Morphologic Analysis of the Scala Tympani Using Synchrotron: Implications for Cochlear Implantation

OBJECTIVES

To use synchrotron radiation phase-contrast imaging (SR-PCI) to visualize and measure the morphology of the entire cochlear scala tympani (ST) and assess cochlear implant (CI) electrode trajectories.

METHODS

SR-PCI images were used to obtain geometric measurements of the cochlear scalar diameter and area at 5-degree increments in 35 unimplanted and three implanted fixed human cadaveric cochleae.

RESULTS

The cross-sectional diameter and area of the cochlea were found to decrease from the base to the apex. This study represents a wide variability in cochlear morphology and suggests that even in the smallest cochlea, the ST can accommodate a 0.4 mm diameter electrode up to 720°. Additionally, all lateral wall array trajectories were within the anatomically accommodating insertion zone.

CONCLUSION

This is the first study to use SR-PCI to visualize and quantify the entire ST morphology, from the round window to the apical tip, and assess the post-operative trajectory of electrodes. These high-resolution anatomical measurements can be used to inform the angular insertion depth that can be accommodated in CI patients, accounting for anatomical variability.

LEVEL OF EVIDENCE

N/A. Laryngoscope, 134:2889-2897, 2024.

Synchrotron Phase-Contrast Imaging and Cochlear Otosclerosis A case-report

BACKGROUND

Otosclerosis is a bone disorder affecting the labyrinthine capsule that leads to conductive and occasionally sensorineural hearing loss. The etiology of otosclerosis remains unknown; factors such as infection, hormones, inflammation, genetics, and autoimmunity have been discussed. Treatment consists primarily of surgical stapes replacement and cochlear implantation. High-resolution computed tomography (HRCT) is routinely used to visualize bone pathology. In the present study, we used synchrotron radiation phase-contrast imaging (SR-PCI) to examine otosclerosis plaques in a temporal bone for the first time. The primary aim was to study their three-dimensional (3D) outline, vascular interrelationships and connections to the middle ear.

MATERIAL AND METHODS

A donated ear from a patient with otosclerosis who had undergone partial stapedectomy with insertion of a stapes wire prosthesis was investigated using SR-PCI and compared with a control ear. Otosclerotic lesions were 3D rendered using the composite with shading technique. Scalar opacity and color mapping were adjusted to display volume properties with the removal of bones to enhance surfaces. Vascular bone channels were segmented, and the communications between lesions and the middle ear were established.

RESULTS

Fenestral, cochlear, meatal and vestibular lesions were outlined three-dimensionally. Vascular bone channels were found to be frequently connected to the middle ear mucosa, perilabyrinthine air spaces, and facial nerve vessels. Round window lesions partly embedded the cochlear aqueduct which was pathologically narrowed, while the inferior cochlear vein was significantly dilated in its proximal part.

DISCUSSION

Otosclerotic/otospongiotic lesions were imaged for the first time using SR-PCI and 3D rendering. The presence of shunts and abnormal vascular connections to the labyrinth appeared to result in hyper-vascularization, overloading the venous system, and leading to sensorineural hearing loss. We speculate about possible local treatments to alleviate the impact of such critical lesions on the labyrinthine micro-circulation.

Immuno-surveillance and protection of the human cochlea

Background

Despite its location near infection-prone areas, the human inner ear demonstrates remarkable resilience. This suggests that there are inherent instruments deterring the invasion and spread of pathogens into the inner ear. Here, we combined high-resolution light microscopy, super-resolution immunohistochemistry (SR-SIM) and synchrotron phase contrast imaging (SR-PCI) to identify the protection and barrier systems in the various parts of the human inner ear, focusing on the lateral wall, spiral ganglion, and endolymphatic sac.

Materials and methods

Light microscopy was conducted on mid-modiolar, semi-thin sections, after direct glutaraldehyde/osmium tetroxide fixation. The tonotopic locations were estimated using SR-PCI and 3D reconstruction in cadaveric specimens. The sections were analyzed for leucocyte and macrophage activity, and the results were correlated with immunohistochemistry using confocal microscopy and SR-SIM.

Results

Light microscopy revealed unprecedented preservation of cell anatomy and several macrophage-like cells that were localized in the cochlea. Immunohistochemistry demonstrated IBA1 cells frequently co-expressing MHC II in the spiral ganglion, nerve fibers, lateral wall, spiral limbus, and tympanic covering layer at all cochlear turns as well as in the endolymphatic sac. RNAscope assays revealed extensive expression of fractalkine gene transcripts in type I spiral ganglion cells. CD4 and CD8 cells occasionally surrounded blood vessels in the modiolus and lateral wall. TMEM119 and P2Y12 were not expressed, indicating that the cells labeled with IBA1 were not microglia. The round window niche, compact basilar membrane, and secondary spiral lamina may form protective shields in the cochlear base.

