Cochlear Fluid Spaces and Structures of the Gerbil High-Frequency Region Measured Using Optical Coherence Tomography (OCT)

Since it has been difficult to directly observe the morphology of the living cochlea, our ability to infer the mechanical functioning of the living ear has been limited. Nearly all our knowledge about cochlear morphology comes from postmortem tissue that was fixed and processed using procedures that possibly distort the structures and fluid spaces of the organ of Corti. In this study, optical coherence tomography was employed to obtain volumetric images of the high-frequency hook region of the gerbil cochlea, as viewed through the round window, with far better resolution capability than had been possible before. The anatomical structures and fluid spaces of the organ of Corti were segmented and quantified in vivo and over a 90-min postmortem period. We find that the arcuate-zone and pectinate-zone widths change very little postmortem. The volume of the scala tympani between the round-window membrane and basilar membrane and the volume of the inner spiral sulcus decrease in the first 60-min postmortem. While textbook drawings of the mammalian organ of Corti and cortilymph prominently depict the tunnel of Corti, the outer tunnel is typically missing. This is likely because textbook drawings are typically made from images obtained by histological methods. Here, we show that the outer tunnel is nearly twice as big as the tunnel of Corti or the space of Nuel. This larger outer tunnel fluid space could have a substantial, little-appreciated effect on cochlear micromechanics. We speculate that the outer tunnel forms a resonant structure that may affect reticular-lamina motion.

Morphological Immaturity of the Neonatal Organ of Corti and Associated Structures in Humans

Although anatomical development of the cochlear duct is thought to be complete by term birth, human newborns continue to show postnatal immaturities in functional measures such as otoacoustic emissions (OAEs). Some of these OAE immaturities are no doubt influenced by incomplete maturation of the external and middle ears in infants; however, the observed prolongation of distortion-product OAE phase-gradient delays in newborns cannot readily be explained by conductive factors. This functional immaturity suggests that the human cochlea at birth may lack fully adult-like traveling-wave motion. In this study, we analyzed temporal-bone sections at the light microscopic level in newborns and adults to quantify dimensions and geometry of cochlear structures thought to influence the mechanical response of the cochlea. Contrary to common belief, results show multiple morphological immaturities along the length of the newborn spiral, suggesting that important refinements in the size and shape of the sensory epithelium and associated structures continue after birth. Specifically, immaturities of the newborn basilar membrane and organ of Corti are consistent with a more compliant and less massive cochlear partition, which could produce longer DPOAE delays and a shifted frequency-place map in the neonatal ear.

Quantitative polarized light microscopy of unstained mammalian cochlear sections

Hearing loss is the most common sensory deficit in the world, and most frequently it originates in the inner ear. Yet, the inner ear has been difficult to access for diagnosis because of its small size, delicate nature, complex three-dimensional anatomy, and encasement in the densest bone in the body. Evolving optical methods are promising to afford cellular diagnosis of pathologic changes in the inner ear. To appropriately interpret results from these emerging technologies, it is important to characterize optical properties of cochlear tissues. Here, we focus on that characterization using quantitative polarized light microscopy (qPLM) applied to unstained cochlear sections of the mouse, a common animal model of human hearing loss. We find that the most birefringent cochlear materials are collagen fibrils and myelin. Retardance of the otic capsule, the spiral ligament, and the basilar membrane are substantially higher than that of other cochlear structures. Retardance of the spiral ligament and the basilar membrane decrease from the cochlear base to the apex, compared with the more uniform retardance of other structures. The intricate structural details revealed by qPLM of unstained cochlear sections ex vivo strongly motivate future application of polarization-sensitive optical coherence tomography to human cochlea in vivo.

Inner ear morphological correlates of ultrasonic hearing in frogs

Three species of anuran amphibians (Odorrana tormota, Odorrana livida and Huia cavitympanum) have recently been found to detect ultrasounds. We employed immunohistochemistry and confocal microscopy to examine several morphometrics of the inner ear of these ultrasonically sensitive species. We compared morphological data collected from the ultrasound-detecting species with data from Rana pipiens, a frog with a typical anuran upper cut-off frequency of ∼3 kHz. In addition, we examined the ears of two species of Lao torrent frogs, Odorrana chloronota and Amolops daorum, that live in an acoustic environment approximating those of ultrasonically sensitive frogs. Our results suggest that the three ultrasound-detecting species have converged on small-scale functional modifications of the basilar papilla (BP), the high-frequency hearing organ in the frog inner ear. These modifications include: 1. reduced BP chamber volume, 2. reduced tectorial membrane mass, 3. reduced hair bundle length, and 4. reduced hair cell soma length. While none of these factors on its own could account for the US sensitivity of the inner ears of these species, the combination of these factors appears to extend their hearing bandwidth, and facilitate high-frequency/ultrasound detection. These modifications are also seen in the ears of O. chloronota, suggesting that this species is a candidate for high-frequency hearing sensitivity. These data form the foundation for future functional work probing the physiological bases of ultrasound detection by a non-mammalian ear.

Phase-sensitive optical coherence tomography imaging of the tissue motion within the organ of Corti at a subnanometer scale: a preliminary study

Hearing loss can mean severe impairment to the quality of life. However, the biomechanical mechanisms of how the hearing organ, i.e., the organ of Corti (OC), responds to sound are still elusive, largely because there is currently no means available to image the 3-D motion characteristics of the OC. We present a novel use of the phase-sensitive spectral domain optical coherence tomography (PSOCT) to characterize the motion of cellular compartments within the OC at a subnanometer scale. The PSOCT system operates at 1310 nm with a spatial resolution of ∼16 μm and an imaging speed of 47,000 A-lines/s. The phase changes of the spectral interferograms induced by the localized tissue motion are used to quantify the vibration magnitude. Fourier transform analysis of the phase changes improves the system sensitivity to sense minute vibrations smaller than 1 nm. We demonstrate that the PSOCT system is feasible to image the meaningful vibration of cellular compartments within the OC with an unprecedented sensitivity down to ∼0.5 Å.

