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PubMedDate not supplied

Structural recovery from sound and aminoglycoside damage in the avian cochlea.

Hair cell regeneration in the mature avian cochlea occurs in response to trauma that causes the death of some or all of the existing hair cell population. In general, this trauma has been introduced experimentally by either sound overexposure or treatment of the bird with high doses of aminoglycoside antibiotics. When injured hair cells are ejected from the sensory epithelium, the nonsensory supporting cells respond by re-entering the cell cycle and proliferating or by transdifferentiating directly into hair cells without a mitotic event. The new hair cells mature in a manner similar to that seen during embryonic development. They make connections with the overlying tectorial membrane and the afferent and efferent cochlear nerve processes within the sensory epithelium. This structural regeneration is accompanied by a significant recovery of auditory function and thus allows the animal to regain its hearing ability. This hair cell regeneration is presumably quite beneficial to birds, whose primary means of communication is based on vocalizations and the ability to hear and comprehend them. The prevalence of hearing loss in our society and the isolating impact it has on affected individuals makes the potential for finding ways to induce a similar hair cell regeneration in humans a very tempting goal. Studies of hair cell regeneration over the last 12 years have focused on the mechanisms that regulate the process and how they could be controlled. This review will examine the structural events involved in regenerating hair cells in the avian cochlea after sound damage and aminoglycoside treatment. It will define how hair cells and nerve endings are lost and the tectorial membrane is damaged by the traumatizing stimuli and how the supporting cells and nerve fibers respond by producing new hair cells, a new tectorial membrane and new synaptic connections during recovery. Finally, it will focus on mechanisms that control the proliferation and transdifferentiation of supporting cells and the differentiation of new hair cells. This structural review is accompanied by a companion review that covers the fundamental issues concerning functional recovery in the avian cochlea associated with hair cell regeneration.

Aminoglycosides↗
PubMed1988

Auditory peripheral tuning: evidence for a simple resonance phenomenon in the lizard Tiliqua.

The origin of the frequency selectivity of neurons in the vertebrate auditory periphery is one of the most important questions in auditory research today. In an attempt to delineate the extent to which structures outside the sensory cells play a role in determining peripheral auditory responses, we measured the mechanical displacement of the basilar membrane and the selectivity of nerve fibres at the same location in the bobtail lizard. These data indicate a contribution to frequency selectivity, the tuning of which resembles a high-pass resonant filter characteristic, arising subsequent to the basilar membrane motion. A comparison of these data with the tuning of auditory-nerve fibres originating from papillar areas in other lizard species without a tectorial membrane, suggests that it is the involvement of the tectorial membrane in a mechanical resonance which increases the frequency selectivity.

Acoustic Stimulation↗
PubMed1993

Glycoconjugates in the cochlea as revealed by the silver methenamine method.

The glycoconjugates in the cochlea of the guinea pig were studied by staining samples by the silver methenamine method as well as after periodic acid-Schiff (PAS) staining. Results obtained by the two methods were similar but not identical. The silver methenamine method was much better in terms of resolution. However, this method of staining seemed less specific than the PAS reaction. When the silver methenamine method was used, the tectorial membrane and outer hair cells were specifically stained. Two types of fibrils were observed in the tectorial membrane. Thick fibrils were located in the fibrous layer. Thin fibrils were situated in the marginal band, the cover net, Hensen's stripe and the fibrous layer. The thick and thin fibrils appeared to correspond to type A and type B protofibrils, respectively. The outer hair cells were found to contain strongly stained particles which, presumably, consisted of glycogen. The basement membrane of the capillaries in the stria vascularis also gave a positive reaction, while that of other capillaries was essentially unstained. This finding suggests structural differences between these capillaries.

Animals↗
PubMedDate not supplied

How OHC lesions can lead to neural cochlear hypersensitivity.

Several laboratories, including ours, have found that damage to OHCs--but not to IHCs--can produce hypersensitivity of cochlear nerve afferents to low-frequency sound in the presence of hyposensitivity at the characteristic frequency (CF). It is possible to explain the paradox by a mechanical coupling of the OHCs and IHCs through the tectorial membrane, for which there is accumulating experimental evidence. No direct contact between the IHC stereocilia and the tectorial membrane is required.

