Regional differences in cochlear nonlinearity across the basal organ of Corti of gerbil: Regional differences in cochlear nonlinearity

Auditory sensation is based in nanoscale vibration of the sensory tissue of the cochlea, the organ of Corti complex (OCC). Motion within the OCC is now observable due to optical coherence tomography. In a previous study (Cooper et al., 2018), the region that includes the electro-motile outer hair cells (OHC) and Deiters cells (DC) was observed to move with larger amplitude than the basilar membrane (BM) and surrounding regions and was termed the "hotspot." In addition to this quantitative distinction, the hotspot moved qualitatively differently than the BM, in that its motion scaled nonlinearly with stimulus level at all frequencies, evincing sub-BF activity. Sub-BF activity enhances non-BF motion; thus the frequency tuning of the OHC/DC region was reduced relative to the BM. In this work we further explore the motion of the gerbil basal OCC and find that regions that lack significant sub-BF activity include the BM, the medial and lateral OCC, and the reticular lamina (RL) region. The observation that the RL region does not move actively sub-BF (already observed in Cho and Puria 2022), suggests that hair cell stereocilia are not exposed to sub-BF activity in the cochlear base. The observation that the lateral and RL regions move approximately linearly sub-BF indicates that linear forces dominate non-linear OHC-based forces on these components at sub-BF frequencies. A complex difference analysis was performed to reveal the internal motion of the OHC/DC region and showed that amplitude structure and phase shifts in the directly measured OHC/DC motion emerge due to the internal OHC/DC motion destructively interfering with BM motion.

Isolation and Culture of Primary Cochlear Hair Cells from Neonatal Mice

Cochlear hair cells are the sensory receptors of the auditory system. These cells are located in the organ of Corti, the sensory organ responsible for hearing, within the osseous labyrinth of the inner ear. Cochlear hair cells consist of two anatomically and functionally distinct types: outer and inner hair cells. Damage to either of them results in hearing loss. Notably, as inner hair cells cannot regenerate, and damage to them is permanent. Hence, in vitro cultivation of primary hair cells is indispensable for investigating the protective or regenerative effects of cochlear hair cells. This study aimed to discover a method for isolating and cultivating mouse hair cells. After manual removal of the cochlear lateral wall, the auditory epithelium was meticulously dissected from the cochlear modiolus under a microscope, incubated in a mixture consisting of 0.25% trypsin-EDTA for 10 min at 37 °C, and gently suspended in culture medium using a 200 µL pipette tip. The cell suspension was passed through a cell filter, the filtrate was centrifuged, and cells were cultured in 24-well plates. Hair cells were identified based on their capacity to express a mechanotransduction complex, myosin-VIIa, which is involved in motor tensions, and via selective labeling of F-actin using phalloidin. Cells reached >90% confluence after 4 d in culture. This method can enhance our understanding of the biological characteristics of in vitro cultured hair cells and demonstrate the efficiency of cochlear hair cell cultures, establishing a solid methodological foundation for further auditory research.

SoxC transcription factors shape the epigenetic landscape to establish competence for sensory differentiation in the mammalian organ of Corti

The auditory organ of Corti is comprised of only two major cell types-the mechanosensory hair cells and their associated supporting cells-both specified from a single pool of prosensory progenitors in the cochlear duct. Here, we show that competence to respond to Atoh1, a transcriptional master regulator necessary and sufficient for induction of mechanosensory hair cells, is established in the prosensory progenitors between E12.0 and 13.5. The transition to the competent state is rapid and is associated with extensive remodeling of the epigenetic landscape controlled by the SoxC group of transcription factors. Conditional loss of Sox4 and Sox11-the two homologous family members transiently expressed in the inner ear at the time of competence establishment-blocks the ability of prosensory progenitors to differentiate as hair cells. Mechanistically, we show that Sox4 binds to and establishes accessibility of early sensory lineage-specific regulatory elements, including ones associated with Atoh1 and its direct downstream targets. Consistent with these observations, overexpression of Sox4 or Sox11 prior to developmental establishment of competence precociously induces hair cell differentiation in the cochlear progenitors. Further, reintroducing Sox4 or Sox11 expression restores the ability of postnatal supporting cells to differentiate as hair cells in vitro and in vivo. Our findings demonstrate the pivotal role of SoxC family members as agents of epigenetic and transcriptional changes necessary for establishing competence for sensory receptor differentiation in the inner ear.

Gradients of glucose metabolism regulate morphogen signalling required for specifying tonotopic organisation in the chicken cochlea

In vertebrates with elongated auditory organs, mechanosensory hair cells (HCs) are organised such that complex sounds are broken down into their component frequencies along a proximal-to-distal long (tonotopic) axis. Acquisition of unique morphologies at the appropriate position along the chick cochlea, the basilar papilla, requires that nascent HCs determine their tonotopic positions during development. The complex signalling within the auditory organ between a developing HC and its local niche along the cochlea is poorly understood. Using a combination of live imaging and NAD(P)H fluorescence lifetime imaging microscopy, we reveal that there is a gradient in the cellular balance between glycolysis and the pentose phosphate pathway in developing HCs along the tonotopic axis. Perturbing this balance by inhibiting different branches of cytosolic glucose catabolism disrupts developmental morphogen signalling and abolishes the normal tonotopic gradient in HC morphology. These findings highlight a causal link between graded morphogen signalling and metabolic reprogramming in specifying the tonotopic identity of developing HCs.