[Science and life--the history of Marquis Alfonso Corti]

Alfonso Corti was born at Gambarana, near Pavia in 1822. A famous friend of Corti's father, Antonio Scarpa, may have kindled his boyhood interest in anatomy and medicine. As a medical student he enrolled first at the University of Pavia. Corti's favorite study there was microanatomy with Bartolomeo Panizza and Mario Rusconi. In 1845, against paternal wishes, Corti moved to Vienna to complete his medical studies and to work in the anatomical institute of Joseph Hirtl. There he received the degree in medicine in 1847 under the supervision of professor Hyrtl, with a thesis on the bloodstream system of a reptile. He was then appointed by Hyrtl to be his Second Prosector. With the outbreak of the 1848 Revolution he left Vienna, and after brief military service in Italy made visits to eminent scientist in Bern, London and Paris. By the beginning of 1850 Corti had received the invitation of the anatomist Albert Kölliker and had moved to Würzburg, where he made friends with Virchow. At the Kölliker Laboratory he began to work on the mammalian auditory system. A short time Corti spent in Utrecht, where he visited Professors Schroeder van der Kolk and Pieter Harting. In Utrecht Corti learned to use methods to preserve several preparations of the cochlea. From Utrecht he returned to Würzburg to complete his study of at least 200 cochlea's' of man and different animals. His famous paper: "Recherches sur l'organe de l'ouïe des mammiferes" appeared in 1851 in Kölliker's journal "Zeitschrift für wissenschaftliche Zoologie". In the same year, after death of his father, he inherited father's title Marchese de San Stefano Belbo and estate and moved back to Italy. In 1855 Corti married the daughter from a neighboring estate, Maria Bettinzoli. His young wife presented him with a daughter Bianca, and a son Gaspare, but in 1861 she died, leaving him with the responsibility of rearing the children. Unfortunately he was gradually developing arthritis deformans. Corti's last 15 years were further darkened by the inexorable progress of his crippling illness. In 1876, on the second of October, he died at Corvino San Quirico.

The general architecture of sensory neuroepithelia

All neuroepithelia are sheets of cells lining an internal or external surface of the body and resting on a basement membrane. They consist of at least two kinds of cell, receptor cells and sustentacular (supporting) cells. Some contain undifferentiated precursor cells and senescent or degenerating cells. The potential for plasticity and regeneration in different sensory neuroepithelia varies widely according to their origins and structure in any individual animal and according to the species in which they occur. Four sensory neuroepithelia are described as examples of the range of construction, complexity, and life history.

Dietary thyroid hormone replacement ameliorates hearing deficits in hypothyroid mice

Thyroid hormone (TH) insufficiency causes variable hearing impairment and mental deficiency in humans. Rodents lacking TH have congenital hearing deficiency that has been attributed to physiologic, morphologic, and developmental abnormalities of the auditory system. We examined four genetically defined strains of hypothyroid mice for development of hearing and response to TH replacement initiated during late gestation and continued through six weeks of age. Auditory brain stem response studies showed variable hearing impairment in homozygous mutants of each strain at three weeks of age relative to normal littermates. Mutants from three of the strains still had hearing deficiencies at six weeks of age. TH-enriched diet significantly improved hearing in three-week-old mutants of each strain relative to untreated mutants. Differences in the level of hearing impairment between the Prop1df and Pit1dw mutants, which have defects in the same developmental pathway, were determined to be due to genetic background modifier genes. Further physiologic and morphologic studies in the Cgatm1Sac strain indicated that poor hearing was due to cochlear defects. We conclude that TH supplement administered during the critical period of hearing development in mice can prevent deafness associated with congenital hypothyroidism of heterogeneous genetic etiology.

Intracochlear pressure and derived quantities from a three-dimensional model

Intracochlear pressure is calculated from a physiologically based, three-dimensional gerbil cochlea model. Olson [J. Acoust. Soc. Am. 103, 3445-3463 (1998); 110, 349-367 (2001)] measured gerbil intracochlear pressure and provided approximations for the following derived quantities: (1) basilar membrane velocity, (2) pressure across the organ of Corti, and (3) partition impedance. The objective of this work is to compare the calculations and measurements for the pressure at points and the derived quantities. The model includes the three-dimensional viscous fluid and the pectinate zone of the elastic orthotropic basilar membrane with dimensional and material property variation along its length. The arrangement of outer hair cell forces within the organ of Corti cytoarchitecture is incorporated by adding the feed-forward approximation to the passive model as done previously. The intracochlear pressure consists of both the compressive fast wave and the slow traveling wave. A Wentzel-Kramers-Brillowin asymptotic and numerical method combined with Fourier series expansions is used to provide an efficient procedure that requires about 1 s to compute the response for a given frequency. Results show reasonably good agreement for the direct pressure and the derived quantities. This confirms the importance of the three-dimensional motion of the fluid for an accurate cochlear model.

Sex differences in the length of the organ of Corti in humans

Sato et al. [Acta. Otolaryngol. 111 (6), 1037-1040 (1991)] reported that the human cochlea is, on average, 15% longer for males than females. This corresponds to 4.7 mm in length and to 2.78 standard deviations (SD). Anatomical measurements of the lengths of cochleas from 148 heads (194 cochleas) from eleven sources are reviewed and summarized. A sex difference of 3.36% is observed. This corresponds to 1.11 mm in length and to 0.49 SD. The mean lengths of the male and female cochleas are approximately 34 and 33 mm, respectively, and the population SD is 2.28 mm. The statistical significance of the observed difference is questionable.

Temporal bone study of development of the organ of Corti: correlation between auditory function and anatomical structure

OBJECTIVE

To study the development of the organ of Corti in the human cochlea, and to correlate our findings with the onset of auditory function.

