Local vs systemic effects of acoustic trauma on cochlear structure and transport

Whether certain effects of acoustic trauma on cochlear structure and function were due to local or systemic agents was investigated in the chinchilla. After unilateral ossicular disarticulation, the animals were given a noise exposure known to cause cochlear damage and vessel transport changes in the stria vascularis. The cochleas with disarticulated ossicular chains received an effective exposure well below one that causes pathologic damage. Only the cochleas with intact ossicular chains showed an increase in the permeability of the strial vessels to horseradish peroxidase tracer and manifested strial edema and organ of Corti damage. These results indicate that the effects of acoustic trauma on cochlear structure and strial vessel transport are induced locally, not systemically. In addition, the minimum time of exposure to 120-dB, 700- to 2800-Hz noise that causes massive cochlear damage in the chinchilla was shown to be between five and ten minutes.     

Reduction in Sharpness of Frequency Tuning but not Endocochlear Potential in Aging and Noise-Exposed BALB/cJ Mice

Schuknecht proposed categories for human age-related hearing loss (ARHL) based upon whether the primary degeneration involves the organ of Corti (sensory ARHL), spiral ganglion cells (neural), stria vascularis (strial), or a combination of these (mixed). Genetically standardized mouse ARHL models can help validate Schuknecht's framework and clarify the underlying cellular processes. Much recent work has focused on the mouse Ahl locus, which promotes both ARHL and noise-induced hearing loss. On the C57BL/6 inbred background, Ahl has been associated with degeneration of organ of Corti, afferent neurons, and stria vascularis/spiral ligament, suggesting that it promotes mixed (sensory/neural/strial) ARHL. Some cochlear degeneration in C57BL/6 mice could be caused by genes other than Ahl, however. The question of what constitutes Ahl-related pathology can be addressed by comparing C57BL/6 mice with other strains that carry the same allele, including BALB/c substrains. We examined the effects of aging and broadband noise exposure in inbred BALB/cJ mice (1.5–13.0 mos) using measures of frequency tuning (compound action potential tuning curves) (CAPTCs), strial function (endocochlear potential recording, EP), and light microscopy. Aging and noise led to generally similar physiological and anatomical changes. Reductions in sensitivity and sharpness of frequency tuning were not consistently linked to hair cell loss, reduction in the EP, or changes in the lateral wall. Instead they appeared best explained by alterations in supporting cells in the basal half of the cochlear and in the spiral limbus in the apex. These results emphasize the importance of cell types other than hair cells in cochlear pathology. They also indicate that Ahl does not necessarily promote a strial form of ARHL.     

Ototoxicity of neomycin and polymyxin B following middle ear application in the chinchilla and baboon

Previous experimental studies have demonstrated structural damage of the organ of Corti and stria vascularis following application of combination antibiotic otic drops to the middle ear. In this investigation the ototoxic effects of neomycin and polymyxin B (two antibiotics often used together in ototopical preparations) were separately evaluated after administration of each agent to the middle ear cavities of chinchillas and baboons. The antibiotics were administered in saline solution at the same concentrations used in Cortisporin Otic Suspension (3.5 mg/ml neomycin base, 10,000 units/ml polymyxin B). In both the rodent and primate, polymyxin B consistently produced greater cochlear damage than did neomycin. In fact, the extent of hair cell loss and strial injury produced by polymyxin B alone was, in many cases, comparable to that previously observed after application of Cortisporin Otic Suspension itself. Hair cell loss in the baboon was markedly less severe than in the chinchilla. It is believed that differences in position and structure of the round window membrane are important factors in the differing levels of ototoxicity observed in the rodent and primate.     