Discussion

The results suggest that the human cochlea is surveilled by dwelling and circulating immune cells. Resident and blood-borne macrophages may initiate protective immune responses via chemokine signaling in the lateral wall, spiral lamina, and spiral ganglion at different frequency locations. Synchrotron imaging revealed intriguing protective barriers in the base of the cochlea. The role of the endolymphatic sac in human inner ear innate and adaptive immunity is discussed.

Microanatomy of the human tunnel of Corti structures and cochlear partition-tonotopic variations and transcellular signaling

Auditory sensitivity and frequency resolution depend on the optimal transfer of sound-induced vibrations from the basilar membrane (BM) to the inner hair cells (IHCs), the principal auditory receptors. There remains a paucity of information on how this is accomplished along the frequency range in the human cochlea. Most of the current knowledge is derived either from animal experiments or human tissue processed after death, offering limited structural preservation and optical resolution. In our study, we analyzed the cytoarchitecture of the human cochlear partition at different frequency locations using high-resolution microscopy of uniquely preserved normal human tissue. The results may have clinical implications and increase our understanding of how frequency-dependent acoustic vibrations are carried to human IHCs. A 1-micron-thick plastic-embedded section (mid-modiolar) from a normal human cochlea uniquely preserved at lateral skull base surgery was analyzed using light and transmission electron microscopy (LM, TEM). Frequency locations were estimated using synchrotron radiation phase-contrast imaging (SR-PCI). Archival human tissue prepared for scanning electron microscopy (SEM) and super-resolution structured illumination microscopy (SR-SIM) were also used and compared in this study. Microscopy demonstrated great variations in the dimension and architecture of the human cochlear partition along the frequency range. Pillar cell geometry was closely regulated and depended on the reticular lamina slope and tympanic lip angle. A type II collagen-expressing lamina extended medially from the tympanic lip under the inner sulcus, here named "accessory basilar membrane." It was linked to the tympanic lip and inner pillar foot, and it may contribute to the overall compliance of the cochlear partition. Based on the findings, we speculate on the remarkable microanatomic inflections and geometric relationships which relay different sound-induced vibrations to the IHCs, including their relevance for the evolution of human speech reception and electric stimulation with auditory implants. The inner pillar transcellular microtubule/actin system's role of directly converting vibration energy to the IHC cuticular plate and ciliary bundle is highlighted.

The ultrastructure of a stria vascularis in the auditory organ of the cuban crocodile (Crocodylus rhombifer)

Background: An endocochlear potential (EP) exists in the mammalian cochlea generated by the stria vascularis and an associated fibrocyte network. It plays an essential role for sensory cell function and hearing sensitivity. In non-mammalian ectothermic animals the endocochlear potential is low and its origin somewhat unclear. In this study, we explored the crocodilian auditory organ and describe the fine structure of a stria vascularis epithelium that has not been verified in birds. Material and Methods: Three Cuban crocodiles (Crocodylus rhombifer) were analyzed with light and transmission electron microscopy. The ears were fixed in glutaraldehyde The temporal bones were drilled out and decalcified. The ears were dehydrated, and embedded and was followed by semi-thin and thin sectioning. Results: The fine structure of the crocodile auditory organ including the papilla basilaris and endolymph system was outlined. The upper roof of the endolymph compartment was specialized into a Reissner membrane and tegmentum vasculosum. At the lateral limbus an organized, multilayered, vascularized epithelium or stria vascularis was identified. Discussion: Electron microscopy demonstrates that the auditory organ in Crocodylus rhombifer, unlike in birds, contains a stria vascularis epithelium separate from the tegmentum vasculosum. It is believed to secrete endolymph and to generate a low grade endocochlear potential. It may regulate endolymph composition and optimize hearing sensitivity alongside the tegmentum vasculosum. It could represent a parallel evolution essential for the adaptation of crocodiles to their diverse habitats.

GJB2 and GJB6 gene transcripts in the human cochlea: A study using RNAscope, confocal, and super-resolution structured illumination microscopy

Background

Gap junction (GJ) proteins, connexin26 and 30, are highly prevalent in the human cochlea (HC), where they are involved in transcellular signaling, metabolic supply, and fluid homeostasis. Their genes, GJB2 and GJB6, are both located at the DFNB1 locus on chromosome 13q12. Mutations in GJB2 may cause mild to profound non-syndromic deafness. Here, we analyzed for the first time the various expressions of GJB2 and GJB6 gene transcripts in the different cell networks in the HC using the RNAscope technique.