Lrrn1 Regulates Medial Boundary Formation in the Developing Mouse Organ of Corti

One of the most striking aspects of the sensory epithelium of the mammalian cochlea, the organ of Corti (OC), is the presence of precise boundaries between sensory and nonsensory cells at its medial and lateral edges. A particular example of this precision is the single row of inner hair cells (IHCs) and associated supporting cells along the medial (neural) boundary. Despite the regularity of this boundary, the developmental processes and genetic factors that contribute to its specification are poorly understood. In this study we demonstrate that Leucine Rich Repeat Neuronal 1 (Lrrn1), which codes for a single-pass, transmembrane protein, is expressed before the development of the mouse organ of Corti in the row of cells that will form its medial border. Deletion of Lrrn1 in mice of mixed sex leads to disruptions in boundary formation that manifest as ectopic inner hair cells and supporting cells. Genetic and pharmacological manipulations demonstrate that Lrrn1 interacts with the Notch signaling pathway and strongly suggest that Lrrn1 normally acts to enhance Notch signaling across the medial boundary. This interaction is required to promote formation of the row of inner hair cells and suppress the conversion of adjacent nonsensory cells into hair cells and supporting cells. These results identify Lrrn1 as an important regulator of boundary formation and cellular patterning during development of the organ of Corti.SIGNIFICANCE STATEMENT Patterning of the developing mammalian cochlea into distinct sensory and nonsensory regions and the specification of multiple different cell fates within those regions are critical for proper auditory function. Here, we report that the transmembrane protein Leucine Rich Repeat Neuronal 1 (LRRN1) is expressed along the sharp medial boundary between the single row of mechanosensory inner hair cells (IHCs) and adjacent nonsensory cells. Formation of this boundary is mediated in part by Notch signaling, and loss of Lrrn1 leads to disruptions in boundary formation similar to those caused by a reduction in Notch activity, suggesting that LRRN1 likely acts to enhance Notch signaling. Greater understanding of sensory/nonsensory cell fate decisions in the cochlea will help inform the development of regenerative strategies aimed at restoring auditory function.

The Dual Roles of Triiodothyronine in Regulating the Morphology of Hair Cells and Supporting Cells during Critical Periods of Mouse Cochlear Development

Clinically, thyroid-related diseases such as endemic iodine deficiency and congenital hypothyroidism are associated with hearing loss, suggesting that thyroid hormones are essential for the development of normal hearing. Triiodothyronine (T3) is the main active form of thyroid hormone and its effect on the remodeling of the organ of Corti remain unclear. This study aims to explore the effect and mechanism of T3 on the remodeling of the organ of Corti and supporting cells development during early development. In this study, mice treated with T3 at postnatal (P) day 0 or P1 showed severe hearing loss with disordered stereocilia of the outer hair cells (OHCs) and impaired function of mechanoelectrical transduction of OHCs. In addition, we found that treatment with T3 at P0 or P1 resulted in the overproduction of Deiter-like cells. Compared with the control group, the transcription levels of Sox2 and notch pathway-related genes in the cochlea of the T3 group were significantly downregulated. Furthermore, Sox2-haploinsufficient mice treated with T3 not only showed excess numbers of Deiter-like cells but also a large number of ectopic outer pillar cells (OPCs). Our study provides new evidence for the dual roles of T3 in regulating both hair cells and supporting cell development, suggesting that it is possible to increase the reserve of supporting cells.

Reconstruction of transverse-longitudinal vibrations in the organ of Corti complex via optical coherence tomography

Optical coherence tomography (OCT) is a common modality for measuring vibrations within the organ of Corti complex (OCC) in vivo. OCT's uniaxial nature leads to limitations that complicate the interpretation of data from cochlear mechanics experiments. The relationship between the optical axis (axis of motion measurement) and anatomically relevant axes in the cochlea varies across experiments, and generally is not known. This leads to characteristically different motion measurements taken from the same structure at different orientations. We present a method that can reconstruct two-dimensional (2-D) motion of intra-OCC structures in the cochlea's longitudinal-transverse plane. The method requires only a single, unmodified OCT system, and does not require any prior knowledge of precise structural locations or measurement angles. It uses the cochlea's traveling wave to register points between measurements taken at multiple viewing angles. We use this method to reconstruct 2-D motion at the outer hair cell/Deiters cell junction in the gerbil base, and show that reconstructed transverse motion resembles directly measured transverse motion, thus validating the method. The technique clarifies the interpretation of OCT measurements, enhancing their utility in probing the micromechanics of the cochlea.

Cochlear motion across the reticular lamina implies that it is not a stiff plate

Within the cochlea, the basilar membrane (BM) is coupled to the reticular lamina (RL) through three rows of piezo-like outer hair cells (OHCs) and supporting cells that endow mammals with sensitive hearing. Anatomical differences across OHC rows suggest differences in their motion. Using optical coherence tomography, we measured in vivo and postmortem displacements through the gerbil round-window membrane from approximately the 40-47 kHz best-frequency (BF) regions. Our high spatial resolution allowed measurements across the RL surface at the tops of the three rows of individual OHCs and their bottoms, and across the BM. RL motion varied radially; the third-row gain was more than 3 times greater than that of the first row near BF, whereas the OHC-bottom motions remained similar. This implies that the RL mosaic, comprised of OHC and phalangeal-process tops joined together by adhesion molecules, is much more flexible than the Deiters' cells connected to the OHCs at their bottom surfaces. Postmortem, the measured points moved together approximately in phase. These imply that in vivo, the RL does not move as a stiff plate hinging around the pillar-cell heads near the first row as has been assumed, but that its mosaic-like structure may instead bend and/or stretch.

Deiters Cells Act as Mechanical Equalizers for Outer Hair Cells

The outer hair cells in the mammalian cochlea are cellular actuators essential for sensitive hearing. The geometry and stiffness of the structural scaffold surrounding the outer hair cells will determine how the active cells shape mammalian hearing by modulating the organ of Corti (OoC) vibrations. Specifically, the tectorial membrane and the Deiters cell are mechanically in series with the hair bundle and soma, respectively, of the outer hair cell. Their mechanical properties and anatomic arrangement must determine the relative motion among different OoC structures. We measured the OoC mechanics in the cochleas acutely excised from young gerbils of both sexes at a resolution fine enough to distinguish the displacement of individual cells. A three-dimensional finite element model of fully deformable OoC was exploited to analyze the measured data in detail. As a means to verify the computer model, the basilar membrane deformations because of static and dynamic stimulations were measured and simulated. Two stiffness ratios have been identified that are critical to understand cochlear physics, which are the stiffness of the tectorial membrane with respect to the hair bundle and the stiffness of the Deiters cell with respect to the outer hair cell body. Our measurements suggest that the Deiters cells act like a mechanical equalizer so that the outer hair cells are constrained neither too rigidly nor too weakly.SIGNIFICANCE STATEMENT Mammals can detect faint sounds thanks to the action of mammalian-specific receptor cells called the outer hair cells. It is getting clearer that understanding the interactions between the outer hair cells and their surrounding structures such as the tectorial membrane and the Deiters cell is critical to resolve standing debates. Depending on theories, the stiffness of those two structures ranges from negligible to rigid. Because of their perceived importance, their properties have been measured in previous studies. However, nearly all existing data were obtained ex situ (after they were detached from the outer hair cells), which obscures their interaction with the outer hair cells. We quantified the mechanical properties of the tectorial membrane and the Deiters cell in situ.