Study of the mechanism of supporting cells repairing the organ of Corti in terms of cell kinetics--nuclear DNA synthesis of supporting cell of the organ of Corti in the cochlea damaged by nitromin administration

3H-thymidine autoradiography in vivo was carried out on normal mature mice, whose organs of Corti were securely damaged by Nitrogen Mustard-N-oxide (nitromin). The cell kinetics of supporting cells of the organ of Corti were examined and relation between these changes and mechanisms of supporting cells to maintain the organ of Corti was discussed in this paper. A few grains in various kinds of supporting cells in S stage, which were not found in control mice without nitromin injection, were discovered. Those were detected in Hensen's cell, Deiters' cell, Claudius' cells, and inner phalangeal cell. Mitosis was seen in one Claudius' cell in addition. Therefore some supporting cells seem to be able to synthesize DNA and proliferate in the acutely damaged organ of Corti even in mature cochlea and those in G0 stage begin to go around cell cycle as occasion demands. Although labelled supporting cells decreased over time, the portion of supporting cell is likely to change in proportion as the extension of damage in the organ of Corti. If this new dynamic change of cell kinetics in the acutely damaged organ of Corti means the reaction to repair the organ of Corti, the supporting cells seem to have a reasonable role to maintain the organ of Corti including repairment of reticular lamina.     

Time sequence of degeneration pattern of the organ of Corti after acoustic overstimulation. A transmission electron microscopy study

Female pigmented guinea pigs were exposed to a 3.85 kHz pure tone of 120 dB SPL for 22.5 min. The exposed animals were sacrificed after varying post-exposure intervals (5 min, 4 and 24 h, 5 and 28 days). Transmission electron microscopical examination was performed after serial-sectioning in areas of maximum damage and both basal to and apical to this region. By serial-sectioning, the degradation pattern of the individual cell structures in the organ of Corti could be studied in relation to post-exposure time and proximity to the well-demarcated area of maximum damage in the organ of Corti. The results provide further information on the non-reversible changes leading to degeneration and loss of sensory and supportive structures in the organ of Corti.     

Cisplatin ototoxicity in guinea pigs with special reference to toxic effects in the stria vascularis

The toxic effects of cisplatin (cis-diamminedichloroplatinum [II]) on the organ of Corti are well established. Few and conflicting data on this drug's effects on the stria vascularis exist. The present study presents animal experiments on the toxic effects of cisplatin in the stria vascularis and in the organ of Corti. Cisplatin-induced toxicity in albino and pigmented guinea pigs was evaluated morphologically and functionally, using light and transmission electron microscopy as well as auditory brainstem-evoked potentials on the organ of Corti and the stria vascularis. The results showed variability in hearing thresholds, ranging from no change to hearing loss of 30 dB, and prominent damage in the organ of Corti and in the stria vascularis. The toxic effects to both the organ of Corti and the stria vascularis should be considered when cisplatin is used in chemotherapy.     

Temporal bone histopathology in alport syndrome

OBJECTIVE: To determine the histopathologic abnormalities within the cochlea in Alport syndrome. BACKGROUND: Alport syndrome, which manifests as hereditary nephritis and sensorineural hearing loss (SNHL), is caused by mutations in genes that code for the proportional, variant3, proportional, variant4, and proportional, variant5 chains of type IV collagen. The proportional, variant3, proportional, variant4, and proportional, variant5 chains of type IV collagen are present in the basement membrane of the organ of Corti. Previous temporal bone studies have failed to identify histopathologic correlates for the SNHL. METHODS: We examined temporal bones from nine individuals with a clinical diagnosis of Alport syndrome. One of our cases also had genetic testing that showed a mutation in the type IV collagen proportional, variant5 chain gene. RESULTS: By light microscopy, eight of nine cases demonstrated two unique pathologic changes: 1) a "zone of separation" between the basilar membrane and overlying cells of the organ of Corti and 2) presence of cells filling the tunnel of Corti and extracellular spaces of Nuel. The cytologic losses of hair cells, stria vascularis, and cochlear neuronal cells were insufficient to account for the observed SNHL in our cases. Electron microscopy was performed in four cases; all four demonstrated the following: 1) the zone of separation that was observed at light microscopy occurred between the basement membrane and the basilar membrane, 2) the cells within the tunnel of Corti and spaces of Nuel were morphologically similar to supporting cells, and 3) the basement membrane of strial capillaries and the spiral vessel (under the basilar membrane) were normal. CONCLUSIONS: The histopathologic correlates of cochlear involvement in Alport syndrome are abnormalities of the basement membrane of cells of the organ of Corti and dysmorphogenesis (cellular infilling of the tunnel and extracellular spaces) of the organ of Corti. We hypothesize that these abnormalities result in SNHL by altering cochlear micromechanics.     