Materials and methods

Archival paraformaldehyde-fixed sections of surgically obtained HC were used to label single mRNA oligonucleotides using the sensitive multiplex RNAscope^® technique with fluorescent-tagged probes. Positive and negative controls also included the localization of ATP1A1, ATP1A2, and KCNJ10 gene transcripts in order to validate the specificity of labeling.

Results

Confocal and super-resolution structured illumination microscopy (SR-SIM) detected single gene transcripts as brightly stained puncta. The GJB2 and GJB6 gene transcripts were distributed in the epithelial and connective tissue systems in all three cochlear turns. The largest number of GJB2 and GJB6 gene transcripts was in the outer sulcus, spiral ligament, and stria vascularis (SV). Oligonucleotides were present in the supporting cells of the organ of Corti (OC), spiral limbus fibrocytes, and the floor of the scala vestibuli. Multiplex gene data suggest that cells in the cochlear lateral wall contain either GJB2 or GJB6 gene transcripts or both. The GJB6, but not GJB2, gene transcripts were found in the intermediate cells but none were found in the marginal cells. There were no GJB2 or GJB6 gene transcripts found in the hair cells and only a few in the spiral ganglion cells.

Conclusion

Both GJB2 and GJB6 mRNA gene transcripts were localized in cells in the adult HC using RNAscope^®in situ hybridization (ISH) and high resolution microscopy. Generally, GJB6 dominated over GJB2, except in the basal cells. Results suggest that cells may contain either GJB2 or GJB6 gene transcripts or both. This may be consistent with specialized GJ plaques having separate channel permeability and gating properties. A reduction in the number of GJB2 gene transcripts was found in the basal turn. Such information may be useful for future gene therapy.

Visualization of macrophage subsets in the development of the fetal human inner ear

Background

Human inner ear contains macrophages whose functional role in early development is yet unclear. Recent studies describe inner ear macrophages act as effector cells of the innate immune system and are often activated following acoustic trauma or exposure to ototoxic drugs. Few or limited literature describing the role of macrophages during inner ear development and organogenesis.

Material and Methods

We performed a study combining immunohistochemistry and immunofluorescence using antibodies against IBA1, CX3CL1, CD168, CD68, CD45 and CollagenIV. Immune staining and quantification was performed on human embryonic inner ear sections from gestational week 09 to 17.

Results

The study showed IBA1 and CD45 positive cells in the mesenchymal tissue at GW 09 to GW17. No IBA1 positive macrophages were detected in the sensory epithelium of the cochlea and vestibulum. Fractalkine (CX3CL1) signalling was initiated GW10 and parallel chemotactic attraction and migration of macrophages into the inner ear. Macrophages also migrated into the spiral ganglion, cochlear nerve, and peripheral nerve fibers and tissue-expressing CX3CL1. The mesenchymal tissue at all gestational weeks expressed CD163 and CD68.

Conclusion

Expressions of markers for resident and non-resident macrophages (IBA1, CD45, CD68, and CD163) were identified in the human fetal inner ear. We speculate that these cells play a role for the development of human inner ear tissue including shaping of the gracile structures.

Regeneration in the Auditory Organ in Cuban and African Dwarf Crocodiles (Crocodylus rhombifer and Osteolaemus tetraspis) Can We Learn From the Crocodile How to Restore Our Hearing?

Background: In several non-mammalian species, auditory receptors undergo cell renewal after damage. This has raised hope of finding new options to treat human sensorineural deafness. Uncertainty remains as to the triggering mechanisms and whether hair cells are regenerated even under normal conditions. In the present investigation, we explored the auditory organ in the crocodile to validate possible ongoing natural hair cell regeneration.

Materials and Methods: Two male Cuban crocodiles (Crocodylus rhombifer) and an adult male African Dwarf crocodile (Osteolaemus tetraspis) were analyzed using transmission electron microscopy and immunohistochemistry using confocal microscopy. The crocodile ears were fixed in formaldehyde and glutaraldehyde and underwent micro-computed tomography (micro-CT) and 3D reconstruction. The temporal bones were drilled out and decalcified.

Results: The crocodile papilla basilaris contained tall (inner) and short (outer) hair cells surrounded by a mosaic of tightly connected supporting cells coupled with gap junctions. Afferent neurons with and without ribbon synapses innervated both hair cell types. Supporting cells occasionally showed signs of trans-differentiation into hair cells. They expressed the MAFA and SOX2 transcription factors. Supporting cells contained organelles that may transfer genetic information between cells, including the efferent nerve fibers during the regeneration process. The tectorial membrane showed signs of being replenished and its architecture being sculpted by extracellular exosome-like proteolysis.