Sound Induced Vibrations Deform the Organ of Corti Complex in the Low-Frequency Apical Region of the Gerbil Cochlea for Normal Hearing : Sound Induced Vibrations Deform the Organ of Corti Complex

Human speech primarily contains low frequencies. It is well established that such frequencies maximally excite the cochlea near its apex. But, the micromechanics that precede and are involved in this transduction are not well understood. We measured vibrations from the low-frequency, second turn in intact gerbil cochleae using optical coherence tomography (OCT). The data were used to create spatial maps that detail the sound-evoked motions across the sensory organ of Corti complex (OCC). These maps were remarkably similar across animals and showed little variation with frequency or level. We identify four, anatomically distinct, response regions within the OCC: the basilar membrane (BM), the outer hair cells (OHC), the lateral compartment (lc), and the tectorial membrane (TM). Results provide evidence that active processes in the OHC play an important role in the mechanical interplay between different OCC structures which increases the amplitude and tuning sharpness of the traveling wave. The angle between the OCT beam and the OCC makes that we captured radial motions thought to be the effective stimulus to the mechano-sensitive hair bundles. We found that TM responses were relatively weak, arguing against a role in enhancing mechanical hair bundle deflection. Rather, BM responses were found to closely resemble the frequency selectivity and sensitivity found in auditory nerve fibers (ANF) that innervate the low-frequency cochlea.

Salicylate-induced changes in organ of Corti vibrations

Intra organ of Corti (OC) vibrations differ from those measured at the basilar membrane (BM), with higher amplitudes and a wide-band nonlinearity extending well below a region's best frequency. The vibrations are boosted by the cochlear amplifier, the active processes within the mammalian hearing organ, and are thus sensitive to metabolic or pharmacological manipulation. We introduced salicylate, a known blocker of outer hair cell (OHC) based electromotility, into the perilymphatic space by applying sodium salicylate onto the round window membrane. Vibration patterns of an area of the OC were mapped with phase sensitive optical coherence tomography before and after treatment; distortion product otoacoustic emissions (DPOAEs) were measured at similar times to assess the cochlear condition. Following treatment, all regions showed a loss of vibration amplitude and tuning while OHC-region vibrations retained their wide-band nonlinearity. OC vibrations, which had been relatively confined in a region including OHCs and extending to the BM at the outer pillar foot, became less confined with structures lateral to the OHCs sometimes exhibiting the highest amplitudes. Vibrations and DPOAEs could recover to baseline levels over approximately three hours post treatment. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

Using volumetric optical coherence tomography to achieve spatially resolved organ of Corti vibration measurements

Optical coherence tomography (OCT) has become a powerful tool for measuring vibrations within the organ of Corti complex (OCC) in cochlear mechanics experiments. However, the one-dimensional nature of OCT measurements, combined with experimental and anatomical constraints, make these data ambiguous: Both the relative positions of measured structures and their orientation relative to the direction of measured vibrations are not known a priori. We present a method by which these measurement features can be determined via the use of a volumetric OCT scan to determine the relationship between the imaging/measurement axes and the canonical anatomical axes. We provide evidence that the method is functional by replicating previously measured radial vibration patterns of the basilar membrane (BM). We used the method to compare outer hair cell and BM vibration phase in the same anatomical cross section (but different optical cross sections), and found that outer hair cell region vibrations lead those of the BM across the entire measured frequency range. In contrast, a phase lead is only present at low frequencies in measurements taken within a single optical cross section. Relative phase is critical to the workings of the cochlea, and these results emphasize the importance of anatomically oriented measurement and analysis.

Comparison of Speech Recognition With an Organ of Corti Versus Spiral Ganglion Frequency-to-Place Function in Place-Based Mapping of Cochlear Implant and Electric-Acoustic Stimulation Devices

OBJECTIVE

To compare acute speech recognition with a cochlear implant (CI) alone or electric-acoustic stimulation (EAS) device for place-based maps calculated with an organ of Corti (OC) versus a spiral ganglion (SG) frequency-to-place function.

PATIENTS

Eleven adult CI recipients of a lateral wall electrode array.

INTERVENTION

Postoperative imaging was used to derive place-based maps calculated with an OC versus SG function.

MAIN OUTCOME MEASURE

Phoneme recognition was evaluated at initial activation with consonant-nucleus-consonant (CNC) words presented using an OC versus a SG place-based map.

RESULTS

For the 9 CI-alone users, there was a nonsignificant trend for better acute phoneme recognition with the SG map (mean 18 RAUs) than the OC map (mean 9 RAUs; p = 0.071, 95% CI [≤-1.2]). When including the 2 EAS users in the analysis, performance was significantly better with the SG map (mean 21 RAUs) than the OC map (mean 7 RAUs; p = 0.019, 95% CI [≤-6.2]).

CONCLUSIONS

Better phoneme recognition with the SG frequency-to-place function could indicate more natural tonotopic alignment of information compared with the OC place-based map.A prospective, randomized investigation is currently underway to assess longitudinal outcomes with place-based mapping in CI-alone and EAS devices using the SG frequency-to-place function.

