The receptor and neuron distribution in the cochlea of the bat, Taphozous kachhensis

Cochlear receptors and spiral ganglion neurons in the inner ear of the bat, Taphozous kachhensis of the family Emballonuridae, were quantitatively analyzed on the basis of surface specimen technique, histological sections, and three-dimensional computer reconstruction. The length of the basilar membrane was 12.1 (+/- 0.4) mm. There were about 23,000 neurons in the spiral ganglion and about 1430 inner hair cells and 4635 outer hair cells in the organ of Corti. Cochlear neurons had two density maxima, one basal and one apical. Inner hair cells had basically the same distribution but less pronounced. Outer hair cells had only one density maximum in the apical region corresponding to the respective maxima of inner hair cells and cochlear neurons. Basal density maxima of inner hair cells and neurons are most probably involved in the analysis of echolocation signals, and presumably correlated with maximum sensitivity of the auditory system. A comparison with the bat, Nyctalus noctula of the family Vespertilionidae revealed a conspicuous similarity in some quantitative aspects of the cochlear structure. These particular similarities may be looked upon as convergent adaptations given by similar flight, hunting, and echolocation strategies in both bat species.     

Tubulin expression in the developing and adult gerbil organ of Corti

In the late stages of inner ear development, the relatively undifferentiated cells of Kollicker's organ are transformed into the elaborately specialized cell types of the organ of Corti. Microtubules are prominent features of adult cells in the organ of Corti, particularly supporting cells. To test the possible role of microtubules in organ of Corti development, the microtubule organization in the organ of Corti has been examined using indirect immunofluorescence to beta-tubulin in the developing gerbil cochlea. Tubulin first appears at post-natal day 0 (P0) as filamentous asters in inner hair cells and by P2, asters are also seen in outer hair cells. Tubulin appears at P3 in inner pillar cells in a tooth crown-like figure. By P6, tubulin expression is also evident in outer pillar cells and by P9, it is seen in Deiters cells. Elaboration of microtubules in pillar cells was observed to proceed from the reticular lamina towards the basilar membrane. The pattern of tubulin expression in the apical organ of Corti lags the base by about 3 days until P6, but by P9, apical and basal organ of Corti appear substantially the same.     

Acoustic lesions in the mammalian cochlea: implications for the spatial distribution of the 'active process'.

The spatial contribution of mechanically active hair cells to tuning and sensitivity at a single point in the mammalian cochlea has been investigated in the basal turn of the guinea pig cochlea. Following the destruction of outer hair cells with acoustic overstimulation it was possible to record apparently normal tuning and sensitivity from spiral ganglion neurones innervating inner hair cells located on the apical edges of substantial lesions. The distance between the recording site, where neurones showed normal sensitivity, and areas of the cochlea showing 60-100% of the outer hair cells either damaged or missing varied between 0.2 and 1.3 mm which incorporates approximately 70 to 450 outer hair cells. In one animal neurones that demonstrated normal sensitivity were recorded within 0.2 mm of a lesion where 67% of the outer hair cells were either missing or showed severe damage to their stereocilia and within 0.5 mm of areas of the organ of Corti showing damage to 97% of the outer hair cells. This distance includes approximately 50 inner hair cells or 180 outer hair cells. The location of these neurones, whose sharp tuning presumably mirrors basilar membrane mechanics, suggests that a substantial proportion of point tuning in the cochlea may be derived over a distance of less than 0.5 mm and involve fewer than 200 active outer hair cells.     

The inner ear as an electrosensory sense organ]

A model for cochlear perception is introduced based on the consideration that the organ of Corti is an electrosensory organ like that found in fish physiology during phylogenetic development. The inner hair cells work as electroreceptors, the outer hair cells as electrocytes. A layer of potassium ions on the lower surface of the tectorial membrane causes the excitation of the inner hair cells as soon as contact with a stereocilium occurs. A model for the basilar membrane motion, based on mathematical considerations and in accordance with the typical tuning curves of single auditory nerve fibres can serve as basis to explain the results of frequency and intensity analyses if we assume an electric excitation of the inner hair cells.     