Discussion: Crocodilians seem to produce new hair cells during their life span from a range of supporting cells. Imposing efferent nerve fibers may play a role in regeneration and re-innervation of the auditory receptors, possibly triggered by apoptotic signals from wasted hair cells. Intercellular signaling may be accomplished by elaborate gap junction and organelle systems, including neural emperipolesis. Crocodilians seem to restore and sculpt their tectorial membranes throughout their lives.

Na/K-ATPase Gene Expression in the Human Cochlea: A Study Using mRNA in situ Hybridization and Super-Resolution Structured Illumination Microscopy

Background

The pervasive Na/K-ATPase pump is highly expressed in the human cochlea and is involved in the generation of the endocochlear potential as well as auditory nerve signaling and relay. Its distribution, molecular organization and gene regulation are essential to establish to better understand inner ear function and disease. Here, we analyzed the expression and distribution of the ATP1A1, ATP1B1, and ATP1A3 gene transcripts encoding the Na/K-ATPase α1, α3, and β1 isoforms in different domains of the human cochlea using RNA in situ hybridization.

Materials and Methods

Archival paraformaldehyde-fixed sections derived from surgically obtained human cochleae were used to label single mRNA gene transcripts using the highly sensitive multiplex RNAscope^® technique. Localization of gene transcripts was performed by super-resolution structured illumination microscopy (SR-SIM) using fluorescent-tagged probes. GJB6 encoding of the protein connexin30 served as an additional control.

Results

Single mRNA gene transcripts were seen as brightly stained puncta. Positive and negative controls verified the specificity of the labeling. ATP1A1 and ATP1B1 gene transcripts were demonstrated in the organ of Corti, including the hair and supporting cells. In the stria vascularis, these transcripts were solely expressed in the marginal cells. A large number of ATP1B1 gene transcripts were found in the spiral ganglion cell soma, outer sulcus, root cells, and type II fibrocytes. The ATP1B1 and ATP1A3 gene transcripts were rarely detected in axons.

Discussion

Surgically obtained inner ear tissue can be used to identify single mRNA gene transcripts using high-resolution fluorescence microscopy after prompt formaldehyde fixation and chelate decalcification. A large number of Na/K-ATPase gene transcripts were localized in selected areas of the cochlear wall epithelium, fibrocyte networks, and spiral ganglion, confirming the enzyme’s essential role for human cochlear function.

The Acute Effects of Furosemide on Na-K-Cl Cotransporter-1, Fetuin-A and Pigment Epithelium-Derived Factor in the Guinea Pig Cochlea

Background

Furosemide is a loop diuretic used to treat edema; however, it also targets the Na-K-Cl cotransporter-1 (NKCC1) in the inner ear. In very high doses, furosemide abolishes the endocochlear potential (EP). The aim of the study was to gain a deeper understanding of the temporal course of the acute effects of furosemide in the inner ear, including the protein localization of Fetuin-A and PEDF in guinea pig cochleae.

Material and Method

Adult guinea pigs were given an intravenous injection of furosemide in a dose of 100 mg per kg of body weight. The cochleae were studied using immunohistochemistry in controls and at four intervals: 3 min, 30 min, 60 min and 120 min. Also, cochleae of untreated guinea pigs were tested for Fetuin-A and PEDF mRNA using RNAscope^® technology.

Results

At 3 min, NKCC1 staining was abolished in the type II fibrocytes in the spiral ligament, followed by a recovery period of up to 120 min. In the stria vascularis, the lowest staining intensity of NKCC1 presented after 30 min. The spiral ganglion showed a stable staining intensity for the full 120 min. Fetuin-A protein and mRNA were detected in the spiral ganglion type I neurons, inner and outer hair cells, pillar cells, Deiters cells and the stria vascularis. Furosemide induced an increased staining intensity of Fetuin-A at 120 min. PEDF protein and mRNA were found in the spiral ganglia type I neurons, the stria vascularis, and in type I and type II fibrocytes of the spiral ligament. PEDF protein staining intensity was high in the pillar cells in the organ of Corti. Furosemide induced an increased staining intensity of PEDF in type I neurons and pillar cells after 120 min.

Conclusion

The results indicate rapid furosemide-induced changes of NKCC1 in the type II fibrocytes. This could be part of the mechanism that causes reduction of the EP within minutes after high dose furosemide injection. Fetuin-A and PEDF are present in many cells of the cochlea and probably increase after furosemide exposure, possibly as an otoprotective response.

