The human cochlea during development and ageing

Linking anatomical and physiological markers of auditory system degeneration with behavioral hearing assessments in a mouse (Mus musculus) model of age-related hearing loss

Age-related hearing loss is a very common sensory disability, affecting one in three older adults. Establishing a link between anatomical, physiological, and behavioral markers of presbycusis in a mouse model can improve the understanding of this disorder in humans. We measured age-related hearing loss for a variety of acoustic signals in quiet and noisy environments using an operant conditioning procedure and investigated the status of peripheral structures in CBA/CaJ mice. Mice showed the greatest degree of hearing loss in the last third of their lifespan, with higher thresholds in noisy than in quiet conditions. Changes in auditory brainstem response thresholds and waveform morphology preceded behavioral hearing loss onset. Loss of hair cells, auditory nerve fibers, and signs of stria vascularis degeneration were observed in old mice. The present work underscores the difficulty in ascribing the primary cause of age-related hearing loss to any particular type of cellular degeneration. Revealing these complex structure-function relationships is critical for establishing successful intervention strategies to restore hearing or prevent presbycusis.

Growth and cellular patterning during fetal human inner ear development studied by a correlative imaging approach

Background

Progressive transformation of the otic placode into the functional inner ear during gestational development in humans leads to the acquisition of hearing perception via the cochlea and balance and spatial orientation via the vestibular organ.

Results

Using a correlative approach involving micro-computerized tomography (micro-CT), transmission electron microscopy and histological techniques we were able to examine both the morphological and cellular changes associated with human inner ear development. Such an evaluation allowed for the examination of 3D geometry with high spatial and temporal resolution. In concert with gestational progression and growth of the cochlear duct, an increase in the distance between some of the Crista ampullaris is evident in all the specimens examined from GW12 to GW36. A parallel increase in the distances between the macular organs - fetal utricle and saccule - is also evident across the gestational stages examined. The distances between both the utricle and saccule to the three cristae ampullares also increased across the stages examined. A gradient in hair cell differentiation is apparent from apex to base of the fetal cochlea even at GW14.

Conclusion

We present structural information on human inner ear development across multiple levels of biological organization, including gross-morphology of the inner ear, cellular and subcellular details of hearing and vestibular organs, as well as ultrastructural details in the developing sensory epithelia. This enabled the gathering of detailed information regarding morphometric changes as well in realizing the complex developmental patterns of the human inner ear. We were able to quantify the volumetric and linear aspects of selected gestational inner ear specimens enabling a better understanding of the cellular changes across the fetal gestational timeline. Moreover, these data could serve as a reference for better understanding disorders that arise during inner ear development.

Electronic supplementary material

The online version of this article (10.1186/s12861-019-0191-y) contains supplementary material, which is available to authorized users.

Presbycusis: a human temporal bone study of individuals with downward sloping audiometric patterns of hearing loss and review of the literature

OBJECTIVE

The purpose of this retrospective case review was to identify patterns of cochlear element degeneration in individuals with presbycusis exhibiting downward sloping audiometric patterns of hearing loss and to correlate these findings with those reported in the literature to clarify conflicting concepts regarding the association between hearing loss and morphologic abnormalities.

METHODS

Archival human temporal bones from individuals with presbycusis were selected on the basis of strict audiometric criteria for downward-sloping audiometric thresholds. Twenty-one temporal bones that met these criteria were identified and compared with 10 temporal bones from individuals with normal hearing. The stria vascularis volumes, spiral ganglion cell populations, inner hair cells, and outer hair cells were quantitatively evaluated. The relationship between the severity of hearing loss and the degeneration of cochlear elements was analyzed using univariate linear regression models.

RESULTS

Outer hair cell loss and ganglion cell loss was observed in all individuals with presbycusis. Inner hair cell loss was observed in 18 of the 21 individuals with presbycusis and stria vascularis loss was observed in 10 of the 21 individuals with presbycusis. The extent of degeneration of all four of the cochlear elements evaluated was highly associated with the severity of hearing loss based on audiometric thresholds at 8,000 Hz and the pure-tone average at 500, 1,000, and 2,000 Hz. The extent of ganglion cell degeneration was associated with the slope of the audiogram.