Anatomical correlates of impulse noise-induced mechanical damage in the cochlea

Changes in the surface morphology of the organ of Corti in the chinchilla were studied following exposure to blast waves at 160 dB peak SPL. The generation and development of a mechanically induced lesion on the organ of Corti was followed over a period of 30 days using scanning electron microscopy. The most prominent feature of the lesion was the complete separation of a 5–7 mm strip of the sensory epithelia consisting of outer hair cells, Deiter cells and Hensen cells from the reticular lamina and the basilar membrane. The inner hair cells in this same area survived for several days in a remarkably normal condition. A spectrum of ciliary changes was observed on the inner and outer hair cells that differ somewhat from those commonly reported following continuous noise exposure. Some of the observed changes in morphology can be related to a variety of inferred mechanical events on the basilar membrane.     

Postnatal development of the organ of Corti in the wild house mouse, laboratory mouse, and their hybrid

Length of the basilar membrane, number and distribution of cochlear receptors, and the width of the triad of outer hair cells were analyzed in the course of the postnatal development and in adult individuals in wild and laboratory house mice and in hybrids of these species. While in newborn animals the triad of outer hair cells was wide at the base and narrow at the apex, the opposite was true for adult animals. The parameter decreased at the base and increased at the apex during postnatal development. The center of differentiation of (the reticular lamina of) the organ of Corti was localized at 40-50% of the basilar membrane length from the base and corresponded to the region with the maximum density of inner hair cells. The reticular lamina in the apical half of the cochlea matured earlier than in the basal half. Distribution of receptors did not change after birth. The shortest basilar membrane and the slowest rate of maturation were found in wild mice. Hybrids had the longest basilar membrane and the highest rate of maturation. These facts are considered an effect of heterosis.     

Comparative anatomy of the cochlea and auditory nerve in mammals

The numbers and structure of hair cells; afferent, efferent, and reciprocal synapses as seen at the base of hair cells; innervation patterns of first order cochlear neurons; and number and morphology of spiral ganglion cells will be discussed and compared in the guinea pig, rat, cat, monkey and man. Despite many similarities both in the organ of Corti and the spiral ganglion in these species, there are a number of differences which may have important physiologic implications. In the organ of Corti, the major differences among species are the length and width of the basilar membrane, the number of inner and outer hair cells, and the length of hairs on both inner and outer hair cells. Significant differences in the innervation pattern of the inner hair cell among these species include the number of afferent nerve terminals per inner hair cell, the degree of branching of afferent fibers, and the number of synapses per afferent nerve terminal. Among outer hair cells, the number of afferent nerve terminals per outer hair cell, presence or absence of a pre-synaptic body, presence or absence of reciprocal synapses, the number of efferent terminals per outer hair cell, and the presence of dendodendritic synapses in outer spiral bundles may be differences important physiologically. In the spiral ganglion, there are significant differences in the number of spiral ganglion cells, the number of cochlear nerve fibers, the percentage of spiral ganglion cells which are myelinated, and the presence of synapses on spiral ganglion cells.     

出生后大鼠听毛细胞纤毛发育过程

目的观察新生大鼠（P1、P7、P14、P21）耳蜗Corti器的形态结构及排列方式,以便了解听毛细胞纤毛发育的过程。方法取新乍SD大鼠四个阶段的耳蜗,采刖扫描电镜对出生后大鼠耳蜗Corti器形态结构进行系统观察。结果发现这四个阶段在体犬鼠耳蜗毛细胞从出生至毛细胞完全发育成熟,绝大部分都是以一排内毛细胞和三排外毛细胞的方式排列的,只有少部分在局部发现多出几个外毛细胞形成第四排。毛细胞的形态结构及排列方式随着时间的改变逐渐发育成熟。P14这个阶段是个重要的时期,Corti器表面柱细胞头板增宽,表面的微绒毛减少,Deiter细胞表面微绒毛也减少,动纤毛从底圄至顶圈逐渐退化已经基本结束,P14这个阶段Corti器各个部分发育已经完全成熟：结论出生后14天之内。大鼠听毛细胞静纤毛和动纤毛与听器其他细胞形态结构仍不同程度地处于由不成熟到成熟的逐渐发育过程,为研究大鼠出生后Corti器的发育、听觉功能的建立和完善提供了比较可靠的形态学证据。     