Distribution of Immune Cells Including Macrophages in the Human Cochlea

Background: The human cochlea was earlier believed to lack capacity to mount specific immune responses. Recent studies established that the human cochlea holds macrophages. The cells appear to surveil, dispose of, and restore wasted cells to maintain tissue integrity. Macrophage activities are believed to be the central elements in immune responses and could swiftly defuse invading microbes that enter via adjacent infection-prone areas. This review updates recent human studies in light of the current literature and adds information about chemokine gene expression. Materials and Methods: We analyzed surgically obtained human tissue using immunohistochemistry, confocal microscopy, and multichannel super-resolution structured illumination microscopy. The samples were considered representative of steady-state conditions. Antibodies against the ionized calcium-binding adaptor molecule 1 were used to identify the macrophages. CD68 and CD11b, and the major histocompatibility complex type II (MHCII) and CD4 and CD8 were analyzed. The RNAscope technique was used for fractalkine gene localization. Results: Many macrophages were found around blood vessels in the stria vascularis but not CD4 and CD8 lymphocytes. Amoeboid macrophages were identified in the spiral ganglion with surveilling "antennae" projecting against targeted cells. Synapse-like contacts were seen on spiral ganglion cell bodies richly expressing single CXC3CL gene transcripts. Branching neurite-like processes extended along central and peripheral axons. Active macrophages were occasionally found near degenerating hair cells. Some macrophage-interacting T lymphocytes were observed between the scala tympani wall and Rosenthal's canal. CD4 and CD8 cells were not found in the organ of Corti. Conclusions: The results indicate that the human cochlea is equipped with macrophages and potentially lymphocytes, suggesting both an innate and adaptive immune capacity. A rich expression of fractalkine gene transcripts in spiral ganglion neurons suggest an essential role for auditory nerve protection, as has been demonstrated experimentally. The findings provide further information on the important role of the immune machinery present in the human inner ear and its potential to carry adverse immune reactions, including cytotoxic and foreign body responses. The results can be used to form a rationale for therapies aiming to modulate these immune activities.

A combined genome-wide association and molecular study of age-related hearing loss in H. sapiens

BACKGROUND

Sensorineural hearing loss is one of the most common sensory deficiencies. However, the molecular contribution to age-related hearing loss is not fully elucidated.

METHODS

We performed genome-wide association studies (GWAS) for hearing loss-related traits in the UK Biobank (N = 362,396) and selected a high confidence set of ten hearing-associated gene products for staining in human cochlear samples: EYA4, LMX1A, PTK2/FAK, UBE3B, MMP2, SYNJ2, GRM5, TRIOBP, LMO-7, and NOX4.

RESULTS

All proteins were found to be expressed in human cochlear structures. Our findings illustrate cochlear structures that mediate mechano-electric transduction of auditory stimuli, neuronal conductance, and neuronal plasticity to be involved in age-related hearing loss.

CONCLUSIONS

Our results suggest common genetic variation to influence structural resilience to damage as well as cochlear recovery after trauma, which protect against accumulated damage to cochlear structures and the development of hearing loss over time.

Aeration of the Human Prussak's Space: A 3D Synchrotron Imaging Study

OBJECTIVES

Prussak's space (PS) is an intricate middle ear region which may play an essential role in the development of middle ear disease. The three-dimensional (3D) anatomy of the human PS and its drainage routes remain relatively unknown. Earlier studies have histologically analyzed PS, by micro-dissection and endoscopy. Here, we used synchrotron-radiation phase-contrast imaging (SR-PCI), 3D reconstructions, and modeling to study the framework of the human PS, including aeration pathways. It may lead to increased understanding of development of middle ear pathology.

DESIGN

Nine human temporal bone specimens underwent in-line SR-PCI at the Canadian Light Source in Saskatoon, Saskatchewan, Canada. Data were processed with volume-rendering software to create 3D reconstructions using scalar opacity mapping and segmentations to visualize its walls in fixed, undecalcified human temporal bones.

RESULTS

The PS was found to be an irregular, variably shaped chamber with different aeration systems. Three different drainage pathways were found: 1) via the posterior malleolar pouch of von Tröltsch in seven of nine ears; 2) directly posterior-inferior into the mesotympanum medial to the posterior malleolar pouch in one ear; and 3) anteriorly in another. The posterior-inferior communications depended on the anatomy of the posterior malleolar fold. In one bilateral case, the aeration differed between the ears. Earlier descriptions of upper ventilation routes between the PS and the epitympanic spaces could not be substantiated.

