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PubMed1977

[Postmortem changes of fine structure in the auditory pathways (organ of Corti and nucleus cochlearis of the guinea pig (author's transl)].

Autolytic changes in the guinea pig cochlear nucleus were studied by means of light- and transmission electron microscopy from 5 min. up to 5 h post mortem. Hyperchromasy and shrinking of neurons was observed already after 5 min. in areas, where the neurons have a high number of primary afferent nerve endings (Kane 1973). A second mode of post mortem degeneration was found. It was characterized by a swelling of the cell, which showed a diminished stain-ability. This lytic swelling seemed to be less rapid and probably less harmful in the beginning. The cochleae were examined by scanning electron microscopy. In contrary to known damaging agents (like noise or antibiotics) the outer haircells showed hardly any changes in the surface structures up to 1 hour in the basal coil, while lytic protrusions were observed in outer and inner haircells already after 5 min. in the apical parts. Post mortem changes in the cochlea and in the nucleus cochlearis show two different modes of autolytic degeneration.

Animals↗

PubMed1992

Postnatal development of the rat organ of Corti. II. Hair cell receptors and their supporting elements.

The development of cochlear receptor cells and their supporting elements was studied by means of semi-thin and ultra-thin sections during the first postnatal weeks in the rat. The temporal and spatial patterns of the receptor cell development were investigated between the 4th and 24th days after birth. At approx. ten equidistant positions along the entire cochlear duct length of inner and outer hair cells, width of outer hair cell triad and stereocilia-length of the outer hair cells were quantitatively analyzed. Striking maturational changes take place before the 12th day after birth, that is, when the onset of hearing occurs. These changes are the formation of the tunnel of Corti, of the Nuel spaces, the appearance of filaments within the supporting elements and the change in cell shape of the hair cells. Between 4 days and 20 days after birth the maturation of outer hair cells is characterized by a decrease of organelles in the cytoplasm and establishment of the subsurface cistern. The quantitative analysis revealed a unique developmental pattern of the length of the outer hair cells, the width of the outer hair cell triad and the stereocilia length of the outer hair cells. Shortly after birth these structures have an almost constant size along the whole cochlear duct, but with increasing age the structures shorten at the cochlear base and enlarge at the apex. This pattern results in the establishment of a baso-apical gradient of the above mentioned structures. We assume that this baso-apical gradient is of central importance for the frequency representation.

Animals↗

PubMed1994

Immunocytochemical detection of choline acetyltransferase in the human organ of Corti.

In the mammalian cochlea acetylcholine has been considered a major neurotransmitter of the lateral and medial efferent fibers. The aims of the present study were to investigate the expression of ChAT in the human cochlea and to develop a new method for immunohistochemical investigations in the human cochlea both at the light and electronmicroscopic level. We thus examined the ChAT-like immunoreactivity in the human inner ear using light and electron microscopy with a pre-embedding technique. Our present results agree with the previously published data acquired in rodent species. The ChAT-like immunostaining could be found in the inner spiral fibers, the inner spiral bundle, tunnel crossing fibers and at the base of the outer hair cells. No staining was noted in the negative controls experiments, while rat cochleas used as positive controls showed the usual ChAT-like immunostaining as described above. The main difference between human and rat cochleas was that the efferent nerve supply seems to be less pronounced in the human cochleas.

Animals↗

PubMed2000

Serotonergic innervation of the organ of Corti.

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

Animals↗

PubMedDate not supplied

Degeneration patterns of actin distribution in the organ of corti in two genotypes of mice.

Alterations of the auditory system during ageing and after noise damage have been studied earlier in two genotypes of mice, CBA/Ca and C57BL/6J, the former of which shows a late-onset hearing loss and the latter a high susceptibility to ageing and acoustic trauma early in life. The two strains display similarities to the different types of human age-related hearing loss. In the present study, the different patterns of hair cell degeneration were visualized by phalloidin-labelled actin in the two strains of mice with increasing age. At 1 month of age there were no major differences in the actin distribution between the two genotypes. With increasing age, CBA mice retained a regular pattern of actin content in the cuticular plate but some hair cells were missing. In contrast, the C57BL mice showed a severely distorted architecture of the cutis plate (CP) with less actin labelling, missing hair cells and scar formation. The instability of actin in the hair cells of C57BL mice might result in their high susceptibility to ageing and noise damage.