Correlation of signal to noise ratio (SNR) value on distortion product otoacoustic emission (DPOAE) and expression of nuclear factor erythroid 2-related factor 2 (NRF2) in cochlear organ of Corti in rat exposed to noise

Aim To investigate the changes in the value of the signal to noise ratio (SNR) and to assess changes in the expression of nuclear factor erythroid 2-related factor 2 (NRF2) in the organ of Corti of rat exposed to noise. Methods This study used a randomized post test only control group laboratory experimental design with 27 male Wistar strain Rattus norvegicus. The study group was divided into 3 groups (n = 9): group I (control), group 2 (2 hours of 100 dB noise exposure) and group 3 (2 hours of 110 dB noise exposure). Results There was no significant difference in the SNR in the group 1 on day 0, 2 and 4 (p>0.05). However, there was a significant difference in the SNR in the group 2 and the group 3 on day 0, 2 and 4 (p<0.05). There was a significant difference in the mean levels of NRF2 expression in the cochlear organ of Rattus norvegicus in all groups (p<0.05). There was no correlation between the SNR and the NRF2 expression in group 2 (p>0.05), but there was a correlation between the SNR and the NRF2 expression in the group 3 (p<0.05). Conclusion There was found a correlation between the SNR value on distortion product otoacoustic emission (DPOAE) examination and NRF2 expression in the cochlear organ of Corti of Rattus norvegicus exposed to 110 dB noise.

The interplay of organ-of-Corti vibrational modes, not tectorial- membrane resonance, sets outer-hair-cell stereocilia phase to produce cochlear amplification

The mechanical motions that deflect outer-hair-cell (OHC) stereocilia and the resulting effects of OHC motility are reviewed, concentrating on high-frequency cochlear regions. It has been proposed that a tectorial-membrane (TM) resonance makes the phase of OHC stereocilia motion be appropriate to produce cochlear amplification, i.e. so that the OHC force that pushes the basilar membrane (BM) is in the same direction as BM velocity. Evidence for and against the TM-resonance hypothesis are considered, including new cochlear-motion measurements using optical coherence tomography, and it is concluded that there is no such TM resonance. The evidence points to there being an advance in the phase of reticular lamina (RL) radial motion at a frequency approximately ½ octave below the BM characteristic frequency, and that this is the main source of the phase difference between the TM and RL radial motions that produces cochlear amplification. It appears that the change in phase of RL radial motion comes about because of a transition between different organ-of-Corti (OoC) vibrational modes that changes RL motion relative to BM and TM motion. The origins and consequences of the large phase change of RL radial motion relative to BM motion are considered; differences in the reported patterns of these changes may be due to different viewing angles. Detailed motion data and new models are needed to better specify the vibrational patterns of the OoC modes and the role of the various OoC structures in producing the modes and the mode transition.

Inner ear is a target for insulin signaling and insulin resistance: evidence from mice and auditory HEI-OC1 cells

OBJECTIVE

The mechanisms underlying the association between diabetes and inner ear dysfunction are not known yet. The aim of the present study is to evaluate the impact of obesity/insulin resistance on inner ear fluid homeostasis in vivo, and to investigate whether the organ of Corti could be a target tissue for insulin signaling using auditory House Ear Institute-Organ of Corti 1 (HEI-OC1) cells as an in vitro model.

METHODS

High fat diet (HFD) fed C57BL/6J mice were used as a model to study the impact of insulin resistance on the inner ear. In one study, 12 C57BL/6J mice were fed either control diet or HFD and the size of the inner ear endolymphatic fluid compartment (EFC) was measured after 30 days using MRI and gadolinium contrast as a read-out. In another study, the size of the inner ear EFC was evaluated in eight C57BL/6J mice both before and after HFD feeding, with the same techniques. HEI-OC1 auditory cells were used as a model to investigate insulin signaling in organ of Corti cells.

RESULTS

HFD feeding induced an expansion of the EFC in C57BL/6J mice, a hallmark of inner ear dysfunction. Insulin also induced phosphorylation of protein kinase B (PKB/Akt) at Ser473, in a PI3-kinase-dependent manner. The phosphorylation of PKB was inhibited by isoproterenol and IBMX, a general phosphodiesterase (PDE) inhibitor. PDE1B, PDE4D and the insulin-sensitive PDE3B were found expressed and catalytically active in HEI-OC1 cells. Insulin decreased and AICAR, an activator of AMP-activated protein kinase, increased the phosphorylation at the inhibitory Ser79 of acetyl-CoA carboxylase, the rate-limiting enzyme in de novo lipogenesis. Furthermore, the activity of hormone-sensitive lipase, the rate-limiting enzyme in lipolysis, was detected in HEI-OC1 cells.

CONCLUSIONS

The organ of Corti could be a target tissue for insulin action, and inner ear insulin resistance might contribute to the association between diabetes and inner ear dysfunction.

Time courses of changes in phospho- and total- MAP kinases in the cochlea after intense noise exposure

Mitogen-activated protein kinases (MAP kinases) are intracellular signaling kinases activated by phosphorylation in response to a variety of extracellular stimuli. Mammalian MAP kinase pathways are composed of three major pathways: MEK1 (mitogen-activated protein kinase kinase 1)/ERK 1/2 (extracellular signal-regulated kinases 1/2)/p90 RSK (p90 ribosomal S6 kinase), JNK (c-Jun amino (N)-terminal kinase)/c-Jun, and p38 MAPK pathways. These pathways coordinately mediate physiological processes such as cell survival, protein synthesis, cell proliferation, growth, migration, and apoptosis. The involvement of MAP kinase in noise-induced hearing loss (NIHL) has been implicated in the cochlea; however, it is unknown how expression levels of MAP kinase change after the onset of NIHL and whether they are regulated by transient phosphorylation or protein synthesis. CBA/J mice were exposed to 120-dB octave band noise for 2 h. Auditory brainstem response confirmed a component of temporary threshold shift within 0–24 h and significant permanent threshold shift at 14 days after noise exposure. Levels and localizations of phospho- and total- MEK1/ERK1/2/p90 RSK, JNK/c-Jun, and p38 MAPK were comprehensively analyzed by the Bio-Plex® Suspension Array System and immunohistochemistry at 0, 3, 6, 12, 24 and 48 h after noise exposure. The phospho-MEK1/ERK1/2/p90 RSK signaling pathway was activated in the spiral ligament and the sensory and supporting cells of the organ of Corti, with peaks at 3–6 h and independently of regulations of total-MEK1/ERK1/2/p90 RSK. The expression of phospho-JNK and p38 MAPK showed late upregulation in spiral neurons at 48 h, in addition to early upregulations with peaks at 3 h after noise trauma. Phospho-p38 MAPK activation was dependent on upregulation of total-p38 MAPK. At present, comprehensive data on MAP kinase expression provide significant insight into understanding the molecular mechanism of NIHL, and for developing therapeutic models for acute sensorineural hearing loss.