CONCLUSIONS

Individuals with downward-sloping audiometric patterns of presbycusis exhibit degeneration of the stria vascularis, spiral ganglion cells, inner hair cells, and outer hair cells that is associated with the severity of hearing loss. This association has not been previously reported in studies that did not use quantitative methodologies for evaluating the cochlear elements and strict audiometric criteria for selecting cases.

Presbycusis: a human temporal bone study of individuals with flat audiometric patterns of hearing loss using a new method to quantify stria vascularis volume

OBJECTIVE

The purpose of this study was to determine the prevalence of stria vascularis atrophy in individuals with presbycusis and flat audiometric patterns of hearing loss. Individuals with presbycusis have historically been categorized by the shape of their audiograms, and flat audiometric thresholds have been reported to be associated with atrophy of the stria vascularis. Stria vascularis volume was not measured in these studies.

STUDY DESIGN

Retrospective case review.

METHODS

Archival human temporal bones from individuals with presbycusis were selected on the basis of strict audiometric criteria for flat audiometric thresholds. Six temporal bones that met these criteria were identified and compared with 10 temporal bones in individuals with normal hearing. A unique quantitative method was developed to measure the stria vascularis volume in these temporal bones. The hair cell and spiral ganglion cell populations also were quantitatively evaluated.

RESULTS

Only one of the six individuals with presbycusis and flat audiometric thresholds had significant atrophy of the stria vascularis. This individual with stria vascularis atrophy also had reduced inner hair cell, outer hair cell, and ganglion cell populations. Three of the individuals with presbycusis had spiral ganglion cell loss, three individuals had inner hair cell loss, and all six individuals had outer hair cell loss.

CONCLUSIONS

The results of this investigation suggest that individuals with presbycusis and flat audiometric patterns of hearing loss infrequently have stria vascularis atrophy. Outer hair cell loss alone or in combination with inner hair cell or ganglion cell loss may be the cause of flat audiometric thresholds in individuals with presbycusis.

OCP2 immunoreactivity in the human fetal cochlea at weeks 11, 17, 20, and 28, and the human adult cochlea

The two most abundant proteins of the organ of Corti, OCP1 and OCP2, are acidic, cytosolic, low molecular weight proteins diffusely distributed within the cytoplasm of supporting cells. A recent study by Henzl et al. (2001) found first, that these two proteins co-localize with connexin 26 along the epithelial gap junction system and second, that OCP2 could participate with OCP1 in an organ of Corti-specific SCF complex (Skp1, cul1in, and Fbp), a ubiquitin ligase complex. Previous study has also implicated OCP2 in the recycling and regulation of intracellular K(+) efflux as well as pH homeostatic mechanisms. In the present study, we document the emergence and distribution features of OCP2 through various stages (weeks 11-28) of gestation in human fetal cochleae. Four fetal cochleae, the cochleae of a normal hearing human adult and a mature rat for positive control were fixed in 4% formalin within 2 h post mortem. Immunohistochemical studies were performed using a rabbit polyclonal antibody raised against a synthetic peptide corresponding to amino acids 3-16. Specimens were mounted in paraffin sections. Results show that OCP2 immunoreactivity is evident at a prenatal age of 11 weeks, peaks in expression at the onset of cochlear function at 20 weeks and achieves adult-like patterns of distribution just prior to histological maturation at 28 weeks. Though this protein could be associated with the development, maturation, and electrochemical maintenance of the cochlear gap junction system, the nature of this protein's function in the developing and mature human cochlea remains unclear.

The role of micro-noise trauma in the etiology of aging-related changes in the inner ear