MATERIAL AND METHODS

Step sections of 81 human fetal temporal bones were studied, from eight weeks of gestation to full term.

RESULTS

By the end of the 10th week, the tectorial membrane primordium could be traced even in the most apical turns. Individual hair cells became identifiable at the basal turn at 14 weeks. At the same time, a small but well formed oval space was observed between the inner and outer hair cells in the basal turn. This does not correspond to the tunnel of Corti, as is erroneously quoted in the literature, as the individual pillar cells develop at later stages. Between 14 and 15 weeks, Hensen's cells were recognised for the first time. Individual pillar cells were identifiable at 17 weeks and the tunnel of Corti opened at 20 weeks. By 25 weeks, the cochlea had reached its adult size, but continued to develop until full term.

DISCUSSION AND CONCLUSIONS

A temporal coincidence of different developmental events is responsible for early fetal audition at 20 weeks, including growth of pillar cells, opening of the tunnel of Corti and regression of Kollicker's organ, with the subsequent formation of the inner spiral sulcus and then separation of the tectorial membrane. The fine structures of the organ of Corti continue to develop well after the 25th week, and this may well alter the mechanical properties of the vibrating parts of the cochlea, which may in turn account for the frequency shift observed in preterm infants. These changes will have to be taken into account in the development of prenatal hearing screening tests.

Frequency map for the human cochlear spiral ganglion: implications for cochlear implants

The goals of this study were to derive a frequency-position function for the human cochlear spiral ganglion (SG) to correlate represented frequency along the organ of Corti (OC) to location along the SG, to determine the range of individual variability, and to calculate an "average" frequency map (based on the trajectories of the dendrites of the SG cells). For both OC and SG frequency maps, a potentially important limitation is that accurate estimates of cochlear place frequency based upon the Greenwood function require knowledge of the total OC or SG length, which cannot be determined in most temporal bone and imaging studies. Therefore, an additional goal of this study was to evaluate a simple metric, basal coil diameter that might be utilized to estimate OC and SG length. Cadaver cochleae (n = 9) were fixed <24 h postmortem, stained with osmium tetroxide, microdissected, decalcified briefly, embedded in epoxy resin, and examined in surface preparations. In digital images, the OC and SG were measured, and the radial nerve fiber trajectories were traced to define a series of frequency-matched coordinates along the two structures. Images of the cochlear turns were reconstructed and measurements of basal turn diameter were made and correlated with OC and SG measurements. The data obtained provide a mathematical function for relating represented frequency along the OC to that of the SG. Results showed that whereas the distance along the OC that corresponds to a critical bandwidth is assumed to be constant throughout the cochlea, estimated critical band distance in the SG varies significantly along the spiral. Additional findings suggest that measurements of basal coil diameter in preoperative images may allow prediction of OC/SG length and estimation of the insertion depth required to reach specific angles of rotation and frequencies. Results also indicate that OC and SG percentage length expressed as a function of rotation angle from the round window is fairly constant across subjects. The implications of these findings for the design and surgical insertion of cochlear implants are discussed.

Promontory Drilling in Stapedectomy

BACKGROUND

Anatomical variants such as an overhanging facial nerve or promontory can impede access to the footplate during stapedectomy. Drilling away bone from the cochlear promontory may be required. In the case of a floating or depressed footplate, it has been recommended that a "pothole" be drilled in the inferior margin of the oval window. There is little published information on the anatomy of the promontory with respect to these maneuvers.

MATERIALS AND METHODS

Twenty temporal bones were studied. A series of measurements was made to assess how much bone may safely be removed without risking damage to the underlying cochlear endosteum and, hence, spiral ligament and stria vascularis.

RESULTS

The bony promontory is thickest posteriorly, and here, the endosteum has least lateral projection. The promontory becomes thinner closer to the oval window. Moving anteriorly, the bone becomes thinner and the underlying endo steum more closely follows the bony contour. The stria vascularis and spiral ligament may be less than 0.2 mm inferior to the inferior margin of the oval window posteriorly. This distance is at least 0.3 mm at the midpoint of the footplate (range, 0.3-0.5 mm).

CONCLUSION

Bone may be removed inferiorly to the posterior one-third of the footplate from lateral to a line that makes an angle of 35 degrees with the superoinferior axis of the footplate. In creating a "pothole" in the case of a floating or depressed footplate, the authors recommend that it be created at the midpoint of the inferior margin of the oval window and should not exceed 0.3 mm in diameter.

Does the geometrical arrangement of the outer hair cell stereocilia perform a fluid-mechanical function?

CONCLUSIONS

Our experiments qualitatively show that the geometrical structure of the inner ear may have the features of a micro-pump. To further substantiate this hypothesis, additional experiments, particularly on in vivo preparations, are needed.

OBJECTIVE

To introduce some new ideas about the functional purpose of the geometric arrangement of the outer hair cell stereocilia. Analogies to some recently developed valveless micro-pumps are pointed out. To illustrate these points, comparative experiments were performed using a simplified macro model.

METHODS

Specific structures of the organ of Corti were simulated in a partially open, partially closed acrylic tank. This rough approximation allows the visualization of fluid flows that are generated as a result of the relative motions between the tectorial membrane and the reticular lamina.

RESULTS

It was shown that the arrangement of the cochlear elements not only forces fluid to flow in a one-way direction, but also generates a fluid stream that flows through the "outlets" between each two V or W formations of stereocilia. These fluid streams are directed towards the inner hair cells.