CONCLUSIONS

The 3D anatomy of the membrane folds organizing the PS in humans was demonstrated for the first time using in-line SR-PCI. The PS was always aerated into the mesotympanum, suggesting its relative independence of attic ventilation. The impact of its various drainage routes on middle ear ventilation and disease were discussed.

The proteome of the human endolymphatic sac endolymph

The endolymphatic sac (ES) is the third part of the inner ear, along with the cochlea and vestibular apparatus. A refined sampling technique was developed to analyse the proteomics of ES endolymph. With a tailored solid phase micro-extraction probe, five ES endolymph samples were collected, and six sac tissue biopsies were obtained in patients undergoing trans-labyrinthine surgery for sporadic vestibular schwannoma. The samples were analysed using nano-liquid chromatography-tandem mass spectrometry (nLC-MS/MS) to identify the total number of proteins. Pathway identification regarding molecular function and protein class was presented. A total of 1656 non-redundant proteins were identified, with 1211 proteins detected in the ES endolymph. A total of 110 proteins were unique to the ES endolymph. The results from the study both validate a strategy for in vivo and in situ human sampling during surgery and may also form a platform for further investigations to better understand the function of this intriguing part of the inner ear.

A Synchrotron and Micro-CT Study of the Human Endolymphatic Duct System: Is Meniere's Disease Caused by an Acute Endolymph Backflow?

Background: The etiology of Meniere's disease (MD) and endolymphatic hydrops believed to underlie its symptoms remain unknown. One reason may be the exceptional complexity of the human inner ear, its vulnerability, and surrounding hard bone. The vestibular organ contains an endolymphatic duct system (EDS) bridging the different fluid reservoirs. It may be essential for monitoring hydraulic equilibrium, and a dysregulation may result in distension of the fluid spaces or endolymphatic hydrops. Material and Methods: We studied the EDS using high-resolution synchrotron phase contrast non-invasive imaging (SR-PCI), and micro-computed tomography (micro-CT). Ten fresh human temporal bones underwent SR-PCI. One bone underwent micro-CT after fixation and staining with Lugol's iodine solution (I2KI) to increase tissue resolution. Data were processed using volume-rendering software to create 3D reconstructions allowing orthogonal sectioning, cropping, and tissue segmentation. Results: Combined imaging techniques with segmentation and tissue modeling demonstrated the 3D anatomy of the human saccule, utricle, endolymphatic duct, and sac together with connecting pathways. The utricular duct (UD) and utriculo-endolymphatic valve (UEV or Bast's valve) were demonstrated three-dimensionally for the first time. The reunion duct was displayed with micro-CT. It may serve as a safety valve to maintain cochlear endolymph homeostasis under certain conditions. Discussion: The thin reunion duct seems to play a minor role in the exchange of endolymph between the cochlea and vestibule under normal conditions. The saccule wall appears highly flexible, which may explain occult hydrops occasionally preceding symptoms in MD on magnetic resonance imaging (MRI). The design of the UEV and connecting ducts suggests that there is a reciprocal exchange of fluid among the utricle, semicircular canals, and the EDS. Based on the anatomic framework and previous experimental data, we speculate that precipitous vestibular symptoms in MD arise from a sudden increase in endolymph pressure caused by an uncontrolled endolymphatic sac secretion. A rapid rise in UD pressure, mediated along the fairly wide UEV, may underlie the acute vertigo attack, refuting the rupture/K+-intoxication theory.

An Approach for Individualized Cochlear Frequency Mapping Determined from 3D Synchrotron Radiation Phase-Contrast Imaging

OBJECTIVE

Cochlear implants are traditionally programmed to stimulate according to a generalized frequency map, where individual anatomic variability is not considered when selecting the centre frequency of stimulation of each implant electrode. However, high variability in cochlear size and spatial frequency distributions exist among individuals. Generalized cochlear implant frequency maps can result in large pitch perception errors and reduced hearing outcomes for cochlear implant recipients. The objective of this work was to develop an individualized frequency mapping technique for the human cochlea to allow for patient-specific cochlear implant stimulation.

METHODS

Ten cadaveric human cochleae were scanned using synchrotron radiation phase-contrast imaging (SR-PCI) combined with computed tomography (CT). For each cochlea, ground truth angle-frequency measurements were obtained in three-dimensions using the SR-PCI CT data. Using an approach designed to minimize perceptual error in frequency estimation, an individualized frequency function was determined to relate angular depth to frequency within the cochlea.

RESULTS

The individualized frequency mapping function significantly reduced pitch errors in comparison to the current gold standard generalized approach.