Triamcinolone acetonide protects auditory hair cells from 4-hydroxy-2,3-nonenal (HNE) ototoxicity in vitro

CONCLUSION: Triamcinolone acetonide crystalline suspension (e.g. Volon A) was not ototoxic to the auditory hair cells present within organ of Corti explants and protected them from an ototoxic molecule, i.e. 4-hydroxy-2,3-nonenal (HNE), that is produced within the organ of Corti as a result of oxidative stress-induced damage. OBJECTIVES: To test the corticosteroid, triamcinolone acetonide, for ototoxicity and otoprotective capacity in organ of Corti explants. MATERIALS AND METHODS: Organ of Corti explants excised from 4-day-old rats were the test system, HNE was the ototoxin challenge. Hair cell integrity counts were performed with fluorescent microscopy on fixed explants stained with FITC-labeled phalloidin. Statistical significance was set at p<0.05. RESULTS: Triamcinolone acetonide did not affect hair cell integrity in the organ of Corti explants and it provided a high level of protection of hair cells against the ototoxic effects of a damaging level of HNE as determined by hair cell density counts.     

J.J. Groen 1912-1973

In the hospital for pulmonary tuberculosis in children, a group of 975 children treated with streptomycin sulphate was audiologically examined and in 36% of children ototoxic lesions were defined. From these, in 24% the damage affected only the vestibular labyrinth, in 8% both the vestibular labyrinth and the organ of Corti and in 4% the organ of Corti alone     

The ototoxic potential of EMLA. A new local anesthetic for the tympanic membrane

Instillation of EMLA, a new local anesthetic, into the middle ear of the guinea pig caused severe morphological damage to the organ of Corti in the first 4 mm from the round window. Further up the cochlea, only derangements of the stereocilia were found. The extent of morphological damage was the same, whether the agent was administered once or several times. The ototoxic potential of EMLA was obvious and is probably due to direct damage in areas where present in high concentrations. Over a short distance of approximately 0.1 mm there is a transition from a total destruction of the organ of Corti to a completely normal morphology.     

Strategies for constructing a guinea pig organ of Corti cDNA library and its potential use.

Mutations of genes common to several tissues or organs can lead to cellular damage, which may result in hearing impairment as part of a syndromic disorder. Mutations of genes that are unique to the organ of Corti would have a high probability of causing nonsyndromic hearing impairment. It is expected that such genes are involved in auditory transduction as well as in maintaining specific hair cell and supporting cell functions in the organ of Corti. Cloning and describing genes involved with nonsyndromic hearing impairment thus require the construction of a guinea pig cDNA library of the organ of Corti.     

Time sequence of degeneration pattern of the organ of Corti after acoustic overstimulation. A light microscopical and electrophysiological investigation in the guinea pig

In order to obtain information about the sequence of events leading to degenerative changes in the cochlea after acoustic trauma, a systematic LM study was performed in the guinea pig organ of Corti. The results were correlated with measurements of CAP N1 thresholds from the same animals. Twenty pigmented guinea pigs were used in the study. Ten of these animals were exposed to a 3.85 kHz pure tone with an intensity varying between 108 and 120 dB SPL for 22.5 to 360 min and were sacrificed after 4 weeks. Eight animals were exposed to 120 dB SPL for 22.5 min and sacrificed at regular intervals after exposure, viz. 5 min, 4, 24 h and 5 days. Prior to sound exposure the animals in the 4-week group were implanted with a permanent round window electrode for measurements of CAP N1 thresholds. The study indicates that the degeneration and reparative processes are not entirely completed after 4 weeks. The development of holes in the reticular lamina may result in further degeneration of cells bilateral to the initial maximum damage. Darkly stained cells seem to be activated in the reparative process in the organ of Corti. Local appearance of dendritic macrophages in the organ of Corti indicate the existence of a local mechanism for disposal of degeneration products from the organ of Corti. The retrograde degeneration of radial nerve fibres seems to be closely associated with that of the pillar cells. A close correlation between structural hair cell damage and CAP N1 threshold shift was found in the region of the organ of Corti corresponding to the exposure frequency. The TS 5 min, 4 h and 24 h after exposure showed a close correlation with swellings in the afferent nerve endings below the IHC in the area of maximum damage and basalwards.     

