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

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

Developmental Changes of Mechanics Measured in the Gerbil Cochlea

This report describes stiffness and best frequency measurements obtained in vitro from the basilar membrane of the gerbil cochlea at the onset of hearing, during hearing maturation, and after hearing has matured. Our stiffness data constitute the first direct experimental evidence of developmental stiffness changes in the basal and middle turns. Stiffness changes by a factor of 5.5 in the basal turn between postnatal day 11 and adult, and the difference from adult is statistically significant for all ages measured up to postnatal day 16. For the middle turn, stiffness changes by a factor of 1.6 between postnatal day 11 and adult. Whereas for postnatal day 12 and beyond there is no statistically significant difference from adult, our data suggest that there may be a significant difference of stiffness between day 11 and adult in the middle turn. For the basal turn, our motion measurements confirm a passive component to the developmental best frequency shift. For the middle turn, changes in best frequency are not statistically significant. Best frequency was determined by stimulating the tissue at audio frequencies with a glass paddle and measuring motion with a computer-based imaging system. Tissue stiffness was measured with a piezoelectric-based sensor system. Tissue stiffness changes have previously been postulated to contribute to the best frequency shift observed in the cochlear base. Incorporating our data into a simple spring-mass resonance model demonstrates that our experimentally measured stiffness change can account for the change of best frequency. These results suggest that a stiffness change is, in fact, a critical component of the best frequency shift observed in the basal turn of the gerbil cochlea after the onset of hearing.     

Endolymphatic hydrops: mechanical causes of hearing loss

Summary An explanation for the mechanical origin of the hearing loss in endolymphatic hydrops is presented that is based on studies in mechanical cochlear models. An elastic bias of the basilar membrane and/or a mass loading of the cochlear duct account for the low-frequency hearing loss, diplacusis, and evenharmonic distortion. In addition, the static shearing displacement between the tectorial membrane and the organ of Corti, caused by the displacement of the basilar membrane, may partially decouple the hair cells from the tectorial membrane, an event that would explain the tinnitus, recruitment, and perhaps even the disportional loss of speech intelligibility associated with endolymphatic hydrops.     

Acoustic overstimulation alters the morphology of the tectorial membrane

After a permanent threshold shift was induced by exposing guinea pigs to a 1 kHz pure tone at 105 dB(A) for 72 h, light microscopic observations of freshly dissected and stained tectorial membranes showed an increased waviness and clumping of the fibers of the middle zone. Hensen's stripe was not seen as a continuous dense structure running through the middle zone but was at times discontinuous and curved. As measured from cross-sections of the cochlea, the thickness of the tectorial membrane was decreased after acoustic overstimulation. The stereocilia of the inner and outer hair cells lie directly under the middle zone. Visual detection levels of threshold of tectorial membrane movement was determined by stimulating the marginal zone of the tectorial membrane of isolated cochlear coils by an oscillating water jet. After acoustic overstimulation the tectorial membrane became more compliant. The tectorial membrane abnormalities were restricted to the regions of the cochlea that demonstrated a 40–50 dB hearing loss.     

A micromechanical model of the cochlea with radial movement of the tectorial membrane.

The function of the tectorial membrane in the cochlear micromechanics is uncertain. In modeling approaches some models have assumed it to be a resonator that participates in the sharp tuning mechanisms of the cochlea with its mass coupled to the ciliary stiffness of outer hair cells, being driven by the shear force between the reticular lamina and itself. This paper presents a different type of micromechanical model which assumes that the tectorial membrane is driven by a lymphatic fluid flow that can be shown to have a substantial radial component. It also assumes that the reticular lamina is relatively stiff and thereby restrains the top end of outer hair cells that exert a force to the basilar membrane via Deiters cells. When combined with a three-dimensional block model, it can simulate the sharp tuning mechanisms of the cochlea well.     

Col11a1 and Col11a2 mRNA expression in the developing mouse cochlea: implications for the correlation of hearing loss phenotype with mutant type XI collagen genotype