Actins↗

PubMed1993

Collagen of accessory structures of organ of Corti.

It was previously demonstrated that about 40% of the protein of the tectorial membrane of the guinea pig consists of collagen type II, with lesser amounts of type IX and XI. In this paper we extend these studies on the tectorial membrane to the basilar membrane and to other accessory structures, the spiral ligament and spiral limbus. Earlier immunohistochemical data indicated that no collagen type II is present in the basilar membrane of the newborn guinea pig, but that it is present in the area of the basilar membrane in the embryo. However, by means of stringent extraction procedures we have determined biochemically that collagen type II and lesser amounts of type XI are present in the basilar membrane of the adult guinea pig, at similar levels (on the basis of total protein) to the tectorial membrane. Levels of collagen type II are much lower in the spiral ligament and spiral limbus. The presented studies demonstrate that classical techniques of collagen chemistry can be applied at the microscale on minute tissue elements. The significance of the presence of collagen in the tectorial membrane and basilar membrane is discussed in the light of known mechanical properties of these structures.

Amino Acids↗

PubMed1999

Morphological and physiological effects of long duration infusion of strychnine into the organ of Corti.

Acute strychnine administration has long been used as a method to eliminate the effects of efferent activity. It has been shown that long after termination of chronic strychnine infusion into the cochlea, the ear becomes more susceptible to acoustic trauma suggesting that chronic strychnine infusion results in long lasting or permanent disruption of efferent function. Much research has been directed towards the functional significance of the olivocochlear system. However, there is little information concerning the effect of long duration inactivation of the medial olivocochlear system in an awake behaving animal. This study was designed to determine the structural and functional consequences of inactivation of the efferents by chronic infusion of strychnine into the cochlear perilymph of guinea pigs for two weeks via an osmotic pump. Physiological evaluations showed that the strychnine infusion eliminated the efferent induced reduction of the cochlear whole-nerve action potential three weeks after cessation of strychnine infusion. Contralateral efferent function remained unaltered. Histological evaluation at the light and electron microscopic levels revealed disoriented efferent synapses under the outer hair cells.

Acoustic Stimulation↗

PubMed1993

The localization of synaptophysin in the organ of Corti of the human as shown by immunoelectron microscopy.

Synaptophysin, or p38, a polypeptide of molecular weight 38 kD, is a calcium-binding membrane protein found in synaptic vesicles of neurons and smooth surfaced vesicles of neuroendocrine cells. Six human neonatal and infant temporal bones were fixed in paraformaldehyde and glutaraldehyde, decalcified in EDTA and were than immunoreacted for synaptophysin (ICN Biomedicals) using the avidin-biotin reaction (ABC kit, Vector Labs). The tissue was then prepared for light microscopic surface preparation, radial sections of 5 microns, and serial section electron microscopy. At a light microscopic level, the inner spiral bundle, tunnel spiral bundle, upper tunnel crossing fibers and the base of outer hair cells were stained. At the base of outer hair cells, the immunoreactivity was seen to decrease from the base to the apex and from the first to third outer hair cells. At an electron microscopic level, immunoreactivity at the base of outer hair cells was limited to vesiculated efferent fibers. The degree of immunoreactivity between adjacent efferent fibers varied significantly. Immunoreactive vesiculated endings were also found in the supranuclear region of outer hair cells.

Hair Cells, Auditory↗

PubMed1987

A functional and histological study of the combined effects of gentamicin and aminooxyacetic acid on the organ of Corti of the guinea pig.

Aminooxyacetic acid (AOAA) is a transaminase inhibitor that has been shown to protect the inner ear from loud noises. This study was done to determine if it can also protect against the cochleotoxic action of gentamicin. Four groups of guinea pigs were injected with gentamicin in doses approximating a clinical therapeutic dose and then in ototoxic doses. Thereafter animals were treated with parenteral AOAA. The effect on hearing was investigated using Preyer's reflex measurements. All animals were sacrificed and their cochleas were examined histologically using the surface preparation technique and mid-modiolar semithin sections. Histocochleograms were plotted to compare the effects of treatment in the animal groups. There was no difference seen among the groups tested. Cochlear damage was nearly equal in all animals, and AOAA was not found to protect the cochlea against gentamicin-induced ototoxicity of gentamicin. The mechanism of the ototoxicity produced is discussed on the basis of the findings. Additionally, hair cell degeneration was studied after therapeutic doses of gentamicin. Changes seen were found to be equal to or less than 5% of the hair cells and were scattered throughout the entire cochlea.

