Effect of high-intensity noise on inner ear sensory epithelia

Pronounced infracuticular endocytosis in mammalian outer hair cells

Endocytosis in cochlear hair cells was investigated by staining with the vital fluorescent dye FM 1-43, that partitions reversibly into membranes and is trapped in vesicles during endocytosis. The temporal development and spatial distribution of FM 1-43 induced fluorescence was investigated using confocal laser-scanning microscopy. FM 1-43 rapidly and intensely stained cochlear hair cells, leaving the supporting cells unstained. For short application (0.2-30 s), only the infracuticular region of outer hair cells (OHCs) was labeled, whereas for long application (30-60 s), the OHCs were also labeled in the infranuclear zone and along a central strand extending from the infracuticular zone down to the nucleus, as well as along the entire cell membrane. Except for the cell membrane, the infracuticular zone, directly below the cuticular plate, showed the most rapid and intense staining, and in most cases staining was spherically shaped with a diameter of 3-7 microm. Localization and size of this infracuticular staining coincided with Hensen's body, a specialized variant of the endoplasmic reticulum. In contrast to the OHCs, apical fluorescence of inner hair cells presented a homogeneous distribution. When OHCs were incubated in FM 1-43 for longer than 1 min, many points of contact between the central strand, the infracuticular zone and the lateral cell membrane were observed. Since Hensen's bodies are a specialty of OHCs and the fluorescent staining pattern of these cells was unique, it is proposed that Hensen's body is involved in the turnover of OHC-specific proteins, such as those involved in the molecular machinery of the motor action of the plasma membrane.

Ultrastructural changes in the hydropic cochlea of the guinea-pig

Transmission electron microscopy of the cochlear organ of Corti in experimental endolymphatic hydrops revealed two principal features. Starting 1 month after induction of hydrops, osmiophilic inclusions thought to represent lipofuscin accumulation were frequently observed in the subcuticular cytoplasm of the outer hair cells along the length of the cochlea. Starting 3 months after induction of hydrops the efferent terminals on the outer hair cells appeared to be vacuolated. These data suggest that oxidative insult is likely to contribute to the pathology associated with endolymphatic hydrops and thus that free radical scavengers might be useful in the treatment of Menière's disease patients. In addition the early modification of the efferent innervation of the hydropic cochlea might underlie the known hypersensitivity to various insults, including noise stimulation, glycerol administration and hypoxia.

Ultrastructural changes in the presynaptic region of outer hair cells after acoustic stimulation

Protection against noise trauma results by sound-conditioning animals to a low-level, long-term acoustic stimulus prior to a damaging exposure. It is known that the outer hair cells are selectively protected by sound-conditioning. The aim of the present study was to determine if the intrinsic properties of the outer hair cell had been modified by sound-conditioning. A stimulus-related increase in the vesicle content in the presynaptic region was found. It is suggested that the outer hair cells are involved in sound conditioning and that changes in the presynaptic region can be one correlate to the protection against noise trauma by sound-conditioning.

Height changes in the organ of Corti after noise exposure

To determine whether or not exposure to noise causes an alteration in the height of the organ of Corti (OC), 16 cochleas which had been exposed for one or two hours to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 108 dB were examined microscopically as whole mounts. These specimens were divided into four groups: early ears (N = 3) recovered less than 0.6 hours following the exposure; intermediate ears (N = 5) recovered 0.6-4.0 hours; 1-day ears (N = 3) recovered 24 hours; and late ears (N = 5) recovered 2-21 days. Height was measured at three positions across the OC and at multiple percentage locations from apex to base. The OC-height data from the noise-exposed cochleas were compared statistically to those from ten control cochleas. A significant reduction (P < or = 0.01) in OC height at the third outer hair cell (OHC) was first evident in the early ears in the region 65-95% distance from the apex. The height was reduced even further in the intermediate ears and included a region from 15-25% distance from the apex as well as the 65-95% region. In the late ears, heights had returned to control values, except within focal OC lesions. Height at the first row of OHCs was less affected than at the third row, and height at the inner hair cell (IHC) was least affected. These height changes were accompanied by distortion of the shape and position of OHCs, the shape of Deiters' cells and buckling of inner and outer pillar bodies. Sometimes IHCs had distorted shapes and were displaced from their usual positions. Although no functional measures were obtained from these ears, data from the literature indicate that the exposure described above would have produced a sizable threshold shift. Transient reduction in OC height likely accounts for some portion of noise-induced threshold shifts.