CONCLUSION AND SIGNIFICANCE

This paper presents for the first time a cochlear frequency map which can be individualized using only the angular length of cochleae. This approach can be applied in the clinical setting and has the potential to revolutionize cochlear implant programming for patients worldwide.

Vestibular Organ and Cochlear Implantation-A Synchrotron and Micro-CT Study

Background: Reports vary on the incidence of vestibular dysfunction and dizziness in patients following cochlear implantation (CI). Disequilibrium may be caused by surgery at the cochlear base, leading to functional disturbances of the vestibular receptors and endolymphatic duct system (EDS) which are located nearby. Here, we analyzed the three-dimensional (3D) anatomy of this region, aiming to optimize surgical approaches to limit damage to the vestibular organ. Material and Methods: A total of 22 fresh-frozen human temporal bones underwent synchrotron radiation phase-contrast imaging (SR-PCI). One temporal bone underwent micro-computed tomography (micro-CT) after fixation and staining with Lugol's iodine solution (I2KI) to increase tissue contrast. We used volume-rendering software to create 3D reconstructions and tissue segmentation that allowed precise assessment of anatomical relationships and topography. Macerated human ears belonging to the Uppsala collection were also used. Drilling and insertion of CI electrodes was performed with metric analyses of different trajectories. Results and Conclusions: SR-PCI and micro-CT imaging demonstrated the complex 3D anatomy of the basal region of the human cochlea, vestibular apparatus, and EDS. Drilling of a cochleostomy may disturb vestibular organ function by injuring the endolymphatic space and disrupting fluid barriers. The saccule is at particular risk due to its proximity to the surgical area and may explain immediate and long-term post-operative vertigo. Round window insertion may be less traumatic to the inner ear, however it may affect the vestibular receptors.

Spike Generators and Cell Signaling in the Human Auditory Nerve: An Ultrastructural, Super-Resolution, and Gene Hybridization Study

Background: The human auditory nerve contains 30,000 nerve fibers (NFs) that relay complex speech information to the brain with spectacular acuity. How speech is coded and influenced by various conditions is not known. It is also uncertain whether human nerve signaling involves exclusive proteins and gene manifestations compared with that of other species. Such information is difficult to determine due to the vulnerable, "esoteric," and encapsulated human ear surrounded by the hardest bone in the body. We collected human inner ear material for nanoscale visualization combining transmission electron microscopy (TEM), super-resolution structured illumination microscopy (SR-SIM), and RNA-scope analysis for the first time. Our aim was to gain information about the molecular instruments in human auditory nerve processing and deviations, and ways to perform electric modeling of prosthetic devices. Material and Methods: Human tissue was collected during trans-cochlear procedures to remove petro-clival meningioma after ethical permission. Cochlear neurons were processed for electron microscopy, confocal microscopy (CM), SR-SIM, and high-sensitive in situ hybridization for labeling single mRNA transcripts to detect ion channel and transporter proteins associated with nerve signal initiation and conductance. Results: Transport proteins and RNA transcripts were localized at the subcellular level. Hemi-nodal proteins were identified beneath the inner hair cells (IHCs). Voltage-gated ion channels (VGICs) were expressed in the spiral ganglion (SG) and axonal initial segments (AISs). Nodes of Ranvier (NR) expressed Nav1.6 proteins, and encoding genes critical for inter-cellular coupling were disclosed. Discussion: Our results suggest that initial spike generators are located beneath the IHCs in humans. The first NRs appear at different places. Additional spike generators and transcellular communication may boost, sharpen, and synchronize afferent signals by cell clusters at different frequency bands. These instruments may be essential for the filtering of complex sounds and may be challenged by various pathological conditions.

Three-dimensional tonotopic mapping of the human cochlea based on synchrotron radiation phase-contrast imaging

The human cochlea transforms sound waves into electrical signals in the acoustic nerve fibers with high acuity. This transformation occurs via vibrating anisotropic membranes (basilar and tectorial membranes) and frequency-specific hair cell receptors. Frequency-positions can be mapped within the cochlea to create a tonotopic chart which fits an almost-exponential function with lowest frequencies positioned apically and highest frequencies positioned at the cochlear base (Bekesy 1960, Greenwood 1961). To date, models of frequency positions have been based on a two-dimensional analysis with inaccurate representations of the cochlear hook region. In the present study, the first three-dimensional frequency analysis of the cochlea using dendritic mapping to obtain accurate tonotopic maps of the human basilar membrane/organ of Corti and the spiral ganglion was performed. A novel imaging technique, synchrotron radiation phase-contrast imaging, was used and a spiral ganglion frequency function was estimated by nonlinear least squares fitting a Greenwood-like function (F = A (10^ax − K)) to the data. The three-dimensional tonotopic data presented herein has large implications for validating electrode position and creating customized frequency maps for cochlear implant recipients.