Eleven chinchillas between 1 and 2.4 years of age had the malleus/incus complex removed from one middle ear and then lived in the Washington University animal facilities for 4 years post-surgery. Each animal had one ear (termed ambient-noise) in which the conductive apparatus was intact; the other ear (termed noise-protected) had a 50-60 dB conductive hearing loss. The background sound level in the animal facility was 59 dBA with periodic brief sounds up to 102 dBA. After the 4-year experimental period, both ears were fixed, embedded in plastic and dissected for microscopic examination as flat preparations. The quantitative and qualitative findings in the noise-protected ears were compared to those in the ambient-noise ears. Both groups of ears sustained losses of sensory and supporting cells throughout the organ of Corti. A variable amount of age pigment was found to have accumulated in the outer hair cells and all supporting cells. In the noise-protected ears, inner hair cell loss ranged from 1.0 to 3.1% and averaged 1.7 +/- 0.8%; outer hair cell loss ranged from 1.8 to 6.4% and averaged 3.6 +/- 1.2%. In the ambient-noise ears, inner hair cell loss ranged from 0.7 to 2.8% and averaged 1.6 +/- 0.7%; outer hair cell loss ranged from 1.3 to 5.4% and averaged 3.6 +/- 1.2%. Within-animal comparison of cell losses in the noise-protected and ambient-noise ears revealed no significant difference between the two groups. It is concluded that long-term exposure to micro-noise does not accelerate the spontaneous loss of sensory cells which occurs with aging. Although not quantified, there was no obvious difference in the amount or cellular distribution of age pigment in the two groups. Thus, it appears that the formation of age pigment in the ear is the result of the cells' basic metabolic processes rather than the wear and tear from sensory transduction.

High-frequency air-conduction and electric bone-conduction audiometry. Age and sex variations

208 subjects representing both sexes and five age groups (15-70 years) were examined to obtain age-related threshold values for high-frequency (HF) electric bone-conduction (EBC) audiometry. The measurements also included conventional pure-tone audiometry and air-conduction (AC) HF (8-18 kHz) audiometry. The measured EBC thresholds were comparable to the values obtained with AC audiometers, and were equal to ISO standards at the frequencies of 0.5-6 kHz. The 15- and 20-year-old groups' EBC thresholds at 8 kHz were equivalent to thresholds of 15-year-old people from a cross-sectional material in Northern Finland. Thresholds deteriorated as a function of age, particularly in the HF range. The males had poorer thresholds than the females, especially in the age groups of 40 and 60 years. This could be attributed mainly to their greater noise exposure. The EBC method is quite practical and reliable for routine clinical measurements, but the dynamic range of the audiometer limits its use to relatively young subjects.

Spiral ganglion cell density in young and old gerbils

The Mongolian gerbil, like other mammalian species, has a decreased number of spiral ganglion cells as a function of age. This loss of cells was first seen in 24- to 30-month old animals in the basal end of the ganglion. In the oldest individuals the apical end of the ganglion was also affected. There were approximately 15-25% fewer cells in the affected areas in the 36- to 42-month old animals. In the oldest animals degeneration of the stria vascularis was seen in the apical turn and some degenerative changes in the organ of Corti were seen throughout the length of the cochlear duct. The aging pattern in the gerbil cochlea, is similar to that described for other species. Vacuoles, previously described in the gerbil cochlear nucleus, were also seen in the auditory nerve within the modiolus, but central to the Schwann-glial border in all animals. Vacuoles were not present within the spiral ganglion or the peripheral processes of the ganglion cells. Because the ganglion cell axons should be similar on either side of the Schwann-glial border, but the vacuoles were confined to the central nervous system, it is concluded that the degenerative process affects glial cells as opposed to neurons.

Ganglion cell and hair cell loss in Coturnix quail associated with aging

Hair cells and ganglion cells were examined in young adult (3 month old) and senescent (3 to 6 year old) quail (Coturnix coturnix). Following sacrifice the basilar papillae, along with the ganglia, were fixed, embedded in plastic and sectioned serially at 100 micron intervals from basal to apical tip. Hair cells and ganglion cells were counted from three micron thick sections at each interval. Hair cell number remained constant between age groups (less than 10% loss even in the oldest group). Ganglion cell number, on the other hand, was considerably reduced in the senescent birds (20-60% loss). These results are similar to quantitative results in senescent mammals and suggest that ganglion cell loss may be generalized response to aging.

Fetal hearing

The fetus can hear during the last trimester of pregnancy. Consistent responses to acoustic stimuli have been observed from 28 weeks onwards. Animal experiments as well as investigations in the human lead to the conclusion that sounds from outside the mother are attenuated, but rarely by more than 30 decibels; external conversations are audible. Only 30% of the phonetic information is available to the fetus, but intonation is almost perfectly transmitted to the amniotic sac. Evidence is accumulating that the mother's voice or different sound patterns from the same voice are learnt by the fetus. Thus there are indications that short-term auditory memory may be present by the end of pregnancy.