Cochlear damage and increased threshold in alpha-difluoromethylornithine (DFMO) treated guinea pigs

Alpha-difluoromethylornithine (DFMO) inhibits polyamine synthesis and is used as an antineoplastic and antiparasitic drug. In early human trials DFMO unexpectedly caused a sensorineural hearing loss. In the current study DFMO was administered to guinea pigs to investigate its effects on organ of Corti histology and on auditory thresholds. Histologic examination revealed that DFMO caused greatest damage in the hook and first turn. Damage in the second and third turns was minimal. Animals treated for 12 weeks with DFMO differed significantly (P less than 0.05) from controls in the hook and first turn in that: 1) DFMO caused a loss of hair cells in all rows. Loss of inner hair cells was greater than that of outer hair cells. 2) The remaining outer hair cells were shorter and contained a greater number of Hensen bodies. 3) The Deiters' cell bodies were longer and this increased length was associated with the decreased length of the corresponding outer hair cells. Brainstem audiometry showed that DFMO produced a hearing loss and the magnitude of this loss increased over twelve weeks of DFMO administration.

Morphologic effects of glycerol and urea on cochlear tissues of the chinchilla

Glycerol and urea are used as test agents in confirming the diagnosis of endolymphatic hydrops. Although both substances act as osmotic diuretics, recent evidence suggests that they may have differing physiologic effects on the inner ear. This study was designed to compare the morphologic effects of urea and glycerol on cochlear tissues, using the chinchilla as an experimental model. Animals were given subcutaneous injections of glycerol (2 g/kg) or urea (1.2 g/kg) over periods of 3 hours, 4 days, or 1 week. Both agents were found to produce ultrastructural changes, including spiral ligament vacuolization, intracellular alterations of the stria vascularis, and increased numbers of Hensen's bodies in outer hair cells. These alterations appeared indicative of metabolic stress, but not toxicity. The morphologic findings provided no evidence that glycerol and urea affect the inner ear by fundamentally different mechanisms of action.

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.

Overview of mechanical damage to the inner ear: noise as a tool to probe cochlear function

The majority of experiments causing mechanical damage to the cochlea involve the use of sound pressure waves to cause overstimulation. This presentation is an overview of the research during the past years on the structural damage produced by noise. The effect of noise on the cochlea depends on the type of noise exposure-impulse or continuous. Experiments have been conducted to determine the effect of increasing intensity, the effect of increasing duration, and the effect of equal energy presented over varying periods of time. The initial mechanism of damage, the progression of damage over time, and the ability of hair cells to recover are discussed. Noise has been used as a tool to probe cochlear function by selectively damaging regions along the length of the sensory epithelium and by selectively damaging one of the two types of hair cells. Results obtained from these types of experiments have given us information on cochlear mechanics, as well as of stereocilia micromechanics and transduction. Information on susceptibility of hair cells to noise confirms previous results, suggesting the presence of structural and metabolic gradients both longitudinally and radially within the sensory epithelium. Moreover, noise lesions have been used to map the afferent innervation pattern to the cochlear nucleus, and noise studies show correlation of hair cell damage with efferent innervation pattern.