Triamcinolone acetonide protects auditory hair cells from 4-hydroxy-2,3-nonenal (HNE) ototoxicity in vitro

Abstract

Conclusion. Triamcinolone acetonide crystalline suspension (e.g. Volon A®) was not ototoxic to the auditory hair cells present within organ of Corti explants and protected them from an ototoxic molecule, i.e. 4-hydroxy-2,3-nonenal (HNE), that is produced within the organ of Corti as a result of oxidative stress-induced damage. Objectives. To test the corticosteroid, triamcinolone acetonide, for ototoxicity and otoprotective capacity in organ of Corti explants. Materials and methods. Organ of Corti explants excised from 4-day-old rats were the test system, HNE was the ototoxin challenge. Hair cell integrity counts were performed with fluorescent microscopy on fixed explants stained with FITC-labeled phalloidin. Statistical significance was set atp <0.05. Results. Triamcinolone acetonide did not affect hair cell integrity in the organ of Corti explants and it provided a high level of protection of hair cells against the ototoxic effects of a damaging level of HNE as determined by hair cell density counts.     

Cryoprobe-induced apical lesions in the chinchilla. I. Morphological effects of lesioning parameters

To create experimental lesions localized to the low frequency region of the organ of Corti, a cryoprobe was applied to the apical area of 37 cochleas from 26 adult chinchillas. Twenty cochleas were exposed to single applications of a cryoprobe for 2.5, 3.0 and 3.5 min; 17 cochleas were exposed to two applications of 1.5, 2.0 and 3.0 min each with about a 3 min interval between applications. Cryoprobe tip temperature rose from about -140 degrees C when placed on the apex to about -80 degrees C after a continuous 3.5 min application. Survival time after lesioning was from 2 to 75 days, with most being 12 days or less. All cochleas except one sustained regions of damage characterized by complete absence of the organ of Corti and by missing hair cells indicated by extensive scarring. Inner hair cells were less susceptible to damage than were outer hair cells. Well-defined lesions which were continuous over the apical organ of Corti were found in some cochleas exposed to single probe applications, but such applications more often resulted in lesions which had areas of less damage. Of the various application protocols used, two applications of 1.5 min each, with an interval to allow the tissue to warm, most consistently produced severe and discrete apical lesions. In 9 of 13 cochleas exposed to two 1.5 min probe applications, such lesions extended about 35% or less of the distance from the apex. In most cochleas, regardless of the severity of the apical lesion, the pattern of hair cell rows and stereocilia configuration appeared normal in the basal 40-50% of the organ of Corti.     

新霉素损伤后小鼠耳蜗毛细胞的改变[论著]

目的　观察新霉素对小鼠听功能及耳蜗毛细胞形态学变化的影响。方法　选取C57BL/6小鼠16只在出生后第8天开始连续10 d皮下注射硫酸新霉素，对照组（10只）皮下注射等量生理盐水。停药后1、4、8、12 w进行听性脑干反应（ABR）检测。每次ABR检测后，新霉素组随机取3只小鼠，对照组随机取2只小鼠，处死后，取耳蜗进行冰冻切片，用免疫荧光方法观察耳蜗Corti器及毛细形态学变化。结果　新霉素可造成小鼠耳蜗毛细胞严重损伤，且小鼠ARB阈值显著增高，不可恢复；新霉素对耳蜗Corti器毛细胞的损伤顶圈最轻，中圈次之，底圈最重，且随着时间推移Corti器形态结构破坏逐渐加重且不可自我修复。结论　硫酸新霉素造成小鼠Corit器毛细胞的损伤是不可逆的。     