HCN channels in the mammalian cochlea: Expression pattern, subcellular location, and age-dependent changes

Neuronal diversity in the cochlea is largely determined by ion channels. Among voltage‐gated channels, hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channels open with hyperpolarization and depolarize the cell until the resting membrane potential. The functions for hearing are not well elucidated and knowledge about localization is controversial. We created a detailed map of subcellular location and co‐expression of all four HCN subunits across different mammalian species including CBA/J, C57Bl/6N, Ly5.1 mice, guinea pigs, cats, and human subjects. We correlated age‐related hearing deterioration in CBA/J and C57Bl/6N with expression levels of HCN1, −2, and −4 in individual auditory neurons from the same cohort. Spatiotemporal expression during murine postnatal development exposed HCN2 and HCN4 involvement in a critical phase of hair cell innervation. The huge diversity of subunit composition, but lack of relevant heteromeric pairing along the perisomatic membrane and axon initial segments, highlighted an active role for auditory neurons. Neuron clusters were found to be the hot spots of HCN1, −2, and −4 immunostaining. HCN channels were also located in afferent and efferent fibers of the sensory epithelium. Age‐related changes on HCN subtype expression were not uniform among mice and could not be directly correlated with audiometric data. The oldest mice groups revealed HCN channel up‐ or downregulation, depending on the mouse strain. The unexpected involvement of HCN channels in outer hair cell function where HCN3 overlaps prestin location emphasized the importance for auditory function. A better understanding may open up new possibilities to tune neuronal responses evoked through electrical stimulation by cochlear implants.

Age-Dependency of Neurite Outgrowth in Postnatal Mouse Cochlear Spiral Ganglion Explants

Background: The spatial gap between cochlear implants (CIs) and the auditory nerve limits frequency selectivity as large populations of spiral ganglion neurons (SGNs) are electrically stimulated synchronously. To improve CI performance, a possible strategy is to promote neurite outgrowth toward the CI, thereby allowing a discrete stimulation of small SGN subpopulations. Brain-derived neurotrophic factor (BDNF) is effective to stimulate neurite outgrowth from SGNs. Method: TrkB (tropomyosin receptor kinase B) agonists, BDNF, and five known small-molecule BDNF mimetics were tested for their efficacy in stimulating neurite outgrowth in postnatal SGN explants. To modulate Trk receptor-mediated effects, TrkB and TrkC ligands were scavenged by an excess of recombinant receptor proteins. The pan-Trk inhibitor K252a was used to block Trk receptor actions. Results: THF (7,8,3′-trihydroxyflavone) partly reproduced the BDNF effect in postnatal day 7 (P7) mouse cochlear spiral ganglion explants (SGEs), but failed to show effectiveness in P4 SGEs. During the same postnatal period, spontaneous and BDNF-stimulated neurite outgrowth increased. The increased neurite outgrowth in P7 SGEs was not caused by the TrkB/TrkC ligands, BDNF and neurotrophin-3 (NT-3). Conclusions: The age-dependency of induction of neurite outgrowth in SGEs was very likely dependent on presently unidentified factors and/or molecular mechanisms which may also be decisive for the age-dependent efficacy of the small-molecule TrkB receptor agonist THF.

Cochlear implantation and residual hearing preservation long-term follow-up of the first consecutively operated patients using the round window approach in Uppsala, Sweden

Objective: We conducted a long-term follow-up study to investigate the time course of residual hearing in our first 21 consecutively operated cochlear implant (CI) patients using the round window (RW) approach . The study may provide additional information about the influence of a flexible lateral wall electrode array on cochlear function. Methods: Data were available for long-term follow-up (>5 years) in 15 patients. Pure tone audiometry (PTA) was assessed at 0.125-8 kHz preoperatively, and at one, three and >5 years postoperatively. Insertion angle, number of electrodes inside the cochlea, user-time of the processor and stimulation strategy were documented. Results: Twelve out of 15 patients had residual hearing after a follow-up period of five years (mean 86 months, range: 61-103 months). Four out of 15 patients had >75% complete hearing preservation (HP), 8 out of 15 had 25-75% partial HP and 3 out of 15 patients had complete loss of hearing. There was a high correlation between insertion angle and HP. Conclusion: Long-term HP was possible in 12 out of 15 cases. Even patients with complete hearing loss at long-term follow-up showed high performance in speech understanding and were full-time users.