Experiments on cochlear cryosurgery in the guinea pig

A method to induce experimental cochlear lesions with cryosurgery is described. Using temperatures on the cochlear surface ranging from 0 degrees C-50 degrees C, lesions were made from the apical turn and down to 8 mm from the round window area. There were no signs of labyrinthine fistula and the remaining parts of the organ of Corti were normal in appearance. The correlation of intracochlear temperature with the spread of anatomical damage is described. It is suggested that this method can be used to achieve non-invasive selective cochlear lesions. With this method it is probably possible to induce sensory-neural lesions not only in the high frequency region of hearing, but also in the middle and low frequency regions.

Cochlear hair cell and vascular changes in the guinea pig following high level pure-tone exposures

Noise is thought to exert metabolic and/or mechanical stress on sensory and vascular tissues of the cochlea, the relative influence of the stressors being influenced by the intensity of the noise. Guinea pigs exposed to either of two pure-tone frequencies, 1.33 or 3.85 kHz for 6 hours at intensity levels ranging from 102 dB to 120 dB SPL, were studied for pathological changes in two spiral lamina vessels--the vessel of the basilar membrane (VSBM) and the vessel of the tympanic lip (VSTL). In general, animals sustaining mild to severe degrees of hair cell destruction one month after noise exposure showed little vascular change in the vessels studied. With respect to the vasculature, the concept of a 'critical level' seems to be dependent on exposure frequency, in that only above 117 dB SPL at 3.85 kHz was there any change in the pattern of damage to the spiral lamina vessels.

Experiments on cochlear cryosurgery in the guinea pig. Light and surface microscopy

The present investigation indicates that with cryosurgery it is possible to produce well defined cochlear lesions. The point of complete ablation can be chosen at will. A great advantage also is the fact that the bony capsule of the labyrinth is left intact. The remarkably slight scarring caused as well as the morphologically intact walls of the cochlear duct indicate an effective healing process, already completed after 6 days. These findings explain the unsuccessful clinical experiences when trying to ablate the labyrinth completely or to induce labyrinthine fistula with the help of cryosurgery (House, 1966). We suggest that for studies on cochlear function after exogenic trauma, cryosurgery gives easy reproductive morphological changes that may help the understanding and correlation of hair cell loss with cochlear physiology.

Giant cilia in the human organ of Corti

Fusion of cilia, the growth of clumps of fused cilia and giant cilium formation have been studied in the normal human organ of Corti using the scanning electron microscope. These unusual forms are found mainly in the apical portions of the cochlea and appear to precede the loss of normal apical cilia which increases and extends in a basal direction with age. These changes may be due to low frequency noise damage or be a phenomenom of ageing. The mechanism of their formation is discussed in the light of recent experimental work on cell fusion.

Cochlear transduction: an integrative model and review

A model for cochlear transduction is presented that is based on considerations of the cell biology of its receptor cells, particularly the mechanisms of transmitter release at recepto-neural synapses. Two new interrelated hypotheses on the functional organization of the organ of Corti result from these considerations, one dealing with the possibility of electrotonic interaction between inner and outer hair cells and the other with a possible contributing source to acoustic emissions of cochlear origin that results from vesicular membrane turnover.

Sensitive period to acoustic trauma in the rat pup cochlea. Histological findings

In a previous paper we demonstrated physiologically a sensitive period for acoustic trauma in the rat pup cochlea, with a maximum at 22 days of age. Seven or 60 days after noise exposure these 22-day exposed rats were used for light and electron microscopy. At 7 days, surface preparation revealed restricted damage to the basal cochlear coil, while with electron microscopy signs of cytoplasmic degeneration appeared in the great majority of basal coil structures. Two months after noise exposure these structures have completely degenerated. These results were discussed in terms of mechanical versus metabolic explanations of the critical period for acoustic trauma.