OBJECTIVE: Mutations in the fibrillar collagen genes COL11A1 and COL11A2 can cause sensorineural hearing loss associated with Stickler syndrome. There is a correlation of hearing loss severity, onset, progression and affected frequencies with the underlying mutated collagen gene. We sought to determine whether differences in spatial or temporal expression of these genes underlie this correlation, and to identify the cochlear cell populations expressing these genes and the structures likely to be affected by mutations. MATERIALS AND METHODS: We used in situ hybridization analysis of C57BL/6J mouse temporal bones. RESULTS: Similar, diffuse expression of Col11a1 and Col11a2 mRNA was first observed in the cochlear duct at embryonic Day 15.5, with increasingly focal hybridization being noted at postnatal Days 1 and 5 in the greater epithelial ridge and lateral wall of the cochlea. The greater epithelial ridge appeared to be the main, if not only, source of mRNA encoding Col11a1 and Col11a2 in the tectorial membrane. At postnatal Day 13, Col11a1 and Col11a2 expression became more focal and co-localized in the inner sulcus, Claudius' cells and cells of Boettcher. CONCLUSIONS: We did not observe spatial or temporal differences in mRNA expression that could account for the auditory phenotype genotype correlation. The expression patterns suggest essential roles for Col11a1 and Col11a2 in the basilar or tectorial membranes.     

Electrical impedance measurements of cochlear structures using the four-electrode reflection-coefficient technique

In individuals with severe-to-profound hearing loss, cochlear implants (CIs) bypass normal inner ear function by applying electrical current directly into the cochlea, thereby stimulating surviving auditory nerve fibers. Although cochlear implants are able to restore some auditory sensation, they are far from providing normal hearing. It has been estimated that up to 75% of the current injected via a CI is shunted along scala tympani and is not available to stimulate auditory neurons. The path of the injected current and the consequent population of stimulated spiral ganglion cells are dependent upon the positions of the electrode contacts within the cochlea and the impedances of cochlear structures. However, characterization of the current path remains one of the most critical, yet least understood, aspects of cochlear implantation. In particular, the impedances of cochlear structures, including the modiolus, are either unknown or based upon estimates derived from circuit models. Impedance values for many cochlear structures have never been measured. By combining the hemicochlea preparation, a cochlea cut in half along its mid-modiolar plane, and the four-electrode reflection-coefficient technique, impedances can be measured for cochlear tissues in a cochlear cross section including the modiolus. Advantages and disadvantages of the method are discussed in detail and electrical impedance measurements obtained in the gerbil hemicochlea are presented. The resistivity values for the cochlear wall in Ωcm are, 528 (range: 432–708) for scala media 3rd turn, 502 (range: 421–616) for scala tympani 3rd turn and scala vestibuli 2nd turn, 627 (range: 531–759) for scala media 2nd turn, 434 (range: 353–555) for scala tympani 2nd turn and scala vestibuli basal turn, 434 (range: 373–514) for scala media basal turn, and 590 (range: 546–643) for scala tympani basal turn. The resistivity was 455 Ωcm (range: 426–487) for the modiolus.     

Regeneration of the tectorial membrane in the chick cochlea following severe acoustic trauma

Damage to the tectorial membrane caused by acoustic trauma was examined with scanning and transmission electron microscopy immediately after exposure and at selected time points over a 10 day recovery period. At 0 h of recovery the structure of the tectorial membrane overlying the region of hair cell damage was severely disrupted and connections between the membrane and the basilar papilla were lost. By 24 h of recovery, regeneration of the tectorial membrane was evident in the secretion of new matrix materials by the supporting cells of the basilar papilla. By 10 days of recovery a new honeycomb-like matrix had replaced the segment of damaged tectorial membrane, re-established connections with hair cell stereocilia and become fused with adjacent regions of undamaged tectorial membrane. However, the regenerated segment included only the honeycomb-like structure of the lower layer of the normal tectorial membrane. The laterally-oriented fibers which form the upper layer of the membrane were not regenerated over the damaged region. These findings indicate that the tectorial membrane is regenerated in parallel with the hair cells during recovery from acoustic trauma but the full extent of this recovery and its effect on cochlear function are not yet clear.     

Inner ear pathologic features following mumps infection. Report of a case in an adult.

Temporal bone studies in an adult with a moderately severe, bilateral sensorineural hearing loss revealed bilateral cochlear changes 13 years after mumps infection. The organ of Corti was completely absent in the greater part of the superior horizontal basal limbs, with occasional hair cell loss throughout the rest of the cochlea. The outer sulcus cell area was degenerated. The stria vascularis was normal, as was the tectorial membrane, except for small hyaline droplets. The number of nerve fibers was extremely decreased in the spiral bony lamina of the basal turns. Basophilic material, possibily representing degeneration of otoliths, was present in the saccule and utricle, bilaterally, with small amounts in all of the ampullae. This was considered to be either a possible result of cytotoxic cancerocidal therapy, or an incidental nonspecific change.     