Developing a Physical Model of the Human Cochlea Using Microfabrication Methods

Advances in micro-machining technology have provided the opportunity to explore possibilities of creating life-sized physical models of the cochlea. The physical model of the cochlea consists of two fluid-filled channels separated by an elastic partition. The partition is micro-machined from silicon and uses a 36-mm linearly tapered polyimide plate with a width of 100 microm at the basal end and 500 microm at the apex to represent the basilar membrane. Thicknesses from 1 to 5 microm have been fabricated. Discrete aluminum fibers (1.5 microm in width) are machined to create direction-dependent properties. A 0.5 x 0.5 mm opening represents the helicotrema. The fluid channels are machined from plexiglas using conventional machining methods. A magnet-coil system excites the fluid channel. Measurements on a model with thickness 4.75 microm show a velocity gain of 4 and phase of 3.5 pi radians at a location 23 mm from the base. Mathematical modeling using a 3-D formulation confirm the general characteristics of the measured response.

Audiogram, body mass, and basilar papilla length: correlations in birds and predictions for extinct archosaurs

The inner ear in the group of archosaurs (birds, crocodilians, and extinct dinosaurs) shows a high degree of structural similarity, enabling predictions of their function in extinct species based on relationships among similar variables in living birds. Behavioral audiograms and morphological data on the length of the auditory sensory epithelium (the basilar papilla) are available for many avian species. By bringing different data sets together, we show that body mass and the size of the basilar papilla are significantly correlated, and the most sensitive frequency in a given species is inversely related to the body mass and the length of the basilar papilla. We also demonstrate that the frequency of best hearing is correlated with the high-frequency limit of hearing. Small species with a short basilar papilla hear higher frequencies compared with larger species with a longer basilar papilla. Based on the regression analysis of two significant correlations in living archosaurs (best audiogram frequency vs body mass and best audiogram frequency vs papillar length), we suggest that hearing in large dinosaurs was restricted to low frequencies with a high-frequency limit below 3 kHz.

Age effects and size effects in the ears of gekkonomorph lizards: inner ear

Audiograms have indicated greater auditory sensitivity in larger than in smaller geckos; part of this difference, interspecifically and intraspecifically, is explained by middle-ear proportions. To investigate the contribution of the inner ear to the variation in sensitivity, we examined it in museum specimens representing 11 species and three subfamilies. We measured papilla basilaris length, and, when intact, the saccular otoconial mass. Papilla length approximated 1% of rostrum-anus length in large geckos but 2% in small geckos; in some species some inter-aural difference was indicated. Over the lumped material, relative papilla length varied as a function of body length, with highly significant correlation. Similar relations prevailed within each subfamily. However, intraspecifically the correlation of papilla basilaris length with animal size was usually nonsignificant. Hair cell populations assessed from SEM photographs were larger in the larger species but intraspecifically did not relate to an individual's size. Hence interspecifically, the dependence of auditory sensitivity on animal size seems supported by inner-ear differences but intraspecifically this relation derives only from the middle ear. Otoconial mass, as measured by its volume, was correlated with animal length both interspecifically and intraspecifically.

The cochlear amplifier: augmentation of the traveling wave within the inner ear

PURPOSE OF REVIEW

There have been many recent advancements in our understanding of cochlear function within the past ten years. In particular, several mechanisms that underlie the sensitivity and sharpness of mammalian tuning have been discovered. This review focuses on these issues.

RECENT FINDINGS

The cochlear amplifier is essentially a positive feedback loop within the cochlea that amplifies the traveling wave. Thus, vibrations within the organ of Corti are sensed and then force is generated in synchrony to increase the vibrations. Mechanisms that generate force within the cochlea include outer hair cell electromotility and stereociliary active bundle movements. These processes can be modulated by the intracellular ionic composition, the lipid constituents of the outer hair cell plasma membrane, and the structure of the outer hair cell cytoskeleton.

SUMMARY

A thorough understanding of the cochlear amplifier has tremendous implications to improve human hearing. Sensorineural hearing loss is a common clinical problem and a common site of initial pathology is the outer hair cell. Loss of outer hair cells causes loss of the cochlear amplifier, resulting in progressive sensorineural hearing loss.

[Are we sectioning the cochlear efferent system during vestibular neurotomy?]

INTRODUCTION

In addition to sensory neurons which transmit information from the inner ear to the brain, there is a system of efferent feedback fibers, called the olivocochlear system, carrying signals from the brain to the ear. Over the past half-century, the efferent system has been extensively studied in animals and results provided theories as to the functional significance of these efferents: to improve signal-to-noise ratio in the auditory periphery, to mediate selective attention, and to protect the inner ear from acoustic overexposure. The results of several studies conducted in man rely on the study of patients who have undergone a vestibular neurectomy. Indeed, anatomical data show that olivocochlear efferents could travel along or inside the vestibular part of the auditory nerve before reaching the organ of Corti. Therefore, these patients may be considered as an experimental model of unilaterally de-efferented subjects. However, to date, none has reported the existence of olivocohlear efferents in the vestibular section following neurectomy.

MATERIALS AND RESULTS

In this study, we present the histological results from 18 vestibular sections and show the absence of olivocochlear efferents.

CONCLUSION

These results provide a reason to reconsider the results of previous experiments conducted in similar patients and ask for further studies on the olivocochlear efferents pathways.