Animals↗
PubMed1992

Mechanical filtering of sound in the inner ear.

We have studied the distortion generated by the cochlea to gain insight into the mechanisms responsible for the sharp tuning or 'frequency selectivity' of the inner ear. We used two stimulating tones of moderate intensity which are progressively separated in frequency, and measured the ear canal cubic distortion components which are generated as a consequence of the stimulus interaction in the cochlea. We inferred that the distortion is generated from the frequency region of the higher of the two stimulus tones and that it is then band-pass filtered by a structure which is tuned to a frequency just over half an octave below that of the high-frequency tone. We suggest that the structure responsible for this band-pass filtering is the tectorial membrane, and we conclude that our results support theories of cochlear mechanics in which resonances due to the tectorial membrane interact with those of the basilar membrane to enhance the frequency selectivity of the inner ear.

Acoustic Stimulation↗
PubMed2005

High resolution scanning electron microscopy of the human organ of Corti. A study using freshly fixed surgical specimens.

Scanning electron microscopy on immediately fixed human cochleae obtained during surgery for life-threatening petro-clival meningioma showed excellently preserved morphology. We compared the morphological findings with those from transmission electron microscopic sections of well preserved human and animal tissue. The characteristics of neural innervation, the pathways of the nerves through the organ of Corti and the intimate relation of nerves to supporting cells along their route could be studied in detail. The lateral membranes of Hensen and Claudius cells were folded creating a surface enlargement. Marginal pillars extended the distal end of the tectorial membrane and correspond to the marginal net or "randfasernetz" described earlier. Stereocilia imprints at the undersurface of the tectorial membrane go as far as to the distal end of the marginal pillars. The presence of an irregularly distributed fourth row of outer hair cell, attached to the marginal pillars, raises questions about differences in the excitation of the last row of outer hair cells. The complex nature of many supporting cells, stria vascularis and Reissner's membrane, intracellular complexities as well as surface features are described. Supernumerary inner hair cells were observed and the different arrangement of outer spiral fibres in contrast to findings in animals and variations of nerve fibres within the organ of Corti between apex and base are discussed.

Animals↗
PubMed1997

The mouse tectorins. Modular matrix proteins of the inner ear homologous to components of the sperm-egg adhesion system.

The cDNA and derived amino acid sequences for the two major non-collagenous proteins of the mouse tectorial membrane, alpha- and beta-tectorin, are presented. The cDNA for alpha-tectorin predicts a protein of 239,034 Da with 33 potential N-glycosylation sites, and that of beta-tectorin a smaller protein of 36,074 Da with 4 consensus N-glycosylation sites. Southern and Northern blot analysis indicate alpha- and beta-tectorin are single copy genes only expressed in the inner ear, and in situ hybridization shows they are expressed by cells both in and surrounding the mechanosensory epithelia. Both sequences terminate with a hydrophobic COOH terminus preceded by a potential endoproteinase cleavage site suggesting the tectorins are synthesized as glycosylphosphatidylinositol-linked, membrane bound precursors, targeted to the apical surface of the inner ear epithelia by the lipid and proteolytically released into the extracellular compartment. The mouse beta-tectorin sequence contains a single zona pellucida domain, whereas alpha-tectorin is composed of three distinct modules: an NH2-terminal region similar to part of the entactin G1 domain, a large central segment with three full and two partial von Willebrand factor type D repeats, and a carboxyl-terminal region which, like beta-tectorin, contains a single zona pellucida domain. The central, high molecular mass region of alpha-tectorin containing the von Willebrand factor type D repeats has homology with zonadhesin, a sperm membrane protein that binds to the zona pellucida. These results indicate the two major non-collagenous proteins of the tectorial membrane are similar to components of the sperm-egg adhesion system, and, as such may interact in the same manner.

Alternative Splicing↗
PubMed1993

Recovery of auditory function and structure in the chick after two intense pure tone exposures.