Pax2 and Pax8 cooperate in mouse inner ear morphogenesis and innervation

BACKGROUND

Pax2;5;8 transcription factors play diverse roles in vertebrate and invertebrate organogenesis, including the development of the inner ear. Past research has suggested various cochlear defects and some vestibular defects in Pax2 null mice but the details of the cochlear defects and the interaction with other Pax family members in ear development remain unclear.

RESULTS

We show that Pax2;8 double null mice do not develop an ear past the otocyst stage and show little to no sensory as well as limited and transient neuronal development, thus indicating that these two family members are essential for overall ear morphogenesis and sustained neurosensory development. In support of functional redundancy between Pax proteins, Pax2 can be substituted by a Pax5 minigene, a gene normally not expressed in the embryonic mouse ear. There is no detectable morphological defect in Pax8 null mice suggesting that Pax2 expression can compensate for Pax8. Conversely, Pax8 cannot compensate for Pax2 leading to a cochlear phenotype not fully appreciated previously: Cochlear development is delayed until E15.5 when the cochlea extrudes as a large sack into the brain case. Immunocytochemistry and tracing from the brain show that a cochlear spiral ganglia form as a small addition to the inferior vestibular ganglion. However, the empty cochlear sack, devoid of any sensory epithelium development as indicated by the absence of Sox2 or MyoVII expression, nevertheless develop a dense innervation network of small neurons situated in the wall of the cochlear sack.

CONCLUSIONS

Combined these data suggest that Pax2 is needed for organ of Corti formation and is directly or indirectly involved in the coordination of spiral ganglion formation which is partially disrupted in the Pax2 null ears. All three Pax genes can signal redundantly in the ear with their function being determined primarily by the spatio-temporal expression driven by the three distinct promoters of these genes.

Planar cell polarity signaling: a common mechanism for cellular polarization

Epithelial cells frequently display--in addition to the common apical-basolateral polarity--a polarization within the plane of the epithelium. This is commonly referred to as planar cell polarity (PCP) or tissue polarity. Examples of vertebrate PCP include epithelial patterning in the skin and inner ear, and also the morphogenetic movements of mesenchymal cells during convergent extension at gastrulation. In Drosophila, all adult epithelial structures of the cuticle are polarized within the plane. This review presents recent results and new insights into the molecular mechanisms underlying the establishment of PCP, and compares and contrasts the intriguing similarities between PCP signaling in Drosophila and vertebrates.

Regulation of polarized extension and planar cell polarity in the cochlea by the vertebrate PCP pathway

The mammalian auditory sensory organ, the organ of Corti, consists of sensory hair cells with uniformly oriented stereocilia on the apical surfaces and has a distinct planar cell polarity (PCP) parallel to the sensory epithelium. It is not certain how this polarity is achieved during differentiation. Here we show that the organ of Corti is formed from a thicker and shorter postmitotic primordium through unidirectional extension, characteristic of cellular intercalation known as convergent extension. Mutations in the PCP pathway interfere with this extension, resulting a shorter and wider cochlea as well as misorientation of stereocilia. Furthermore, parallel to the homologous pathway in Drosophila melanogaster, a mammalian PCP component Dishevelled2 shows PCP-dependent polarized subcellular localization across the organ of Corti. Taken together, these data suggest that there is a conserved molecular mechanism for PCP pathways in invertebrates and vertebrates and indicate that the mammalian PCP pathway might directly couple cellular intercalations to PCP establishment in the cochlea.

Two contrasting roles for Notch activity in chick inner ear development:specification of prosensory patches and lateral inhibition of hair-cell differentiation

Lateral inhibition mediated by Notch is thought to generate the mosaic of hair cells and supporting cells in the inner ear, but the effects of the activated Notch protein itself have never been directly tested. We have explored the role of Notch signalling by transiently overexpressing activated Notch (NICD) in the chick otocyst. We saw two contrasting consequences, depending on the time and site of gene misexpression: (1)inhibition of hair-cell differentiation within a sensory patch; and (2)induction of ectopic sensory patches. We infer that Notch signalling has at least two functions during inner ear development. Initially, Notch activity can drive cells to adopt a prosensory character, defining future sensory patches. Subsequently, Notch signalling within each such patch mediates lateral inhibition, restricting the proportion of cells that differentiate as hair cells so as to generate the fine-grained mixture of hair cells and supporting cells.

Cochlear Degeneration in Leigh Disease: Histopathologic Features

OBJECTIVE

To describe pathologic findings from temporal bones acquired from an infant with Leigh disease.

STUDY DESIGN

Retrospective case review.

MATERIALS AND METHODS

Temporal bones were taken at autopsy from an 8-month-old infant with Leigh disease. The right temporal bone was studied by microdissection. The middle ear was examined and the inner ear sensory organs dissected for study by light microscopy. The left temporal bone was embedded in celloidin, and sections were cut for microscopic examination.

RESULTS

Middle ear structures were normal bilaterally. There was, however, evidence of otitis media in both middle ears, which was more severe on the left side. Inner and outer hair cell loss, patchy degeneration of organ of Corti, and loss of nerve fibers in the osseous spiral lamina were found in the basal and middle turns of both cochleas. Basophilic deposits in the stria vascularis were observed in the apical portion of the left cochlea.