Motion analysis in the hemicochlea

Optical flow techniques are often used to estimate velocity fields to represent motion in successive video images. Usually the method is mathematically ill-posed, because the single scalar equation representing the conservation of local intensity contains more than one unknown velocity component. Instead of regularizing the problem using optimization techniques, we formulate a well-posed problem for the gerbil hemicochlea preparation by introducing an in-plane incompressibility constraint, and then show that local brightness is also conserved. We solve the resulting system using a Lagrangian description of the conservation equations. With this approach, the displacement of isointensity contours on sequential images determines the normal component of velocity of an area element, while the tangential component is computed from the local constant area constraint. We have validated our method using pairs of images generated from our calculations of the vibrational deformation in a cross section of the organ of Corti and tectorial membrane in the mammalian cochlea, and quantified the superior performance of our method when complex artificial motion is applied to a noisy image obtained from the hemicochlea preparation. The micromechanics of the organ of Corti and the tectorial membrane is then analyzed by our new method.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

Efferent protection from acoustic injury is mediated via alpha9 nicotinic acetylcholine receptors on outer hair cells

Exposure to intense sound can damage the mechanosensors of the inner ear and their afferent innervation. These neurosensory elements are innervated by a sound-activated feedback pathway, the olivocochlear efferent system. One major component of this system is cholinergic, and known cholinergic effects are mediated by the alpha9/alpha10 nicotinic acetylcholine receptor (nAChR) complex. Here, we show that overexpression of alpha9 nAChR in the outer hair cells of bacterial artificial chromosome transgenic mice significantly reduces acoustic injury from exposures causing either temporary or permanent damage, without changing pre-exposure cochlear sensitivity to low- or moderate-level sound. These data demonstrate that efferent protection is mediated via the alpha9 nAChR in the outer hair cells and provide direct evidence for a protective role, in vivo, of a member of the nAChR family.

Anatomical differences in the peripheral auditory system of mammals and man. A mini review

The major anatomical differences among animal models and man are briefly reviewed. Differences are described in the length and width of the basilar membrane, the number of inner (IHCs) and outer hair cells (OHCs), and the length of cilia on both cell types. Significant differences in the innervation pattern of the IHCs among these species include the number of afferent nerve terminals per IHC, the degree of branching of afferent fibers and the number of synapses per afferent nerve terminal. At the OHCs, the number of afferent and efferent nerve terminals, the presence or absence of presynaptic bodies, reciprocal synapses and the presence of dendrodendritic synapses in the outer spiral bundles may have important physiological functions. In the cochlear nerve, significant differences are described in the number of spiral ganglion cells (SGCs) and cochlear nerve fibers. Furthermore the percentage of myelinated SGCs and the presence of synapses on SGCs varies enormously.

[Atomic force microscope (AFM). A nanomanipulator for biophysical studies of stereocilia of the cochlear hair cells]

OBJECTIVES

Studies of the mechanoelectrical sensor system of the hair cell bundle in the cochlea require a manipulation device that enables controlled force application and movement of individual stereocilia in the nanometer range.

METHODS

In our atomic force microscope (AFM) setup, the scan is directly controlled in an upright differential interference contrast (DIC) infrared video microscope with a water immersion objective and in the measured AFM image. Here we present studies on hair cells of the mammalian cochlea.

RESULTS AND CONCLUSIONS

The resulting images revealed the tips of individual stereocilia of living sensory cells of the organ of Corti and the typical shape of the ciliary bundle. Scanning electron-microscopic (SEM) images of the identical hair bundles obtained after AFM investigation demonstrated that up to four AFM manipulations on the same cell did not cause obvious damage to the surface morphology of the stereocilia.

Cochlear dimensions obtained in hemicochleae of four different strains of mice: CBA/CaJ, 129/CD1, 129/SvEv and C57BL/6J

Because homologies between mice and human genomes are well established and hereditary abnormalities are similar in both, mice present a valuable animal model to study hereditary hearing disorders in humans. One of the manifestations of hereditary hearing disorders might be in the structure of cochlear elements, such as the gross morphology of the cochlea. Cochlear dimensions, however, are one factor that determines inner ear mechanics and thus hearing function. Therefore, gross cochlear dimension might be important when different strains of mice are compared regarding their hearing. Although several studies have examined mouse inner ear structures on a sub-cellular level, only few have studied cochlear gross morphology. Moreover, the sparse data available were acquired from fixed and dehydrated tissue. Dehydration, however, produces severe distortion of gel-like cochlear structures such as the tectorial membrane and the basilar membrane hyaline matrix. In this study, the hemicochlea technique, which allows fresh mouse cochlear material to be viewed from a radial perspective, was used to provide an itemized study of the dimensions of gross cochlear structures in four mouse strains (CBA/CaJ, 129/SvEv, 129/CD1 and C57BL/6J). Except for the CBA/CaJ, these strains are known to possess genes for age-related hearing loss. The measurements showed no major differences among the four strains. However, when compared with previous data, the thickness measures of the basilar membrane were up to 10 times larger. Such differences are likely to result from the different techniques used to process the material. The hemicochlea technique eliminates much of the distortion caused by dehydration, which was present in previous experiments.

Direct evidence of nitric oxide production in the guinea pig organ of Corti

Production of nitric oxide (NO) in the organ of Corti of the guinea pig was investigated using the new fluorescence indicator 4,5-diaminofluorescein diacetate for direct detection of NO. The organ of Corti, lateral wall of the cochlea and isolated outer and inner hair cells were examined to locate NO production sites. The fluorescence intensities were augmented by stimulation with L-arginine or glutamate, and significantly increased after inoculation with lipopolysaccharide. This is the first direct evidence of NO production in the cochlea. NO may play an important role in the physiology of the organ of Corti and may also be involved in hearing disorders.

[Serotonergic innervation of the auditory receptor]

The serotonergic inervation of the auditory receptor is composed by nerve fibres of multipolar neurons located on the superior lateral oliva. These neurons could correspond to outer elements of the nucleus reticularis caudalis. The peripheral projection on the auditory receptor is composed of varicose fibres distributed within the inner spiral bundle connecting on inner hair cells. Other fibres, after a spiral pathway within the Corti's tunnel, reach the outer hair cells. This new fascicle, with a particular origin and peripheral distribution, characterized as serotonergic and of unknown function, could be considered as a previously undescribed projection of the reticular formation on the auditory receptor.