Acetates↗

PubMed1989

Asymmetry in voltage-dependent movements of isolated outer hair cells from the organ of Corti.

The electrically induced movements of outer hair cells (OHC) were studied using the whole-cell voltage-clamp technique and video analysis. Cell shortening occurs during depolarization and elongation occurs during hyperpolarization from holding potentials near -70 mV. However, a marked asymmetry in response magnitude exists such that depolarization produces larger cell length changes than do comparable levels of hyperpolarization. The response is such that at normal resting potentials in vivo, displacements are about 2 nm/mV, but increase to about 15 nm/mV as the cell is depolarized. This mechanical rectification in the depolarizing direction manifests itself during symmetrical sinusoidal voltage stimulation as a "DC" reduction in cell length superimposed upon "AC" length changes. The observed OHC mechanical rectification may be involved in the reported production of "DC" basilar membrane displacements during suprathreshold acoustic stimulation (LePage, 1987). Estimates of the magnitude of OHC movements at acoustic threshold levels induced by receptor potentials in the high-frequency region of the cochlea indicate a disparity between basilar membrane and OHC movements on the order of 21 dB. Thus, it appears questionable whether OHC mechanical movements solely underlie the "active process" thought to be responsible for the high degree of neural tuning at sound pressures near 0 dB.

Animals↗

PubMed1987

Qualitative and quantitative changes in the guinea pig organ of Corti after pure tone acoustic overstimulation.

A direct relationship between delivered sound energy and morphological overstimulation has been formulated in the so-called equal energy principle. The validity of this principle was systematically tested by studying the effects of acoustic overstimulation on the guinea pig cochlea by means of scanning electron microscopy (SEM) and light microscopy (LM) techniques. The animals in the present study were exposed to 3.85 kHz pure tones of various durations and intensities and allowed a recovery period of 4 weeks before the morphological changes in the cochlea were assessed. A 4-grade scale was devised in order to achieve a qualitative as well as a quantitative SEM evaluation of the cochlear damage. The results of the examinations by LM and SEM were compared and analysed statistically. No significant difference was observed between groups of animals exposed to a sound energy of 9.4 and 37.5 Pa2.h, irrespective of sound intensity (114 or 120 dB SPL). Groups of animals exposed to a higher energy level (150 Pa2.h) differed significantly from those exposed to higher and also lower sound energy levels. Groups exposed to different intensities (108, 114 and 120 dB SPL) did not differ significantly within this energy level.

Acoustic Stimulation↗

PubMed2005

Mechanical responses of the organ of corti to acoustic and electrical stimulation in vitro.

The detection of sound by the cochlea involves a complex mechanical interplay among components of the cochlear partition. An in vitro preparation of the second turn of the jird's cochlea provides an opportunity to measure cochlear responses with subcellular resolution under controlled mechanical, ionic, and electrical conditions that simulate those encountered in vivo. Using photodiode micrometry, laser interferometry, and stroboscopic video microscopy, we have assessed the mechanical responses of the cochlear partition to acoustic and electrical stimuli near the preparation's characteristic frequency. Upon acoustic stimulation, the partition responds principally as a rigid plate pivoting around its insertion along the spiral lamina. The radial motion at the reticular lamina greatly surpasses that of the tectorial membrane, giving rise to shear that deflects the mechanosensitive hair bundles. Electrically evoked mechanical responses are qualitatively dissimilar from their acoustically evoked counterparts and suggest the recruitment of both hair-bundle- and soma-based electromechanical transduction processes. Finally, we observe significant changes in the stiffness of the cochlear partition upon tip-link destruction and tectorial-membrane removal, suggesting that these structures contribute considerably to the system's mechanical impedance and that hair-bundle-based forces can drive active motion of the cochlear partition.