Groups of neonatal chicks were examined in three experimental conditions that differed in the age and number of times they were exposed to a pure tone of 0.9 kHz at 120 dB SPL for 48 h. Several animals were exposed once at 2 or 16 days of age, while others were subjected twice to the above stimulus, first at 2 days and then at 16 days. Evoked potential measures of threshold shift, obtained at 0, 12 or 26 days post-exposure, were used to determine the degree of hearing loss and recovery. The average threshold loss in the mid-range frequencies was about 57 dB at 0 days for all three conditions. This level was reduced to about 15 dB in all three groups at 12 days of recovery, while in birds exposed once at 2 days, but allowed 26 days to recover, the post-exposure thresholds returned to pre-exposure levels. Scanning electron microscopic analysis of cochlear structure was conducted in groups of similarly exposed and recovered animals. Twelve days post-exposure, the structural analysis revealed regeneration of a single honeycomb-like tectorial membrane layer in both the once and twice-exposed cochleae. However, damage to, and repair of, the tectorial membrane after the second exposure revealed the production of a second honeycomb layer in about half the animals examined. The results indicated that chicks retain the capacity to repair receptor epithelium damage and recover considerably from hearing loss after multiple exposures to intense sound.

Acoustic Stimulation↗
PubMedDate not supplied

A note on development of Corti's organ.

The structure and postnatal transformation of Kölliker's organ in the cat were studied by means of both light and electron microscopes. Kölliker's organ, which is located in the inner sulcus area of the cochlear duct during embryonic development of the kitten, appeared to undergo a major transformation during the first 30 days after birth. On the level of the organ as a whole the possible nature of this transformation is briefly discussed. On the cellular level, the transformation of this organ was found to involve a process of cellular autophagocytosis. Kölliker's organ consists of numerous tightly packed, tall columnar cells filling the inner sulcus. Autophagic vacuoles containing cell organelles were observed in these cells in all stages of transformation of the organ. Cellular autophagocytosis reduced the number of cells present in each section from approximately 50 in the newborn, to approximately 12 in the 30-day-old kitten. The apparent transformation of Kölliker's organ was observed as progressing from base to apex and from the limbus to the inner hair cell. The relationship of the tectorial membrane to Kölliker's organ and that of the tectorial membrane to the area in and around the inner hair cells are discussed.

Animals↗
PubMed1992

Quantitative carbohydrate analyses of the tectorial and otoconial membranes of the guinea pig.

Carbohydrate composition of the tectorial membrane (TM) and the otoconial membrane (OM) of the guinea pig was analyzed after hydrolysis, using high-performance anion-exchange chromatography and pulsed amperometric detection. Both of the tissues were highly glycosylated; the carbohydrate content being 24-42% of protein. GlcN, Gal, Glc and Man were found to be the major component sugars of TM, whereas little GalN was found. Fuc and NANA were also present, but NGNA was not detectable. After digestion with thermolysin for solubilization, OM was separated into two fractions: insoluble mineral particles of the otoconia (OM-ppt) and a soluble fraction from the gelatinous layer (OM-sup). These two fractions showed distinct carbohydrate composition from each other. Further analyses using glycosidases revealed that TM contained asialyl and monosialyl but little di-, tri- and tetrasialyl N-glycosides, and OM-sup did not seem to be susceptible to endo-beta-galactosidase, which is known to cleave some N-acetyl-polylactosamine and keratan sulfate. Based on these analyses, it can be suggested that most of the carbohydrates in TM are likely to be asialyl and monosialyl N-glycosides. N-Glycosides may be predominant in the otoconia as well, and a polymer structure consisting of GlcN(Ac) and Gal other than N-acetyl-polylactosamine may exist in the gelatinous layer of OM. O-Glycosylation of the usual type appeared to be minor in all the fractions.

Acetylation↗
PubMed1993

Development of the tectal cells in the mouse cochlea.

Tectal cells appear at birth in the outer part of the developing organ of Corti. At first they are attached to the basilar membrane, but later they ascend through the auditory epithelium. During the 1st postnatal week (coinciding with the development of the minor tectorial membrane), the newly formed tectal cells show several cytological characteristics suggesting increased metabolic and secretory activities, which include: (1) a large Golgi complex, (2) abundant amorphous material inside the cisterns of rough endoplasmic reticulum, and (3) dense granules inside the mitochondrial matrix. All these features gradually disappear, and by the 14th postnatal day the tectal cells show a dark cytoplasm and few and short microvilli. In addition, tectal cells were stained selectively by some lectins. These findings suggest that tectal cells may participate in the secretion of some components of the minor tectorial membrane, different from those produced by Deiters' cells, Hensen's cells and pillar cells.