The origin of eponyms used in cochlear anatomy

The anatomy of the cochlea, one of the most complicated parts of the inner ear, is associated with numerous eponyms, mostly originating from German anatomists of the 19th century. In addition to the organ of Corti and Reissner's membrane, which are the best known, nine other eponymic anatomic terms are associated with the cochlea: Deiters' cells, Hensen's cells, Hensen's strip, Claudius' cells, Boettscher's cells, Rosenthal's canal, Hardesty's membrane, Huschke's teeth, and Nuel's space. All of these anatomic eponyms are described, with their original references.

Expression of p75NTR and its associated protein NADE in the rat cochlea

OBJECTIVES/HYPOTHESIS

To investigate the expression of the low-affinity neurotrophin receptor p75 (p75NTR) and its associated protein NADE in the cochlea of the developing and the adult rat. Studies such as this one will help to predict the functional role of p75NTR and NADE in cochlear development.

STUDY DESIGN

Histochemical evaluation of p75NTR and NADE in the rat cochlea was performed.

METHODS

Immunohistochemical analysis was used to localize p75NTR and NADE in the rat cochlea at postnatal (PN) days PN0, PN2, PN4, PN6, PN8, PN10, and PN13 and in the adult. Confocal laser scanning microscopy was used to analyze whole-mount specimens.

RESULTS

Immunoreactivity of both p75NTR and NADE was observed in pillar cells. However, these proteins displayed reciprocal expression patterns. Expression of p75NTR was detected at PN0 and PN2, but disappeared after PN4. In contrast, NADE expression was initially detected at PN2 and persisted into adulthood.

CONCLUSIONS

The neurotrophin receptor p75NTR and NADE have distinct and independent roles in developing and mature cochlea.

Perilymphatic fluid compartments and intercellular spaces of the inner ear and the organ of Corti

An in vitro preparation of the inner ear cochlea has been used to visualize the structural relationships of unfixed, living sensory cells and structural components within the intact hearing organ. By perfusing perilymphatic compartments of the cochlea with fluorochrome-conjugated dextran, the extracellular spaces were clearly outlined. The staining pattern illustrated the large fluid compartments formed by the tunnel of Corti, the space of Nuel, and the outer tunnel. The dextran solution also indicated the spaces between the outer hair cell rows, the inner hair cells, and the surrounding supporting cells. The staining pattern demonstrates that the organ of Corti has a loose structure, suggesting a weak mechanical coupling between the cells. Moreover, it is evident that substances applied to the perilymph (e.g., therapeutic drugs) will readily reach all the cells of the hearing organ. In addition to the intraorgan fluid compartments, the spiral limbus was shown to contain significant volumes of perilymph within the intercellular spaces forming the so-called teeth of Huschke between the interdental cells. An extensive system of bundles following the teeth of Huschke was shown to be completely immersed in perilymph. The bundles were stained by a potentiometric dye, which in the inner ear primarily stains nerve fibers and sensory cells, which may indicate a nervous control of cells in this region.

Perilymphatic fluid compartments and intercellular spaces of the inner ear and the organ of Corti

An in vitro preparation of the inner ear cochlea has been used to visualize the structural relationships of unfixed, living sensory cells and structural components within the intact hearing organ. By perfusing perilymphatic compartments of the cochlea with fluorochrome-conjugated dextran, the extracellular spaces were clearly outlined. The staining pattern illustrated the large fluid compartments formed by the tunnel of Corti, the space of Nuel, and the outer tunnel. The dextran solution also indicated the spaces between the outer hair cell rows, the inner hair cells, and the surrounding supporting cells. The staining pattern demonstrates that the organ of Corti has a loose structure, suggesting a weak mechanical coupling between the cells. Moreover, it is evident that substances applied to the perilymph (e.g., therapeutic drugs) will readily reach all the cells of the hearing organ. In addition to the intraorgan fluid compartments, the spiral limbus was shown to contain significant volumes of perilymph within the intercellular spaces forming the so-called teeth of Huschke between the interdental cells. An extensive system of bundles following the teeth of Huschke was shown to be completely immersed in perilymph. The bundles were stained by a potentiometric dye, which in the inner ear primarily stains nerve fibers and sensory cells, which may indicate a nervous control of cells in this region.

Thyroid hormone deficiency before the onset of hearing causes irreversible damage to peripheral and central auditory systems

Both a genetic or acquired neonatal thyroid hormone (TH) deficiency may result in a profound mental disability that is often accompanied by deafness. The existence of various TH-sensitive periods during inner ear development and general success of delayed, corrective TH treatment was investigated by treating pregnant and lactating rats with the goitrogen methimazole (MMI). We observed that for the establishment of normal hearing ability, maternal TH, before fetal thyroid gland function on estrus days 17-18, is obviously not required. Within a crucial time between the onset of fetal thyroid gland function and the onset of hearing at postnatal day 12 (P12), any postponement in the rise of TH-plasma levels, as can be brought about by treating lactating mothers with MMI, leads to permanent hearing defects of the adult offspring. The severity of hearing defects that were measured in 3- to 9-mo-old offspring could be increased with each additional day of TH deficiency during this critical period. Unexpectedly, the active cochlear process, assayed by distortion product otoacoustic emissions (DPOAE) measurements, and speed of auditory brain stem responses, which both until now were not thought to be controlled by TH, proved to be TH-dependent processes that were damaged by a delay of TH supply within this critical time. In contrast, no significant differences in the gross morphology and innervation of the organ of Corti or myelin gene expression in the auditory system, detected as myelin basic protein (MBP) and proteolipid protein (PLP) mRNA using Northern blot approach, were observed when TH supply was delayed for few days. These classical TH-dependent processes, however, were damaged when TH supply was delayed for several weeks. These surprising results may suggest the existence of different TH-dependent processes in the auditory system: those that respond to corrective TH supply (e.g., innervation and morphogenesis of the organ of Corti) and those that do not, but require T3 activity during a very tight time window (e.g. , active cochlear process, central processes).