CONCLUSIONS

Inner ear sensorineural degeneration may occur in Leigh disease. Possible cochlear dysfunction caused by the degenerative changes needs to be considered in the hearing assessment of patients with Leigh disease.

A genetic approach to understanding auditory function

Little is known of the molecular basis of normal auditory function. In contrast to the visual or olfactory senses, in which reasonable amounts of sensory tissue can be gathered, the auditory system has proven difficult to access through biochemical routes, mainly because such small amounts of tissue are available for analysis. Key molecules, such as the transduction channel, may be present in only a few tens of copies per sensory hair cell, compounding the difficulty. Moreover, fundamental differences in the mechanism of stimulation and, most importantly, the speed of response of audition compared with other senses means that we have no well-understood models to provide good candidate molecules for investigation. For these reasons, a genetic approach is useful for identifying the key components of auditory transduction, as it makes no assumptions about the nature or expression level of molecules essential for hearing. We review here some of the major advances in our understanding of auditory function resulting from the recent rapid progress in identification of genes involved in deafness.

Electron microscope observations on in vitro cultures of the isolated fowl embryo otocyst

In vitro cultures of isolated fowl embryo otocysts were studied with the electron microscope. Hair cells of the developing organ of Corti and crista ampullaris have been examined with particular reference to the structure of the cilia and of the cell membrane. Two types of hair cells could be distinguished on the basis whether or not they possessed a "kinocilium" and "stereocilia," or "stereocilia" only. The cytoplasmic membranes were simple and there were no multiple vesicular layers in any of the hair cells. The supporting elements consisted of supporting cells flanking the hair cells, fibroblasts, and the cartilaginous otic capsule. Both the cochlear and vestibular sensory area showed rich innervation by mainly non-myelinated fibers with partial myelinization in others. There were well developed ganglion cells present. Bare axons penetrated the basement membrane and spread, amongst the supporting cells sheltering them, to the base of the hair cells where they formed bud-shaped nerve endings but, at the stage of development examined, no calyces. These in vitro cultures of the isolated fowl embryo otocyst provided convenient and suitable material for the electron microscope study of the sensory epithelium of the ear and revealed further that the isolated fowl embryo otocyst possesses great powers of self-differentiation also at the ultrastructural level.

Improved and simplified methods for specifying positions of the electrode bands of a cochlear implant array

OBJECTIVE

To develop techniques for measuring the positions of the individual electrodes of a multiple channel cochlear implant and for estimating associated characteristic frequencies.

BACKGROUND

Information concerning the positions of the individual electrodes of a cochlear implant array is important for analyzing speech perception or psychophysical data and for optimizing speech-processing strategies. This study presents two techniques for obtaining such information from postoperative plain film radiographs.

METHODS

A template spiral shape, derived from analysis of the radiographs of 30 cochlear implant patients, is used to obtain measurements of the angular positions of the electrode bands within scala tympani. A research technique measures angular positions and estimates characteristic frequencies for all electrode bands but requires that the positions of two cochlear landmarks and all electrode bands be digitized. A clinical technique provides similar angle and frequency estimates but requires a minimum of information to be extracted visually from the radiograph. The lateral positions of the bands are estimated, in the research technique, using mean outer and inner wall functions obtained from 11 Silastic molds of scala tympani.

RESULTS

The mean position of the implanted array relative to the mean scala tympani outer wall function was consistent with published histologic observations of implanted temporal bones. Measured angles did not vary greatly with experimenter or with rotation of the cochlea relative to the radiographic beam by up to 20 degrees.

CONCLUSIONS

The techniques described allow, principally, measurement of the longitudinal positions of the bands of a cochlear implant in scala tympani and estimation of corresponding characteristic frequencies.

Immunoreactivity of sensory hair bundles of the guinea-pig cochlea to antibodies against elastin and keratan sulphate

The stereociliary bundles of hair cells contain cross-linking extracellular filaments which have been suggested to play a role in mechanoelectrical transduction. To investigate the composition of these filaments, antibodies to the extracellular matrix molecules elastin and keratan sulphate have been used for light- and electron-microscopic immunocytochemistry of the guinea-pig organ of Corti. With the antibody to elastin, no immunoreactivity was found in hair bundles. This implies either that the epitope recognised by this antibody is not present in the links or that it is obscured. The antibody to keratan sulphate labelled the stereociliary bundles of both inner and outer hair cells but not supporting cells. The tips of the tallest stereocilia, especially on outer hair cells, the tips of the shorter stereocilia where the tip links attach to the stereociliary membrane, and the attachments of the lateral links, were labelled. This suggests that the links contain keratan sulphate proteoglycans, molecules which in other tissues are known to maintain structural integrity and fibrillar spacing, and to influence the microenvironment of the cell surface.

Basic fibroblast growth factor (FGF-2) protects rat cochlear hair cells in organotypical culture from aminoglycoside injury

Given the evidence that basic fibroblast growth factor (FGF-2) can protect neural and retinal cells from degeneration, we evaluated the potential of this growth factor to protect sensory cells in the inner ear. When sensory cells of the organ of Corti are exposed to aminoglycoside antibiotics such as neomycin either in vivo or in vitro, significant ototoxicity is observed. The in vitro cytotoxic effects of neomycin are dose and time dependent. In neonatal rat organ of Corti cultures, complete inner and outer hair cell destruction is observed at high (mM) concentrations of neomycin while inner hair cell survival and severely damaged outer hair cells are noted at moderate (microM) concentrations, with a maximal effect observed after 2 days of culture. Approximately 50% of cochlear outer hair cells are lost at a dose of 35 microM neomycin, and most surviving cells show disorganized stereocilia. Inner hair cells show primarily disorganization of their stereocilia. A significant protective effect is observed when the organ of Corti is pre-treated with FGF-2 (500 ng/ml) for 48 hours, and then FGF-2 is included with neomycin in the culture medium. A greater extent of outer hair cell survival and a significant decrease in stereociliary damage are noted with FGF-2. However, disorganization of inner hair cell stereocilia is unaffected by FGF-2. The protective effect of FGF-2 is specific, since interleukin-1B, nerve growth factor, tumor necrosis factor, and epidermal growth factor are ineffective, while retinoic acid and transforming growth factor alpha show only a moderate protective effect. These results confirm the potential of molecules like FGF-2 for preventing cell death due to a variety of causes.