Acoustic Stimulation↗

PubMedDate not supplied

Apoptosis in auditory brainstem neurons after a severe noise trauma of the organ of Corti: intracochlear GDNF treatment reduces the number of apoptotic cells.

We have studied the morphological and cellular changes in the cochlear nucleus (CN) after cochlear nerve degeneration and whether these changes can be prevented by rescuing the primary cochlear neurons from degeneration with local glial cell line derived neurotrophic factor (GDNF) treatment. Degeneration of spiral ganglion neurons was seen to lead to a reduction of the volume of the anteroventral cochlear nucleus (AVCN); the size of the cell nuclei in the AVCN also was reduced. No differences were observed in cell density. After intrascalar GDNF treatment the volume of the AVCN was significantly larger when compared to the untreated side, and the size of the cell nuclei in the AVCN was significantly larger on the treated side. After degeneration of spiral ganglion neurons, an increased number of apoptotic cell nuclei were seen in ipsilateral CN and superior olivary complex. This increase was significantly smaller after intrascalar GDNF treatment. Degeneration of primary cochlear neurons seems to lead to an increase in the number of CN neurons undergoing apoptotic cell death. This can be prevented partially by rescuing primary cochlear neurons from degeneration with local GDNF treatment.

Animals↗

PubMed1999

Direct visualization of organ of corti kinematics in a hemicochlea.

The basilar membrane in the mammalian cochlea vibrates when the cochlea receives a sound stimulus. This mechanical vibration is transduced into hair cell receptor potentials and thereafter encoded by action potentials in the auditory nerve. Knowledge of the mechanical transformation that converts basilar membrane vibration into hair cell stimulation has been limited, until recently, to hypothetical geometric models. Experimental observations are largely lacking to prove or disprove the validity of these models. We have developed a hemicochlea preparation to visualize the kinematics of the cochlear micromechanism. Direct mechanical drive of 1-2 Hz sinusoidal command was applied to the basilar membrane. Vibration patterns of the basilar membrane, inner and outer hair cells, supporting cells, and tectorial membrane have been recorded concurrently by means of a video optical flow technique. Basilar membrane vibration was driven in a direction transversal to its plane. However, the direction of the resulting vibration was found to be essentially radial at the level of the reticular lamina and cuticular plates of inner and outer hair cells. The tectorial membrane vibration was mainly transversal. The transmission ratio between cilia displacement of inner and outer hair cells and basilar membrane vibration is in the range of 0.7-1.1. These observations support, in part, the classical geometric models at low frequencies. However, there appears to be less tectorial membrane motion than predicted, and it is largely in the transversal direction.

Animals↗

PubMed2003

Olivocochlear efferent innervation of the organ of corti in hypothyroid rats.

Congenital hypothyroidism induces developmental abnormalities in the auditory receptor, causing deafness due to a poor development of the outer hair cells (OHCs) and a lack of synaptogenesis between these cells and the olivocochlear axons. This efferent innervation is formed by two separate systems: the lateral system, which originates in the lateral superior olive (LSO) and reaches the inner hair cells; and the medial system, which originates in the ventral nucleus of the trapezoid body (VNTB) and innervates the OHCs. A previous study carried out in our laboratory showed that in congenitally hypothyroid animals, the neurons which give rise to the efferent system are normal in number and distribution, although smaller in size. The aim of the present work was to study the efferent fibers in the auditory receptor of hypothyroid animals, by means of stereotaxic injections of biotinylated dextran amine in the nuclei that give rise to the olivocochlear system: LSO and VNTB. In hypothyroid animals, injections in LSO gave rise to lateral olivocochlear fibers lacking their characteristic dense terminal arbors, while injections in the VNTB-labeled fibers terminating in the spiral bundle region, far from the OHCs with which they normally contact. In the latter case, only a small percentage of labeled fibers reached the OHCs area, giving off only two radial branches maximum. Because the number of neurons which develop into the efferent innervation was normal in hypothyroid animals, we conclude that medial fibers may contact a new target.

Animals↗

PubMed1990

NCAM in the organ of Corti of the developing mouse.