Animals↗
PubMed1983

[Hearing receptor of the white rat].

The cochlea of albino rats was studied on the fresh and fixed but not dehydrated specimens, as well as on radial sections of Epon embedded material, and by scanning electron microscopy. Obtaining sections of the non-dehydrated tectorial membrane made it possible to reconstruct its shape and position with respect to the organ of Corti and sulcus spiralis. The quantitative description of geometry of the tectorial membrane is given in addition to that of the surface of the organ of Corti and of bundles of stereocilies of both outer and inner hair cells. The three parts of the cochlea are shown (apical, medial and basal) to manifest characteristic gradients of the analyzed parameters.

Animals↗
PubMed2006

Soluble megalin is accumulated in the lumen of the rat endolymphatic sac.

The endolymphatic sac (ES) is believed to play an important role in maintaining homeostasis in the inner ear by the absorption and endocytosis of endolymph. Megalin is a 600-kDa multiligand endocytic receptor expressed in certain types of absorptive epithelia including kidney proximal tubules. We analyzed the immunoreactivity for megalin in rat ES by immunofluorescence, immunogold electron microscopy, and immunoblotting. With immunostaining, the luminal substances of the ES were strongly stained for megalin. Megalin was also localized in luminal macrophage-like cells and both types of epithelial cell (mitochondria-rich cells and ribosome-rich cells). In these cells, the megalin was localized in the lumen of endosomes, but was not membrane associated. This localization pattern indicates that the megalin in these cells is not a membrane receptor, but merely one of the constituents that are endocytosed from the lumen of the ES. Immunoblotting indicated that the megalin in the ES is a 210-kDa molecule lacking a cytoplasmic domain. This suggests that the megalin in the ES may be a soluble form, different from the 600-kDa membrane-bound receptor expressed in kidneys. Taken together, it is likely that the megalin in the ES lumen is a soluble component and may be endocytosed by the ES epithelial cells. Furthermore, we found that the tectorial membrane, an acellular structure in the cochlea, gave a strong megalin immunoreaction. Since the cochlea is connected to the ES, the megalin may be transported alone or with the components of the tectorial membrane from the cochlea to the ES lumen through longitudinal flow.

Animals↗
PubMed2003

Motion analysis in the hemicochlea.

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

Algorithms↗
PubMed1996

Cochlear nerve activity after intense sound exposure in neonatal chicks.

1. Single-neuron behavior in the cochlear nerve of neonatal (3-day-old) chicks was examined after exposure to a 120-dB SPL pure tone (0.9 kHz) for 48 h. Exposed animals were tested after 0 days or 12 days of recovery. Nonexposed chicks, age-matched to the exposed animals, formed two control groups. 2. Spectral response plots were obtained from each cell. These plots described the neuron discharge rates in response to 1,767 tone burst stimuli, each with a unique frequency-intensity combination. The tone bursts were presented at frequencies between 0.1 and 4.5 kHz and for intensities between 0 and 100 dB SPL. From these plots the characteristic frequency (CF), CF threshold, and sharpness of tuning (Q10 dB) were derived for each cell. Frequency response-area functions at selected stimulus levels and rate-intensity functions at the CF were also constructed from the spectral response plots. In addition, spontaneous activity was determined. Data were obtained from 903 cells. 3. Neuron activity in the control cells revealed no differences between CF thresholds, Q10 dB, or spontaneous activity in the two age groups. However, age differences at all frequencies were noted in the rate-intensity functions. 4. A frequency-dependent loss in CF threshold was observed in the 0-day recovered cells. The threshold shift (relative to age-matched control cells) was 55-65 dB between 0.8 and 1.5 kHz, but only 10-15 dB between 0.1-0.4 kHz and 2.5-3.5 kHz. The exposed cells showed no loss in frequency selectivity (Q10 dB) at < 0.5 kHz, whereas above this frequency an increasing deterioration in tuning was noted. Spontaneous activity in the 0-day cells was suppressed across the entire range of CFs. The rate-intensity function of exposed cells had a steeper growth rate than that of control cells. 5. At 12 days of recovery, CF threshold, Q10 dB, and spontaneous activity all recovered to the levels exhibited by age-matched control cells. However, the rate-intensity function for cells with CFs between 0.8 and 1.0 kHz showed abnormal growth and higher discharge rates at saturation than the control cells. Outside of this frequency range the rate-intensity functions of control and exposed cells were similar to each other. 6. Recovery of function in the sound-damaged chick ear is accompanied by almost complete repair of the basilar papilla. The tectorial membrane, however, retains a major defect and only the lower layer of this membrane regenerates. An important observation in this presentation was the abnormal rate-intensity functions (in the 12-day recovered cells) reported for frequencies served by that region of the sensory epithelium where the tectorial membrane defect was found. This observation may be related to sustained structural damage to the short hair cell region of the papilla and/or alterations in the efferent control of papilla function mediated by the short hair cells.