Effects of cochlear ablation on local cerebral glucose utilization in fetal sheep

Local cerebral glucose utilization was measured by the [14C]-deoxyglucose method in five near-term fetal sheep in whom bilateral ablation of the cochleae had been accomplished aseptically 5 to 8 days earlier. The tympanic membrane and ossicles were removed and all turns of each cochlea were unroofed with destruction carried to the modiolus. Mean local cerebral glucose utilization of 33 of 34 gray matter structures and four of four white matter structures in operated animals were significantly lower (p less than 0.05) than that in unoperated control fetuses. The depression in local cerebral glucose utilization was greatest (p less than 0.002) in brain stem auditory nuclei, in which the mean rate of glucose utilization was approximately 25% of the levels in unoperated fetuses. The pattern of glucose utilization in these structures was clearly altered, with a reversal of the normal distribution in density of the inferior colliculus. Tonotopic bands of high local cerebral glucose utilization frequently seen in autoradiographs of inferior colliculus in unoperated fetuses were not observed in operated fetuses. These results show that the glucose utilization of the brain, and by implication the normal growth and maturation of the brain, depends on an intact auditory system during prenatal life.

Hair cell counts in an age-graded series of rat cochleas

Hair cells of Sprague-Dawley rats aged 2-33 months were counted in order to assess the magnitude, location and time course of cell degeneration. The mean number of hair cell places (hair cells plus phalangeal scars) was approximately 4700: 960 inner hair cell places and 3470 outer hair cell places. These numbers do not vary systematically with age. Hair cell degeneration was observed in all animals. At 31-33 months of age, animals had inner hair cell losses ranging from 1.6 to 4.2% and outer hair cell losses ranging from 2.1 to 23.3%. The loss of hair cells was greatest in the upper apex, where the 31-33-month-old animals had 3.1-9.2% inner hair cell losses and 7.4-46.8% outer hair cell losses. Outer hair cell losses were also large in the basal end, where inner hair cell losses were small. In the older animals, hair cell losses were consistently most prominent in the third row of outer hair cells. Following examination of the hair cell population, the ganglion cells in the apical region were evaluated in a number of cochleas. No significant correlation was found between the magnitude of inner hair cell and ganglion cell losses.

Spiral ganglion cell counts in an age-graded series of rat cochleas

Spiral ganglion cells in the cochleas of Sprague-Dawley rats in various age groups were counted in order to assess the extent and location of cell degeneration with age. Cells in every tenth section of serially sectioned plastic embedded cochleas were counted in the light microscope. The median cell number for the 1- to 2-month-old animals was 15,800 cells. This number was first seen to be significantly reduced (-14%) in the 23-month-old animals. At 27 to 29 months the ganglion cell number was reduced by 20%, while at 33 to 34 months there was a 17% loss. Losses were found throughout the length of the ganglion with the greatest losses at the lower basal and apical ends. In the oldest group, these losses amounted to 28% and 33%, respectively. Type II ganglion cells first showed a significant decrease in number in the 27- to 29-month-old group, when a 32% loss was seen. The same loss was seen in the 33- to 34-month-old group. Unlike the type I cells which are lost throughout the length of the ganglion, type II cells were not significantly reduced in number at the basal end, but decreased by as much as 42% in the middle and apical regions.

Organ of Corti in the human fetus: scanning and transmission electronmicroscope studies

The organ of Corti in the five-month human fetus was studied by transmission and scanning electronmicroscopy. Differentiation of the surface organization of the organ of Corti into a single row of inner and three to four rows of outer hair cells was complete at this stage except at the apical end. The morphological aspects of the hair bundles changed with maturation of the sensory cells; the inner hair cells preceded the outer hair cells in cytodifferentiation at a given location.

Release from masking as a means of studying hair cell function

If mutual inhibition exists among the neural elements of the auditory pathway, then tonal contours should be enchanced by analogy with the mechanisms of Mach bands in vision. Experimental evidence is presented for enhancement or sharpening of contours by means of direct masking of pure tones by narrow-band noise. These results suggested that sharply defined regions of loss of hair cells in sharply delimited regions of the basilar membrane ought to be associated with gains in hearing level due to release from inhibition. Support for this notion was found in both the experimental and clinical literature.