[Optokinetic nystagmus and visual-vestibular interaction in subjects with "whiplash injuries"]

We studied the behaviour of Vestibular Nystagmus (VOR), of Optokinetic Nystagmus (OKN) and of Visuo-Vestibular-Ocular-Reflex (VVOR) in seven normal subjects and in thirty-two patients who had undergone cervical trauma in an automobile accident with the so called "whiplash mechanism". Thirteen subjects underwent examination within the first three months after the accident (first group), six subjects between the third and the sixth months (third group). Ocular movements was recorded according to the usual method by means of a Tonnies electronystagmograph with eight channels. The subjects, head blocked, sat on a Tonnies rotatory chair Pro model which was placed in the middle of a rotatory cylindrical chamber 2 metres in diameter and 1.9 metres in height. The width internal area was covered with thirty-two black vertical contrast. The rotatory cylinder was lighted from above by a 100 W bulb and was driven by a direct current engine which turned it clockwise and counterclockwise up to 200 degrees/sec., maximum speed, with preset acceleration ranging from 1 degree to 2 degrees/sec. All the subjects underwent to Rotatory Vestibular Stimulation by Stop test from a constant angular velocity of 90 degrees/sec. with clockwise and counterclockwise rotation, "stare type" Optokinetic stimulation with a cylinder rotation velocity of 30 degrees/sec. for 60 seconds and to contemporary Rotatory Vestibular and Optokinetic Stimulation (VVOR) so that OKN was VOR counterdirectional. The results of our experience show a statistically significative mean gain decrease of VOR and VVOR nystagmus (beating OKN direction) calculated on the first three beats in the patients of the first group and a significative increase of OKN mean gain in all the patients of the three groups. Furthermore, in sixteen out of thirty-two patients (seven in the first group, two in the second and seven in the third) we observed (during VVOR examination, immediately after stop) a nystagmus beating VOR direction lasting from 3 to 15 seconds.

Vein of the vestibular aqueduct in the gerbil

The vein of the vestibular aqueduct in the gerbil runs along the lateral side of the endolymphatic duct, deviates from this course to enter the middle ear cavity, and connects with the lateral sinus. In 20 animals, the vein of the vestibular aqueduct was obliterated by drilling from the middle ear side. Histopathology of these specimens after 2 months' survival revealed consistent sensory cell atrophy in the posterior canal cristae, frequent loss of sensory cells in a small superior portion of the macula sacculi and in the basal end of the cochlea, and fibrosis and osteogenesis in the three semicircular canals. The endolymphatic sacs contained colloidal substances in some specimens which were otherwise normal. Endolymphatic hydrops was absent except in some specimens which showed additional surgical damage to the endolymphatic sac and canals. The blocked vein re-opened occasionally and connected with the vessel formed in new bone from which it attached to the lateral sinus. The sensory cell degeneration and canal fibrosis reflects the pattern of blood drainage by the vein of the vestibular aqueduct. Vascular disorder in the vestibular labyrinth initiates vestibular symptoms; however, it will not produce endolymphatic hydrops unless function of the endolymphatic sac is impaired.

Characterization of cochlear outer hair cell turgor

The cochlear outer hair cell (OHC) is a cylindrical cell with structural features suggestive of a hydraulic skeleton, i.e., an elastic shell with a positive internal pressure. This study characterizes the role of the OHC elevated cytoplasmic pressure in maintaining the cell shape. Intracellular pressure of OHCs from guinea pig is estimated by measuring changes in cell morphology in response to increasing or decreasing osmolarity. Cells collapse when subjected to a continuous increase in osmolarity. Collapse occurs at an average of 8 mosM above the standard medium, suggesting that normal cells have an effective intracellular pressure of 128 mmHg. Fewer cells collapse when exposed to slow rates of osmolarity increase than cells exposed to fast rates of osmolarity increase, although the final change in osmolarity in the perfusion chamber is similar. Furthermore, cells undergo a slow, spontaneous increase in volume on exposure to either no osmolarity change or slow rates of osmolarity increase, suggesting that the cell's internal osmolarity increases in vitro. After volume reduction or elevation, cells do not return to their initial volume.

Measures of auditory brain-stem responses, distortion product otoacoustic emissions, hair cell loss, and forward masked tuning curves in the waltzing guinea pig

Measures of the auditory brain-stem response (ABR), distortion product otoacoustic emission (2f1-f2), hair cell loss, and forward masked tuning curves were obtained from waltzing guinea pigs and their age-matched controls at postnatal day 2, 9, 15, and 30. A mild ABR threshold shift (10-15 dB) is seen by 2 days postnatal and gradually increases to a more severe threshold shift (40-50 dB) by postnatal day 15. Already by 30 days the auditory brain-stem response, being beyond the output of the instrumentation, could not be elicited. The mean distortion product otoacoustic emission (DPOE) amplitude as a function of f1 amplitude for the postnatal day 2, 9, and 15 waltzing guinea pigs were only between 3 and 8 dB below the control values for stimulus levels below 65 dB SPL. The DPOE audiogram constructed for the waltzing guinea pigs shows no more than an 8-dB mean difference from the control values when the intensity of f1 was 50 dB SPL, and no more than 10 dB when f1 was 60 dB SPL. Analysis of the individual cases revealed that the DPOE amplitude could be greater than control values. On the contrary, when f1 stimulus levels were below 65 dB SPL, DPOEs could not be detected for the postnatal day 30 waltzing guinea pigs. At stimulus levels above 65 dB SPL, DPOEs could be recorded yet these responses were depressed from control values by 10 to 25 dB. Analysis of surface preparations of the organ of Corti from the day 15 waltzing guinea pig reveals that the prominent alteration occurs on the third row outer hair cells. To a lesser extent, the second row outer hair cells, and then the first row outer hair cells are affected while the inner hair cells appear normal. In contrast, the organ of Corti from the postnatal day 30 waltzing guinea pig showed a more extensive outer hair cell loss among all three rows as well as a considerable degree of inner hair cell loss. Forward masked auditory brain-stem response tuning curves were generated at 8, 4, and 1 kHz for control and waltzing guinea pigs between 2 and 12 days of age. All tuning curves obtained from waltzing guinea pigs showed progressive decreases in sensitivity with increasing age. The Q 10 dB values of the 1- and 4-kHz tuning curves were not different between the controls and the waltzing guinea pigs at any age.(ABSTRACT TRUNCATED AT 400 WORDS)