人Corti‘s器扫描电镜观察

目的 了解正常人Corti’s器的超微结构特征。方法 用扫描电镜观察成年人和胎儿的Corti’s器。结果成人和胎儿Corti’s器基底圈内、外毛细胞排列规则,中圈和顶圈毛细胞排列不规则,内毛细胞以一排,外毛细胞以三排排列。可观察到列外的内、外毛细胞,四排排列和自然缺失的外毛细胞。外毛细胞粗大的静纤毛以及由于固定不及时引起的外毛细胞静纤毛气球样改变。结论 用扫描电镜观察成人和胎儿Corti’s器的精细结构,发现了毛细胞排列的基本规律和一些异常的形态。及时固定是保存Corti’s器结构完好的必要条件。     

Kanamycin and bumetanide ototoxicity: Anatomical,physiological and behavioral correlates

Severe hair-cell degeneration and cochlear dysfunction was observed in chinchillas examined at 60 days (or longer) after administration of a single injection of 150 mg/kg kanamycin, followed 2 h later by a single injection of 20 mg/kg bumetanide. Outer hair cells in the cochlear base were most severely affected. While inner and outer hair-cell loss was common, some animals showed large regions along the basilar membrane where almost all inner hair cells were present and almost all outer hair cells were absent. Wherever areas of complete degeneration of the organ of Corti occurred, a small, diffuse population of nerve fibers within the spiral lamina was always present. Single-unit tuning curves correlated best with anatomical observations, compared with the other functional measures of auditory sensitivity that were obtained (behavioral audiogram and compound action potential thresholds). Results indicated that behavioral detection of auditory stimuli is relatively independent of innervation density as long as a few inner hair cells are present. Thus, the cross-fiber threshold envelope of the single-unit tuning curves appeared very similar to the behavioral audiogram.     

Aspartate aminotransferase immunoreactivity in cochlea of guinea pig

The distribution of aspartate aminotransferase-like immunoreactivity in the cochlea of the guinea pig was studied at the light microscopy level. Indirect immunofluorescence histochemistry using antisera against cytoplasmic aspartate aminotransferase prepared from pig heart was applied to surface preparations of the organ of Corti and cryostat sections of the cochlea. In the modiolus, immunofluorescence was localized to spiral ganglion cells and myelinated fibers of the auditory nerve and intraganglionic spiral bundles. In the organ of Corti, immunofluorescence was seen in upper tunnel crossing fibers and at the base of outer hair cells, following a distribution similar to that of the efferent innervation of the outer hair cells. Weak immunofluorescence was seen in the inner spiral bundle and tunnel spiral bundle, but was not present in all preparations. Immunofluorescence was not seen in inner hair cells, nor at the base of inner hair cells, and may have been absent from outer hair cells.It is concluded that spiral ganglion cells and myelinated auditory nerve axons contain aspartate aminotransferase-like immunoreactivity; such immunoreactivity has previously been determined in auditory nerve endings in the cochlear nucleus. Olivocochlear neurons that innervate outer hair cells also contain such immunoreactivity while other cochlear efferents contain little or none.     

Postnatal development of the organ of Corti in cats: a light microscopic morphometric study

This study quantitatively characterizes the development of the major morphological features of the organ of Corti during the first 2 weeks postnatal, the period when the cat auditory system makes the transition from being essentially non-functional to having nearly adult-like responses. Four groups of kittens (n=3) were studied at one day postnatal (P1), P5, P10, P15, and compared to adults. Measurements were made of the organ of Corti at 3 cochlear locations: 20%, 60% and 85% of basilar membrane length from the base – cochlear locations which in the adult correspond to best frequencies of ≈20 kHz, 2 kHz and 500 Hz, respectively. In addition, measurements of basilar membrane length and opening of the tunnel of Corti were made in 20 cochlear specimens from kittens aged P0–P6. Results indicate that: (i) at P0 the basilar membrane has attained adult length, and the tunnel of Corti is open over approximately the basal one-half of the cochlea; (ii) the initial opening of the tunnel of Corti occurs at a site about 4 mm from the cochlear base (best frequency of ≈25 kHz in the adult cochlea); (iii) the thickness of the tympanic cell layer decreases markedly at the basal 20-kHz location; (iv) the areas of the tunnel of Corti and space of Nuel and the angulation of the inner hair cells (IHC) relative to the basilar membrane all show marked postnatal increases at both the middle and apical locations; (v) IHC are nearly adult-like in length and shape at birth, whereas the OHC (at 2-kHz and 500-Hz locations) undergo marked postnatal changes; (vi) disappearance of the marginal pillars and maturation of the supporting cells are not yet complete by P15.     