Aging↗
PubMed1990

Central projections of cochlear nerve fibers in the alligator lizard.

The auditory (cochlear) ganglion cells of the alligator lizard (Gerrhonotus multicarinatus) give rise to two types of peripheral fibers: tectorial fibers, which contact hair cells covered by a tectorial membrane, and free-standing fibers, which contact hair cells without a tectorial membrane. To determine the central projections of these fibers, we applied intracellular and extracellular injections of horseradish peroxidase (HRP) to the peripheral component of the cochlear nerve. After histological processing with diaminobenzidine, individual cochlear nerve fibers could be traced through serial sections with the aid of a light microscope and drawing tube. The projection patterns formed two morphologically distinct groups. Neurons whose peripheral processes contacted tectorial hair cells in the cochlea projected to three divisions of the cochlear nucleus: nucleus magnocellularis lateralis (NML), nucleus magnocellularis medialis (NMM), and nucleus angularis lateralis (NAL). Neurons whose peripheral processes contacted free-standing hair cells projected primarily to the nucleus angularis medialis (NAM), although some also sent a single, thin branch to the NML; these neurons never projected to NAL or NMM. Morphometric comparisons of tectorial and free-standing fibers demonstrate that tectorial fibers have a larger axonal diameter, form a greater number of terminal swellings, and make proportionally more somatic contacts. By correlating the morphologically defined groups with previously reported physiologically defined groups, we conclude that different divisions of the cochlear nucleus are associated with separate frequency ranges and that stimuli in the different frequency ranges may be processed separately in the brain.

Acoustic Stimulation↗
PubMed1977

Histopathologic changes in the temporal bone resulting from measles infection.

We report the pathologic changes in the temporal bones of four children who died of bronchoppenumonia secondary to acute measles infection. All suffered from severe necrotizing otitis media characterized by round cell infiltration. In one case, multilobulated giant cells (Warthin-Finkelday) were found in the middle ear. These cells appear to be similar to those found elsewhere within the body during acute measles infection. Inner ear changes were seen in two cases. Severe loss of nerve fibers and ganglion cells with atrophy of the striae vascularis was present in one case. Adhesions were seen between the Reissner membrane and the tectorial membrane in one case and the Reissner membrane and the inner spiral limbus in a second case. Such findings are characteristic of those seen in the cochlear duct of persons suffering from prenatal rubella.

Child, Preschool↗
PubMed1989

Lectin detection of carbohydrates in the endolymphatic sac.

The carbohydrate contents of the guinea pig endolymphatic sac were investigated by the use of lectins. The lumen of the endolymphatic sac was filled with stainable precipitate containing N-acetyl glucosamine, mannose, glucose, galactose and fucose. N-Acetyl galactosamine was also detected but in minute amounts. This composition corresponded to other areas in the inner ear, such as the cupula, the otolithic membrane and the tectorial membrane. The function of these carbohydrates may play an important role in preventing the lumen of the endolymphatic sac from collapsing as well as in regulating transepithelial fluid transport.

Acetylgalactosamine↗