Serotonergic innervation of the organ of Corti

The olivocochlear efferent system of the mammalian cochlea, which is divided into two lateral and medial bundles, contains numerous neuroactive substances (acetylcholine, GABA, dopamine, enkephalins, dynorphins and CGRP). These have been located at the brainstem in neurons belonging to the lateral superior olive (lateral efferent system) or in neurons of the periolivary region around the medial superior olive and the trapezoid body (medial efferent system). All of these substances were found in well-characterized projections corresponding to lateral and medial nerve fibres and terminals which connect to the type I afferent dendrites and the outer hair cells, respectively. All could be involved in the modulation of the auditory process, as is suggested by the cochlear turnover increases observed in some of them (i.e. enkephalins or dopamine) induced by sound stimulation. Recently, the presence and distribution of serotonin-containing fibres has been included in the long list of cochlear neuroactive substances. However, its highly particular peripheral pattern of distribution together with the lack of response to sound stimulation could suggest that serotonergic fibres constitute a previously unknown cochlear innervation.

Toward three-dimensional analysis of cochlear structure

Recent results from a three-dimensional model of the cochlea are summarized. The features include physically realistic values of basilar membrane stiffness, mass, and fluid viscosity. The simple 'feed-forward' principle for the active process yields results in qualitative agreement with recent measurements in the cochlea. The limitation is a simplified representation of the organ of Corti, with two degrees of freedom representing the motion of the pectinate and arcuate zones of the basilar membrane. However, the inner sulcus fluid flow is included. The new feature presented in this paper is an approach to treat all the structural detail of the organ of Corti, with the sole input to the calculation in a form easily understood by anyone familiar with the cochlea. Specific results are shown for the Pakistani water buffalo, since a fairly complete anatomical description of this cochlea is available. The static stiffness from the calculation, based on only the anatomy and known values for the protein elastic moduli, are in remarkable agreement with recent measurements in the gerbil cochlea. Only preliminary results for the dynamic response with inviscid fluid are reported. Of interest, however, are the propagation modes related to significant fluid displacement and pressure in the different compartments of the organ of Corti.

Reticular lamina vibrations in the apical turn of a living guinea pig cochlea

The reticular lamina of the apical turn of a living guinea pig cochlea was viewed through the intact Reissner's membrane using a slit confocal microscope. Vibrations were measured at selected identified locations with a confocal heterodyne interferometer, in response to tones applied with an acoustic transducer coupled to the ear canal. The position coordinates of each location were recorded. Mechanical tuning curves were measured along a radial track at Hensen's cells, outer hair cells, inner hair cells and at the osseous spiral lamina, over a frequency range of 3 kHz, using five sound pressure levels (100, 90, 80, 70 and 60 dB SPL). The carrier to noise ratio obtained throughout the experiments was high. The response shape at any measuring location was not found to change appreciably with signal level. The response shape also did not change significantly with the radial position on the reticular lamina. However, the response magnitude increased progressively from the inner hair cell to the Hensen's cell. The observed linearity of response at the fundamental frequency is explained by the presence of negative feed back in the apical turn of the cochlea.

Comparing in vitro, in situ, and in vivo experimental data in a three-dimensional model of mammalian cochlear mechanics

Normal mammalian hearing is refined by amplification of the motion of the cochlear partition. This partition, comprising the organ of Corti sandwiched between the basilar and tectorial membranes, contains the outer hair cells that are thought to drive this amplification process. Force generation by outer hair cells has been studied extensively in vitro and in situ, but, to understand cochlear amplification fully, it is necessary to characterize the role played by each of the components of the cochlear partition in vivo. Observations of cochlear partition motion in vivo are severely restricted by its inaccessibility and sensitivity to surgical trauma, so, for the present study, a computer model has been used to simulate the operation of the cochlea under different experimental conditions. In this model, which uniquely retains much of the three-dimensional complexity of the real cochlea, the motions of the basilar and tectorial membranes are fundamentally different during in situ- and in vivo-like conditions. Furthermore, enhanced outer hair cell force generation in vitro leads paradoxically to a decrease in the gain of the cochlear amplifier during sound stimulation to the model in vivo. These results suggest that it is not possible to extrapolate directly from experimental observations made in vitro and in situ to the normal operation of the intact organ in vivo.

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 approximately 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 approximately 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.

Morphology of the unfixed cochlea

Our knowledge of cochlear geometry is based largely upon anatomical observations derived from fixed, dehydrated, embedded and/or sputter-coated material. We have now developed a novel preparation, the hemicochlea, where for the first time living cochlear structures can be observed in situ and from a radial perspective. The experiments were performed on the Mongolian gerbil. Ion substitution experiments suggest that no significant swelling or shrinkage occurs when the preparation is bathed in normal culture medium, so long as calcium concentration is kept at endolymph-like (20 microM) levels. The tectorial membrane-reticular lamina relationship appears to remain well preserved. Hensen's stripe maintains a close relationship with the inner hair cell stereociliary bundle, unless the mechanical coupling becomes disturbed. In addition, standard fixation and/or dehydration procedures are used to quantify changes due to shrinkage artifacts. Various morphometric gradients are examined in unfixed specimens from apical, middle, and basal turns.

Importance of type IV collagen, laminin, and heparan sulfate proteoglycan in the regulation of labyrinthine fluid in the rat cochlear duct

The distribution of major components of the basement membrane, such as type IV collagen, laminin, and heparan sulfate proteoglycan (HSPG), was investigated in the rat cochlear duct. Immunofluorescence demonstrated that type IV collagen, laminin and HSPG were distributed along capillaries in the cochlear duct, including the stria vascularis, spiral ligament, spiral prominence and spiral limbus. Additionally, type IV collagen, laminin and HSPG were found to be distributed from the basement membrane of Reissner's membrane to that of the spiral prominence in a linear pattern. The scala media was surrounded by these basement membrane components, demarcating endolymph from perilymph, along epithelial cells except at the stria vascularis. These findings suggest that type IV collagen, laminin and HSPG create the anatomical separation between endolymph and perilymph, thus indicating that they may be involved in the regulation of fluid transport between the endolymph and perilymph.

