. Pigmentation within the vestibular system did not increase with age in mouse specimens and only increased within the utricular wall with age in human specimens. Individuals who identified as African–American had higher pigment content within the SV and vestibular system, both as children and as adults. These results highlight how similar age-related pigmentary changes occur in the auditory and vestibular systems across species and underscore the importance of racial/ethnic diversity in human temporal bone studies.

The Stria Vascularis: Renewed Attention on a Key Player in Age-Related Hearing Loss

“…A correlation also exists between the ethnicity and pigmentation in the SV. African American SV samples contain significantly more pigmentation than Caucasian specimens, both as children and as adults [126,127]. Consistent with this, several indicators of melanin content, such as eye color and skin sun-sensitivity, are correlated with susceptibility to temporary auditory threshold shift after noise overexposure [128][129][130].…”

“…In aging humans, as well as in C57BL/6 and CBA/CaJ mice, strial melanin content significantly increases with age. It is currently unclear whether this increase with age is related to the potential protective role of melanin or whether it is simply a metabolic by-product [126,127,131].…”

Sex and Race Representation in Temporal Bone Histopathology Studies in the United States: A Systematic Review

“…In 2011, a National Health and Nutritional Examination Survey study found that individuals who self-identify as Black had decreased odds of hearing loss (Arnegard et al 2020). Further, studies have described an association between skin color and inner ear pigmentation, where race was often used as a surrogate for skin color (Barrenäs & Axelsson 1992;Erbele et al 2016;Andresen et al 2021). TBH studies in animals, then humans, postulated that melanin in the stria vascularis may be responsible for this effect by acting like a free-radical scavenger and metal ion chelator (Strain 2004;Ohlemiller et al 2009Ohlemiller et al , 2010Erbele et al 2016;Oakes & Kaufman 2017).…”

Hearing Function, Degeneration, and Disease: Spotlight on the Stria Vascularis

“…Using the Fitzpatrick skin colour scale and audiometry, Lin et al showed that individuals with darker skin have better hearing than those with lighter skin (Lin et al, 2012). Skin pigmentation appears to correlate with melanin content in the SV, and Andresen et al demonstrated that SV melanin is indeed higher in African Americans when compared to Caucasians (Andresen et al, 2021).…”

“…However, an important observation is that melanin does not protect against agerelated SV degeneration. In fact, melanin content increases in the aging ear of mice and humans despite the prevalence of agerelated hearing loss (Kobrina et al, 2020;Andresen et al, 2021).…”