Audiometric and histological differences between the effects of continuous and impulsive noise exposures

An experiment was designed to determine if, for equal SPL and power spectrum, the effects on hearing of high-kurtosis noise exposures and a Gaussian noise exposure are different and the extent to which any differences measured in terms of audiometric and histological variables are frequency specific. Three groups of chinchillas with 10 animals/group were exposed for 5 days at 90 dB SPL to one of three types of noise, each with the same power spectrum. The impulsiveness, defined by the kurtosis, and the region of the spectrum from which the impulsive components of the noise were created differed for two of the noises, while the third was a continuous Gaussian noise. The results show that the most impulsive noise produced up to 20 dB greater permanent threshold shift at the high frequencies than did the Gaussian noise exposure. However, these audiometric results were difficult to reconcile with the pattern of sensory cell losses that showed statistically significant larger losses of outer hair cells for the impulsive exposure in the 0.25-kHz region. When the impacts in a high-kurtosis noise were created from the energy in the 1- through 6-kHz region of the spectrum, the audiometric profile of hearing loss was similar to that produced by the Gaussian noise; however, inner hair cell losses were significantly greater in the 4-kHz octave band region of the cochlea.

Complex noise exposures: an energy analysis

Industrial noise environments usually present a complex stimulus to the exposed individual. These environments often contain mixtures of multiply reflected impact noises and a relatively Gaussian broadband noise. Noise exposure standards do not consider the possibility of interactions between the two classes of noise that can exacerbate the amount of hearing trauma. This paper presents the results of a large series of experiments designed to document the hazard posed to hearing from complex noise exposures. Twenty-three groups of chinchillas with 5 to 11 animals per group (total N = 135) were exposed for 5 days to either octave bands of noise, impacts alone, or combinations of impact and octave bands of noise. Evoked potential measures of hearing thresholds and cochleograms were used to quantify the noise-induced trauma. The results show that, for sound exposure levels (SEL) which produce less than approximately 10 dB PTS (permanent threshold shift) or 5% total sensory cell loss, equal-energy exposures tend to produce equivalent effects on hearing. However, there is a range of at least 10 dB in the SEL parameter where hearing loss from equal-energy exposures at a particular SEL can be exacerbated by increasing the repetition rate of the impacts or by the addition of a Gaussian low-level noise. The exacerbation of trauma from the addition of a Gaussian continuous noise is dependent upon the spectrum of that noise.

Two-tone suppression of inner hair cell and basilar membrane responses in the guinea pig

Recordings of receptor potentials from inner hair cells (IHCs) and the basilar membrane (BM) motion were made in pigmented guinea pigs. The acoustic stimuli were single tones near best frequency (BF) and two-tone complexes. Single tone input/output (I/O) functions had a saturating growth for the magnitude and their phase shifts were strongly dependent on the tone frequency relative to BF. For IHCs, a BF tone stimulus produced no phase shift in the ac receptor potential response. Phase lag or lead occurred for tones below or above BF, respectively. BM velocity I/O functions were not as compressively saturating as IHC ac I/O curves. BM phase shifts (in relation to BF) were similar to those of the IHCs. Two-tone suppression was observed in both IHC and BM response measures. Suppressor tones on the low-frequency side of BF produced complex suppression results, which were inconsistent with a simple attenuation model for suppression. The growth of suppression was faster than the attenuation from equivalent level reductions of the probe tone, and phase shifts were phase lead. Depending upon experimental conditions, phase change with suppression may be in the opposite direction from phase change observed from pure attenuation of the probe tone. High-frequency suppressors (relative to BF) are consistent with an attenuation model of suppression for the IHCs of the current study. High side suppression of basilar membrane velocity, however, differed from the IHCs in a systematic way. The phase change caused by suppression of BM velocity was always smaller than that of an equivalent reduction in the probe tone level.

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

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

[Alfonso Corti (1822-1876), some of his biographers and his relationship to Vienna]

Biographical data and the evaluation of the scientific work of Corti taken from Austrian, German, Italian, Swiss and American medical historiographers are summarised and completed by details and documents of the time Corti spent in Vienna. It is pointed out that Corti received important impulse for his later research when he was Hyrtl's assistant.

The effect of impulse noise on cochlear vessels

Short- and long-term changes in the cochlear vasculature and long-term changes in the sensorineuroepithelium were studied in guinea pigs after they were exposed to impulse noise. Vessel histology and cochlear hair cell loss were assessed, using a surface-preparation technique, and the results showed considerable variability. Hair cell loss and radial tears in the organ of Corti were a common finding in the animals killed four weeks after impulse-noise exposure. Impulse-noise exposure resulted in few cochlear vascular changes in the acutely and chronically affected groups. Compared with the results of our previous studies using continuous-noise exposure of different characteristics and in different mammals, this impulse-noise experiment resulted in a nonsignificant damaging effect on the cochlear vasculature.

Early post-mortem change of the crista ampullaris. A light and electron microscopic study of the Guinea-pig

Post-mortem change (autolysis) of the sensory and the secretory epithelia was studied by means of the light and the electron microscope. The first ultrastructurally recognizable sign of autolysis occurred 10-15 min after death in the mitochondria of the hair cells and their adjacent nerve endings. The morphology of hair cells was occasionally well preserved 30 min post-mortem, but after 90 min or more autolytic changes were invariably severe. The secretory epithelium, however, was considerably less affected by autolysis than the hair cells during the first 5 h after death.

Ototoxicity with children caused by streptomycin

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