Cell and molecular basis of hearing

The mammalian auditory organ is a specialized sensory epithelium capable of detecting subnanometer movements produced by sound and transducing them into electrical signals. It is a dual system consisting of two types of sensory cells: the inner hair cells, which provide afferent input to the central nervous system, and the outer hair cells, which provide frequency resolution and signal amplification. The critical element in mechano-reception is the hair bundle, a cluster of stereocilia located in the apical end of the sensory cells. Mechanical stimulation causes deflection of ciliary bundles that leads to the opening or closing of transduction channels located in the apical part of the plasma membrane of the stereocilia. The current concept of auditory function requires an active process of signal amplification within the cochlea. It is generally believed that outer hair cells, functioning as both sensor and motor elements, are responsible for this amplification process. The motor function consists of changes in cell length produced by the concerted action of a large number of independent molecular motors distributed along the length of the outer hair cell lateral plasma membrane. Although experimental evidence suggests its association with conformational changes in transmembrane proteins, the exact nature of the outer hair cell force generation mechanism on a molecular level is still unknown.

Electron microscopic localization of nitric oxide I synthase in the organ of Corti of the guinea pig

Nitric oxide synthase (NOS) activity has been detected previously in the mammalian cochlea at a light microscopic level. Here we present results of electron microscopic analysis for post-embedding immunoreactivity of neural-type NOS I in the cochlea of the guinea pig. Strong enzyme immunoreactivity was identified in the cytoplasm of inner and outer hair cells. Gold-labeled NOS I antibodies were mainly located in electron-dense areas of the cytoplasm, whereas electron-lucent regions of the receptor cells were nearly free from any immunoreactivity. In both types of hair cells anti-NOS I antibodies were also visible in the cuticular plates, hair bundles and nuclei. Further ultrastructural analysis revealed that the submembranous cisternae of the outer hair cells were nearly free from any reaction product, demonstrating that the whole cytoplasm of this hair cell was not immunoreactive. Other NOS I immunoreactivity was identified in the cuticular plates of the inner and outer pillar cells and in the cytoskeletal elements located in the apical parts of Deiter cells, forming the lamina reticularis or in cytoskeletal-containing regions in basal Deiter cells. Anti-NOS antibodies were visible in the nuclei of various cell types. Our findings suggest that nitric oxide produced by NO I synthase in the organ of Corti may act as a modulator of hair cell physiology during the processes of signal transduction with frequency selectivity.

Alternatively spliced isoforms of the Na+/Ca2+ exchanger in the guinea pig cochlea

The cochlea has been suggested to express some Na+/ Ca2+ exchangers (NCX), since efficient acoustic transduction requires cytosolic calcium homeostasis. The present study revealed that several spliced isoforms of NCX are expressed in the guinea pig cochlea. Moreover, to determine their localization in the cochlea, microdissected RT-PCR was performed. The guinea pig cochlea was microdissected into three parts (lateral wall, the organ of Corti and modiolus). The cochlear lateral wall and the organ of Corti expressed only a single isoform of NCX1. On the other hand, five isoforms of NCX1 and four isoforms of NCX3 were detected in the cochlear modiolus. The alternative splicing may provide diverse functions for NCX in the cochlea.

[High resolution scanning electron microscopy of isolated outer hair cells]

Isolated hair cell preparations have gained wide acceptance as a model for studying physiological and molecular properties of the sensory cells involved in the hearing process. Ultrastructural details, such as stereocilia links, lateral membrane substructure or synaptic links are of crucial importance for normal sensory transduction. For this reason, we developed a high-resolution scanning electron microscopy (SEM) procedure to study the surface of isolated hair cells. Cells were mechanically and/or enzymatically separated, isolated and immobilized on cover slips by alcian blue and fixed by 2% glutardialdehyde or 1% OsO4. After dehydration, preparations were critical point-dried and sputter-coated with gold-palladium (2-4 nm). Up to 5 nm resolution was achieved. Optimal fixation kept the cells in their typical cylindrical forms. Preservation of the stereocilia and the apical plates of the outer hair cells depended strongly on the fixation process. Tip- and side-links were observed only sporadically because of the aggressive preparation procedure. The lateral plasma membranes of the cell bodies showed regular granular structures of 5-7 nm diameter at maximal magnification. The granular structure of the cell membrane seemed to correspond to putative transmembrane proteins believed to generate membrane-based motility. The remnants of the nerve endings and/or supporting cells usually covered the cell base. The preservation of the cells was better when enzymatic isolation was omitted. The technique used allowed for high resolution ultrastructural examination of isolated hair cells and, when combined with immunological labeling, may permit the identification of proteins at a molecular level.

Computer-aided three-dimensional reconstruction in human cochlear maps: measurement of the lengths of organ of Corti, outer wall, inner wall, and Rosenthal's canal

This paper describes the application of computer-aided three-dimensional reconstruction to measurements of the length of the organ of Corti (mean +/- SD, 35.58 +/- 1.41 mm), scala tympani outer wall (40.81 +/- 1.97 mm), scala tympani inner wall (18.29 +/- 1.47 mm), and Rosenthal's canal center (15.98 +/- 1.33 mm) in eight adult male cochleas. The Rosenthal's canal center ranged between 1 3/4 and 2 turns, did not appear to be linearly related to the organ of Corti, and lay near the basal end of the latter. The length of the organ of Corti measured with three-dimensional reconstruction differed by 7.6% +/- 3.2% ("cutting angle difference") from that derived from traditional two-dimensional reconstruction on the plane perpendicular to the plane of section, and by 2.6% +/- 1.7% ("geometric difference") from that derived from two-dimensional reconstruction on the axial plane at right angles to the modiolar axis.