Rapid Hair Cell Loss: A Mouse Model for Cochlear Lesions

In comparison to other mammals, mice have proved extremely resistant to aminoglycoside-induced hair cell ablation in vivo. In this paper we examine the pattern and extent of cochlear lesions rapidly induced with a combination of a single dose of aminoglycoside (kanamycin) followed by a loop diuretic (bumetanide). With this protocol, the vestibular system was unaffected, but in the cochlea, there was extensive loss of outer hair cells (OHC) that commenced in the basal coil and progressed apically so that, by 48 h, OHC loss was almost complete. TUNEL-positive nuclei and activated caspase-3 labeling demonstrated that most OHC died via a classical apoptotic pathway. However, scattered debris within the OHC region suggested that many apoptotic cells ruptured prior to completion of apoptosis. Following lesion repair, supporting cells retained characteristics of differentiated cells but positional shift occurred. In comparison to OHC loss, inner hair cell (IHC) death was delayed and only observed in 50% of all cochleae examined even after extensive reorganization of the tissue. The coadmininstration of diuretic with FM1-43, used as a tracer for aminoglycoside uptake, indicated entry into IHC as readily as OHC, suggesting that the differential response to aminoglycoside was not due to differential uptake. Where IHC death was ongoing, there were indications of different modes of cell death: cells with morphological features of autophagy, necrosis, and apoptosis were apparent. In addition to damage to the organ of Corti, there was a significant and progressive decrease in strial thickness beginning as early as 7 days posttreatment. This was due predominantly to degeneration of marginal cells. The strial pathology resembled that reported after noise damage and with aging. This in vivo protocol provides a robust model in which to obtain extensive OHC loss in the mature cochleae of mice and is a means with which to examine different aspects of cochlear pathology in transgenic or mutant strains.     

Pathologic features of the inner ear in congenital deafness.

We present the morphologic findings of the temporal bones and brain of a patient with congenital deafness. We discuss these findings in relation to pathologic observations in other reported cases of congenital deafness. Morphologic abnormalities in the patient were mainly in the pars inferior of the membranous labyrinths. The osseous labyrinths were well developed. There was severe dilation of the cochlear duct with herniation of the Reissner membrane, extensive atrophy of the stria vascularis that was associated with calcified thrombi to the strial vessels, encasement of the tectorial membrane in a syncytium, and dyspiastic or regressive degeneration of the organ of Corti. Absence of spiral ganglion cells and their fibers was a prominent feature. The extensive and varied pathologic changes that were present in our patient simultaneously suggest a congenital abnormality in endolymph production and raise the possibility of anomalous development of the labyrinthine vasculature.     

Identification of factors that maintain mammalian outer hair cells in adult organ of Corti explants

Both outer hair cells (OHCs) and inner hair cells (IHCs) survive and mature in 3 days old rat organ of Corti explants cultured for 1 month in a minimal essential medium. In contrast, under the same culture conditions, only IHCs survive in explants from adult guinea pig organ of Corti while many of the OHCs are lost within the first 48 h. Hair cell counts show OHCs loss to be greater in the lower portion (i.e. middle turn) of the cochlea than at the apex. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) indicates that there is DNA damage in adult OHCs within 8 h of explantation. Treatment of the adult organ of Corti explants with either actinomycin D (10⁻⁷ M) or cycloheximide (10⁻⁶ M) prevents most OHC losses. According to these results apoptosis may be the mechanism of OHC loss in adult organ of Corti explants. Stable membrane potentials recorded from the OHCs in both uncultured and actinomycin D-treated organ of Corti explants cultured for 72 h demonstrate the functional integrity of these hair cells. OHC losses in the adult guinea pig organ of Corti cultures can also be prevented by treatment with several of the growth factors tested, i.e. acidic fibroblast growth factor (aFGF), insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), transforming growth factor-β1 (TGF-β1), and glial cell-derived neurotrophic factor (GDNF). The results of this study suggest that growth factor therapy may be applicable to the treatment of some hearing disorders.     

Development of the human stria vascularis.

Fifteen human fetal cochleas were investigated by light microscopy and transmission electron microscopy in order to observe the development of the stria vascularis. The earliest signs of strial cell differentiation take place during the 11th week of gestation. Subsequently, the first stages of the stria vascularis development occur quickly. At week 14 the three types of cells, namely, marginal, intermediate and basal cells are discernable. Moreover at this stage, signs of specific activity are already present. The adult-like appearance of the stria vascularis is reached by week 21 but its maturation is completed only during the last trimester of pregnancy. This is in good agreement both with the development of the organ of Corti structures and with the maturation of the human auditory function.