During development in the cochlea, NCAM is present on all neuronal systems--radial fibres, inner spiral bundle, inner pillar bundle (described by Sobkowicz & Emmerling, 1989), and outer spiral fibres. In cochleae from E17 mice, NCAM-positive puncta, possibly associated with growth cones, dot the area of the inner spiral bundle. The region overlying the basilar membrane is also NCAM positive. Until about P7, NCAM immunoreactivity surrounds the inner and outer hair cells. During the second postnatal week, NCAM no longer covers the hair cell surfaces. At this time, NCAM is present on the inner spiral and inner pillar bundles. In the outer hair cell region, immunoreactivity concentrates in neuronal endings with 'Y'-like or 'dot'-like appearances typical of cholinergic efferents. At a time when neuronal fibres are regenerating in the cochlea of the Bronx waltzer mutant mouse, many NCAM-positive puncta dot the regions of the inner spiral bundle and inner spiral sulcus, suggesting that these puncta correspond to growth cones. In organotypic culture, hair cells and nerve fibres retain their immunoreactivity up to 7 days. In conclusion, NCAM is present on the surfaces of neurons and on their cochlear targets at the time when nerve-target recognition is proceeding. NCAM disappears from targets during the time of maturation of efferent synapses. NCAM is also present along the growth path of the nerve fibres. These studies suggest that NCAM is properly positioned in the cochlea for possible roles in axon guidance and nerve-target interactions.

Animals↗

PubMed1994

Pilocarpine-induced changes in the saccharide composition of the tectorial membrane and interdental cells of the organ of Corti: a study with gold-labeled lectins.

The glycoconjugates in the cytoplasm of inner ear interdental cells and those constituting the limbal tectorial membrane were identified by a post-embedding cytochemical method using low-temperature embedding in Lowicryl K4M and labeling with biotinylated lectins, goat anti-biotin antibody, rabbit anti-goat antibody, and gold-labeled protein A in control animals, and after the systemic injection of pilocarpine. The lectins used were ConA, PHA-E, PSA, RCA, SBA, Succ-WGA, UEA, and WGA. In control animals, a semiquantitive analysis of gold particles showed that Succ-WGA produced the strongest labeling on the tectorial membrane, followed by SBA, ConA, WGA, RCA, PHA-E, and PSA. The lowest values were obtained with UEA. The cytoplasm of the interdental cells was also labeled with all the lectins, but the number of particles/microns2 was lower than on the tectorial membrane. The concentration of gold particles on the limbal tectorial membrane in pilocarpine-treated animals was higher than in control animals for some lectins (RCA, PSA, UEA) but lower for others (WGA, SBA, PHA-E, Succ-WGA). The changes in the labeling pattern of the cytoplasm of the interdental cells paralleled those in the tectorial membrane. These results demonstrate that the saccharide composition of the limbal tectorial membrane can be modified by systemic injection of pilocarpine. This action may take place through a change in either the secretion rate or the amount of some glycoconjugates by the interdental cells.

Acrylic Resins↗

PubMed2006

Inhibitors of differentiation and DNA binding (Ids) regulate Math1 and hair cell formation during the development of the organ of Corti.

The basic helix-loop-helix (bHLH) transcription factor Math1 (also called Atoh1) is both necessary and sufficient for hair cell development in the mammalian cochlea (Bermingham et al., 1999; Zheng and Gao, 2000). Previous studies have demonstrated that a dynamic pattern of Math1 expression plays a key role in regulating the number and position of mechanosensory hair cells. However, the factors that regulate the temporal and spatial expression of Math1 within the cochlea are unknown. The bHLH-related inhibitors of differentiation and DNA binding (Id) proteins are known to negatively regulate many bHLH transcription factors, including Math1, in a number of different systems. Therefore, Id proteins are good candidates for regulating Math1 in the cochlea. Results from PCR and in situ hybridization indicate that Id1, Id2, and Id3 are expressed within the cochlear duct in a pattern that is consistent with a role in regulation of hair cell development. In particular, expression of Ids and Math1 overlapped in cochlear progenitor cells before cellular differentiation, but a specific downregulation of Id expression was observed in individual cells that differentiated as hair cells. In addition, progenitor cells in which the expression of Ids was maintained during the time period for hair cell differentiation were inhibited from developing as hair cells. These results indicate a key role for Ids in the regulation of expression of Math1 and hair cell differentiation in the developing cochlea.

Animals↗