Epidermal Growth Factor Upregulates Production of Supernumerary Hair Cells in Neonatal Rat Organ of Corti Explants

The organ of Corti is highly ordered, with a single row of inner hair cells and three rows of outer hair cells. The number of hair cells produced was thought to be limited by the time of their terminal mitosis (i.e. E14 in the mouse). However, exogenous application of retinoic acid has been shown to stimulate the formation of supernumerary hair cells in organ of Corti explants from E13 to E16 mouse embryos. Using late embryonic and neonatal rat organ of Corti explants, we investigated the potential for production of supernumerary hair cells in more mature auditory sensory epithelia. When newborn rat organ of Corti explants were cultured under control conditions, an area of supernumerary hair cells was observed in a segment of organ of Corti that was at the junction between the basal and middle turns. In these areas of supernumerary hair cells the number of hair cells increased per unit of length, but remained constant per surface unit, further demonstrating the supernumerary character of this phenomenon. Organ of Corti explants treated with epidermal growth factor (EGF) showed a 50% increase in the length of the organ of Corti segment containing supernumerary hair cells. Upregulation of supernumerary hair cell formation by EGF was found to start and be maximal at birth (P0) and to disappear by 2 days after birth (P2). Treatment of EGF stimulated P0 explants with an antimitotic drug, cytosine arabinoside (ARAc), demonstrated that the production of supernumerary hair cells occurred independently of cell division.     

Epidermal growth factor upregulates production of supernumerary hair cells in neonatal rat organ of corti explants

The organ of Corti is highly ordered, with a single row of inner hair cells and three rows of outer hair cells. The number of hair cells produced was thought to be limited by the time of their terminal mitosis (i.e. E14 in the mouse). However, exogenous application of retinoic acid has been shown to stimulate the formation of supernumerary hair cells in organ of Corti explants from E13 to E16 mouse embryos. Using late embryonic and neonatal rat organ of Corti explants, we investigated the potential for production of supernumerary hair cells in more mature auditory sensory epithelia. When newborn rat organ of Corti explants were cultured under control conditions, an area of supernumerary hair cells was observed in a segment of organ of Corti that was at the junction between the basal and middle turns. In these areas of supernumerary hair cells the number of hair cells increased per unit of length, but remained constant per surface unit, further demonstrating the supernumerary character of this phenomenon. Organ of Corti explants treated with epidermal growth factor (EGF) showed a 50% increase in the length of the organ of Corti segment containing supernumerary hair cells. Upregulation of supernumerary hair cell formation by EGF was found to start and be maximal at birth (P0) and to disappear by 2 days after birth (P2). Treatment of EGF stimulated P0 explants with an antimitotic drug, cytosine arabinoside (ARAc), demonstrated that the production of supernumerary hair cells occurred independently of cell division.     

Abrupt (sharp cut) type sensorineural hearing loss--a human temporal bone study--

Histopathology of a case of bilateral sensorineural hearing loss of abrupt (sharp cut) type is reported. While there was a 45 dB gap in threshold between 1,000 Hz and 2,000 Hz bilaterally, the patient had a good hearing at 1,000 Hz and lower frequencies. The patient was suffering from Takayasu's arteritis. Major histopathological findings were as follows: Almost complete loss of the outer hair cells from the basal end to 12 mm area in the left cochlea (length: 30.5 mm) and 13 mm in the right (length: 31.5 mm). The inner hair cell of the same region was also missing in the left cochlea, and to a lesser degree in the right. There was a clear separation between the normal and the pathological organ of Corti. Marked loss of the cochlear neuron was noted in the same region. Blood vessels within the cochlea and the internal auditory meatus were normal. Bilateral abrupt (sharp cut) type sensorineural hearing loss with unknown etiology is a group of inner ear disease due to abiotrophy of the organ of Corti and cochlear neuron. Disposition or hereditary factor possibly plays an important role in the development of hearing loss.     