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

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

Wheat germ agglutinin binding sites in the organ of Corti as revealed by lectin-gold labeling

The distribution of wheat germ agglutinin(WGA)-binding sites in the organ of Corti of the guinea pig and mongolian gerbil was studied. WGA was conjugated with gold particles and applied on thin sections of the cochlea embedded in Spurr's resin and in Lowicryl K4M. WGA-binding sites were found on the plasma membrane, lysosomes and cytoskeletons of hair and supporting cells as well as on the tectorial and basilar membranes. No distinct difference was discovered between hair cells and supporting cells in terms of WGA-binding activities.     

Predicting the effect of post-implant cochlear fibrosis on residual hearing

Intracochlear scarring is a well-described sequela of cochlear implantation. We developed a mathematical model of passive cochlear mechanics to predict the impact that this might have upon residual acoustical hearing after implantation. The cochlea was modeled using lumped impedance terms for scala vestibuli (SV), scala tympani (ST), and the cochlear partition (CP). The damping of ST and CP was increased in the basal one half of the cochlea to simulate the effect of scar tissue. We found that increasing the damping of the ST predominantly reduced basilar membrane vibrations in the apex of the cochlea while increasing the damping of the CP predominantly reduced basilar membrane vibrations in the base of the cochlea. As long as intracochlear scarring continues to occur with cochlear implantation, there will be limitations on hearing preservation. Newer surgical techniques and electrode technologies that do not result in as much scar tissue formation will permit improved hearing preservation.     

一氧化氮合酶和心钠素在豚鼠耳蜗的分布

目的 观察豚鼠耳蜗局部心钠素（atral natruretc peptde，ANP）和一氧化氮合酶（nitric oxide synthase，NOS）免疫组化反应产物的分布，为研究ANP和NOS在豚鼠耳蜗局部血流、淋巴以及神经调节中的相互作用提供形态学依据。方法 采用免疫组织化学双标法检测ANP和NOS在正常豚鼠耳蜗的分布特征。结果 在耳蜗各转螺旋动脉和血管纹．螺旋缘、螺旋韧带和Corti器显示双阳性染色，螺旋神经节细胞及囊斑神经上皮细胞膜及轴突NOS阳性染色，胞质ANP阳性染色；盖膜、前庭膜阴性染色。结论 ANP和NOS在内耳血 流调节，内、外淋巴平衡调节以及神经信号传递等方面可能具有重要作用，二者之间可能存在密切的相互作用机制，其分布特点与功能密切相关。     

Basilar Membrane and Tectorial Membrane Stiffness in the CBA/CaJ Mouse

The mouse has become an important animal model in understanding cochlear function. Structures, such as the tectorial membrane or hair cells, have been changed by gene manipulation, and the resulting effect on cochlear function has been studied. To contrast those findings, physical properties of the basilar membrane (BM) and tectorial membrane (TM) in mice without gene mutation are of great importance. Using the hemicochlea of CBA/CaJ mice, we have demonstrated that tectorial membrane (TM) and basilar membrane (BM) revealed a stiffness gradient along the cochlea. While a simple spring mass resonator predicts the change in the characteristic frequency of the BM, the spring mass model does not predict the frequency change along the TM. Plateau stiffness values of the TM were 0.6 ± 0.5, 0.2 ± 0.1, and 0.09 ± 0.09 N/m for the basal, middle, and upper turns, respectively. The BM plateau stiffness values were 3.7 ± 2.2, 1.2 ± 1.2, and 0.5 ± 0.5 N/m for the basal, middle, and upper turns, respectively. Estimations of the TM Young’s modulus (in kPa) revealed 24.3 ± 25.2 for the basal turns, 5.1 ± 4.5 for the middle turns, and 1.9 ± 1.6 for the apical turns. Young’s modulus determined at the BM pectinate zone was 76.8 ± 72, 23.9 ± 30.6, and 9.4 ± 6.2 kPa for the basal, middle, and apical turns, respectively. The reported stiffness values of the CBA/CaJ mouse TM and BM provide basic data for the physical properties of its organ of Corti.     

Recent developments in cochlear physiology

Recent findings in cochlear physiology have caused many of our long held ideas about how sound is analyzed by the ear to be reevaluated. This article describes changes which have occurred in three classical ideas of cochlear transduction: (1) There is a gradient of frequency representation along the cochlea with high frequencies being represented at the base and lower frequencies represented progressively toward the apex. It is now known that the specific frequency which is represented at a given location along the cochlea is not invariant but changes systematically during the normal development of hearing. (2) The place code and frequency tuning along the cochlea are due to the conventional traveling wave of von Békésy and basilar membrane mechanics. Experiments in nonmammalian vertebrates which lack a traveling wave have shown that other mechanisms, including the mechanical resonance of hair cell stereocilia, may contribute to tonotopic organization and frequency tuning. It is possible that hair cell stereocilia also contribute to frequency representation and tuning in the mammalian cochlea. (3) The vibration of the basilar membrane to sound is determined by its passive mechanical properties. It is now known that the response of the basilar membrane, and that of the cochlear partition as a whole, is influenced by physiological processes which utilize metabolic energy. The active processes are likely expressed through the motile activity of outer hair cells.     