Neural damage in the guinea pig cochlea after noise exposure. A light microscopic study

Neural damage produced by exposure to 8 kHz octave-band noise was studied by light microscopy in guinea pigs. Abnormal nerve endings were found beneath both inner and outer hair cells within 24 hours after exposure at either 118 or 120 dB SPL. In areas where the organ of Corti was destroyed degeneration of myelinated nerve fibers was seen immediately after 24-hour exposures. Nerve degeneration secondary to hair cell loss was observed. However, degeneration of myelinated fibers occurred in some cases without significant hair cell loss. At survival times as long as two months nerve fibers coursing near capillaries were still present in extensively damaged areas. In three animals nerve fibers which had apparently regenerated into regions where the sensory epithelium was destroyed were found distal to the habenula perforata. Evdence of regenerative activity was also noted in the osseous spiral lamina.

The cytochochleogram in atoxyl-treated guinea pigs

The earliest atoxyl induced changes in the cochlea appeared in the upper and medial parts of the 4th coil, whence the changes spread progressively downwards towards the round window, the extent of the changes depending on the amount of atoxyl administered and the duration of the treatment. The inner hair cells were more resistant to the effects of atoxyl than the outer hair cells which were affected first. The sensory cells in the 2nd and 3rd rows appeared more sensitive than the outer hair cells in the first row.

Toxic effects of gentamicin on the basilar papilla in the lizard Calotes versicolor. A surface study

Gentamicin in a dose of 100 mg and in some cases 150 mg per kg bodyweight and day was given intraperitoneally to healthy lizards, belonging to the species Calotes Versicolor. The animals were injected for 7, 14 and 21 days. After completed injections the animals were sacrificed and their hearing organ, the basilar papilla, was processed for scanning electron microscopy. Animals treated for 7 days did not show any significant surface damage in the basilar papilla. When gentamicin was administered for 14 days the normal appearance of the surface structure was lost. The ventral (apical) type A cells were relatively intact while the type B cell-population in the dorsal (basal) part of the organ showed sensory hair fusions and cytoplasmic herniations. Lizards treated for 21 days showed a severely damaged basilar papilla. The ventral (apical) type A cells still only were moderately damaged with some hair fusions and cytoplasmic herniations while the dorsal (basal) type B cells were more or less destroyed. Only occasional cells were left and some of these were severely damaged. The surface of the dorsal (basal) part of the organ instead was covered by supporting cells thus forming a sort of scar tissue.

Cochlear potentials and electron microscopy applied to the study of small cochlear lesions

Many investigators have attempted to define the relations among behavioral auditory thresholds, cochlear potentials and cochlear pathology after excessive exposure to noise. We have observed that some noise exposed chinchillas have fewer missing sensory cells yet lower cochlear potentials than nonexposed controls. By comparing the cochleae of animals with the above characteristics, we found that the noise exposed ear had many misshapen outer hair cells, possible loss of some of the peripherally arranged mitochondria in the same cells and an increase in vesicles, vacuoles and smooth endoplasmic reticulum in the afferent nerve fibers in the region of the organ of Corti from which the low cochlear potentials were recorded. It is clear from these findings that hair cell counts alone do not always give a true indication of the functional status of the cochlea. In describing the permanent anatomical effects of a particular noise exposure on the inner ear, the condition of the cells and nerve fibers which remain after injury must be determined in addition to the number of sensory cells and nerve fibers which were lost.

Electron microscopical studies on the effect of lapsed time on the nerve endings of the outer hair cells in acoustically exposed rabbits

Rabbits were exposed to pure tone (2 kHz, 100 dB for 2 h) and the changes in the nerve endings of the outer hair cells over a period of 1 month were studied using the electron microscope. In the afferent nerve endings mitochondria dilated and small vesicles decreased in number immediately after acoustic exposure. However, the afferent nerve endings recovered during the period of 1 month. In the efferent nerve endings mitochondria dilated and were distributed throughout the nerve endings, and small vesicles decreased in number immediately after acoustic exposure. The efferent nerve endings recovered more slowly than afferent nerve endings. In addition, the changes in the agglomerations of synaptic vesicles along the synaptic membrane and the gap-structures in the synaptic cleft in the efferent nerve endings after acoustic exposure are briefly discussed.