Absence of Voltage-Dependent Compliance in High-Frequency Cochlear Outer Hair Cells

Cochlear outer hair cells are the key element in a mechanical amplification process that enhances auditory sensitivity and tuning in the mammalian inner ear. The electromotility of outer hair cells, that is, their ability to extend or contract at acoustic frequencies, is proposed to be the source of the mechanical amplification. For amplification to take place, some stiffness is required for outer hair cells to communicate force to the organ of Corti, the sensory epithelium of the inner ear. Modulation of this stiffness would be expected to have a significant effect on inner ear function. Outer hair cell compressive stiffness has recently been shown to be dependent on membrane potential, but this has only been demonstrated for cells originating in the apical, low-frequency segment of the cochlea, whereas cochlear amplification is arguably more important in the more basal high-frequency segment. The voltage-dependent compliance (the reciprocal of stiffness) of high-frequency outer hair cells was investigated by two methods in cells isolated from the basal turns of the guinea pig cochlea. In contrast to previous findings, no evidence was found for voltage-dependent changes in compliance. The results call into question the importance of outer hair cell voltage-dependent compliance as a component of cochlear amplification.     

Acetylcholinesterase activity in the chinchilla organ of Corti in normal and acoustically overstimulated animals.

The distribution of acetylcholinesterase (AChE) positive nerve fibers and endings in the organ of Corti of normal and acoustically overstimulated chinchillas is described. The strongest activity was located in the inner spiral bundle, ane remained almost unchanged even after all the outer and the majority of the inner hair cells had degenerated as a result of intense exposure. In the chinchilla even the apical coil had three rows of AChE-positive nerve endings under the outer hair cells.     

Organ of Corti Kinematics

The internal workings of the organ of Corti and their relation to basilar membrane motion are examined with the aid of a simple kinematic model. It is shown that, due to the lever system embodied in the organ of Corti, there is a significant transformer gain between basilar membrane and cilia displacements. While this transformation is nonlinear, linear response prevails in the narrow physiologically relevant operating range of the ciliary transducer. The model also simulates cilia deflection when the mechanical stimulus is the length change of outer hair cells.     

Hair cell loss in the aged guinea pig cochlea

The various effects of ageing on the auditory system, collectively termed presbycusis, are being studied across a wide range of animal species, including humans. One contributing factor to presbycusis is thought to be losses of the sensory hair cells in the cochlea. In this study, hair cell counts were obtained from cochleas of pigmented guinea pigs (Cavia porcellus) at ages ranging from 11 days to 4 years 7 months, using scanning electron microscopy to visualize the organ of Corti. Representative samples of the basal, middle and apical turn of the cochlea were photographed for analysis. Hair cell loss was observed, even in young animals. However, the loss was greater in the aged animals, but was not distributed evenly throughout the length of the cochlea. No significant loss of hair cells was seen in the basal (high frequency) or middle turn of the cochlea of the aged animals. In the apical (low frequency) turn, there was a significant loss of hair cells in all rows of outer hair cells (up to around 20%), and was most severe in the third row. There was no loss of apical inner hair cells in the aged animals.     

The cytokeratin skeleton of the human organ of Corti and its functional significance]

In the adult human organ of Corti cytokeratin (CK) is expressed by all supporting cells enclosing it like a shell. The pattern of immunoreactivity clearly demonstrates a quantitative gradient in the expression of CK, with more CK at the apex than at the base of the cochlea. Predominantly in the apical cochlear turns, the CK-shell separates the compartment of the inner hair cells from that of the outer hair cells. Ultrastructurally, the supporting cells contain a loose fibrillary network which apically is oriented toward the desmosome chain and which can be clearly distinguished from the well-known tubular filaments (microtubuli). Some supporting cells show centrioles. Both the expression of CK and the presence of centrioles indicate a possible potential for cell regeneration. Ultrastructurally, outer hair cells and Deiters' cells show features of a specialized contact zone which might be of functional significance regarding the contractile abilities of the outer hair cells. In the human organ of Corti, vimentin is expressed only by the inner and outer pillar cells. These cells thus express CK together with vimentin. The distinct shell configuration of the CK network in the organ of Corti gives it a tonotopically related difference in rigidity which not only must be of importance for cochlear perception of sound but also could explain the reduced vulnerability of the ear to low frequencies.