Immunolocalization of calbindin D28k and calretinin in the dog cochlea during postnatal development

The calbindin (CB) and the calretinin (CR) immunoreactivities were studied in the dog cochlea during its postnatal maturation from birth to the 33rd postnatal day. At birth, CB was expressed in the K?lliker's organ, in the immature inner (IHC) and outer hair cells (OHC), in neurons of the spiral ganglion, and in nerve fibers running in the basilar membrane of the apical turn. During the cochlear maturation, non-sensorineuronal structures, such as the K?lliker's organ, the rods of Corti, and the inner sulcus cells, displayed a transient CB-staining. In the adult-like dog cochlea, CB was found in the cytoplasm, the cuticular plate, and the stereocilia of the IHC and OHC. All the neurons of the spiral ganglion and some nerves fibers in the modulius were CB-positive. At birth, CR exhibited a neuronal distribution: about 75% of the spiral ganglion neurons, some nerve fibers in the modulius and nerve fibers running in the basilar membrane were CR-labeled. During the postnatal maturation, a CR-immunostaining appeared around the IHC body and CR was expressed transiently in the OHC. In the adult-like dog cochlea, a CR-positive network surrounded the unlabeled IHC. The neuronal CR-labeling remained unchanged from birth.     

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

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

Mechanical transduction in outer hair cells

The outer hair cells are responsible for the exquisite sensitivity, frequency selectivity and dynamic range of the cochlea. These cells are part of a mechanical feedback system involving the basilar membrane and tectorial membrane. Transverse displacement of the basilar membrane results in relative motion between the tectorial membrane and the reticular lamina, causing deflection of the stereocilia and modulation of the open probability of their transduction channels. The resulting current causes a change of membrane potential, which in turn produces mechanical force, that is fed back into the motion of the basilar membrane. Experiments were conducted to address mechanical transduction mechanisms in both the stereocilia and the basolateral cell membrane, as well as modes of coupling of the outer hair cell force to the organ of Corti.     

小鼠出生后早期耳蜗柯蒂氏器巨噬细胞形态的变化

目的 揭示小鼠出生后早期耳蜗柯蒂氏器是否存在巨噬细胞及柯蒂氏器巨噬细胞形态和分布的变化.方法 选1～4周龄的C57BL/6J小鼠,解剖取耳蜗基底膜.CD45抗体(一种全白细胞标记物)染色耳蜗基底膜,F4/80(巨噬细胞专有蛋白标记物)确认巨噬细胞,碘化丙锭标记细胞核,荧光显微镜下观察CD45染色阳性细胞的形态和分布变化...     

Electrical resistivity measurements in the mammalian cochlea after neural degeneration

OBJECTIVES/HYPOTHESIS: In the present series of experiments, the effect of neural degeneration on the cochlear structure electrical resistivities was evaluated to test if it alters the current flow in the cochlea and if increased current levels are needed to stimulate the impaired cochlea. In cochlear implants, frequency information is encoded in part by stimulating discrete populations of spiral ganglion cells along the cochlea. However, electrical properties of the cochlear structures result in shunting of the current away from the auditory neurons. This consumes energy, makes cochlear implants less efficient, and drastically reduces battery life. Models of the electrically stimulated cochlea serve to make predictions on current paths using modified and improved cochlear implant electrodes. However, one of the model's shortcomings is that most of the values for tissue impedances are not direct measurements. They are derived from bulk impedance measurements, which are fitted to lumped-element models. STUDY DESIGN: The four-electrode reflection-coefficient technique was used to measure resistivities in the gerbil cochlea. In vivo and in vitro (the hemicochlea) models were used. Measurements were made in normal and in deafened animals. Cochlear damage was induced by neomycin injection into the animals' middle ears. Neural degeneration was allowed to occur over 2 months before performing the measurements in the deafened animals. RESULTS: The resistivity values in deafened animals were smaller than in the normal-hearing animals, thus altering the current flow within the cochlea. CONCLUSIONS: Resistivity changes and subsequent changes in current path should be considered in future designs of cochlear implants.