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PubMed2005

Age effects and size effects in the ears of gekkonomorph lizards: inner ear.

Audiograms have indicated greater auditory sensitivity in larger than in smaller geckos; part of this difference, interspecifically and intraspecifically, is explained by middle-ear proportions. To investigate the contribution of the inner ear to the variation in sensitivity, we examined it in museum specimens representing 11 species and three subfamilies. We measured papilla basilaris length, and, when intact, the saccular otoconial mass. Papilla length approximated 1% of rostrum-anus length in large geckos but 2% in small geckos; in some species some inter-aural difference was indicated. Over the lumped material, relative papilla length varied as a function of body length, with highly significant correlation. Similar relations prevailed within each subfamily. However, intraspecifically the correlation of papilla basilaris length with animal size was usually nonsignificant. Hair cell populations assessed from SEM photographs were larger in the larger species but intraspecifically did not relate to an individual's size. Hence interspecifically, the dependence of auditory sensitivity on animal size seems supported by inner-ear differences but intraspecifically this relation derives only from the middle ear. Otoconial mass, as measured by its volume, was correlated with animal length both interspecifically and intraspecifically.

Age Factors↗
PubMed2005

Morphology of the endolymphatic sac in the guinea pig after an acute endolymphatic hydrops.

The role of the endolymphatic sac (ES) in endolymph volume homeostasis is speculative. The present study investigates changes of the ES's epithelia and luminal filling after induction of an acute endolymphatic hydrops. After microinjection of 1.1 mul artificial endolymph into scala media of the cochlea, guinea pigs were terminated immediately (n = 6) or after different time intervals ; 1/2 h (n = 3), 1 h (n = 4) and 2 h (n = 4). Inner ear specimens were processed for light and/or transmission electron microscopy. The non-injected contralateral ear served as a histological control. Correct injection was confirmed by detection of microspheres in the endolymphatic compartment after the same microinjection procedure. In all specimens, ribosome rich cells and intraluminal macrophages appeared to be actively involved in degradation of homogeneous substance (HS) by secreting lytic enzymes and digestion, respectively. Amazingly, in our study no ES differences were found between injected and non-injected ears and no distinct changes were observed in guinea pigs terminated after different time intervals. The ES's luminal HS was always present and often to a large extent. This is in contrast with [Hear. Res. 138, 81] dramatic changes were observed. Endolymph volume homeostasis is a complex mechanism, in which the role of HS remains obscure.

Acute Disease↗
PubMed2006

Expression of CLC-K chloride channels in the rat cochlea.

Current models of the lateral K+ recycling pathway in the mammalian cochlea include two multicellular transport networks separated from one another by three interstitial gaps. The first gap is between outer hair cells and Deiters cells, the second is between outer sulcus cells and type II spiral ligament fibrocytes and the third is between intermediate and marginal cells in the stria vascularis. K+ taken up by cells bordering these interstitial spaces is accompanied by Cl-. Maintaining appropriate electrolyte balance and membrane potentials in these cells requires a mechanism for exit of the resorbed Cl-. One possible candidate for regulating this Cl- efflux is ClC-K, a chloride channel previously thought to be kidney specific. Here, we demonstrate the expression of both known isoforms of ClC-K in the organ of Corti, spiral ligament and stria vascularis of the rat cochlea by immunohistochemical, Western blot and RT-PCR analysis. These results indicate a role for ClC-K in mediating Cl- recycling in the cochlea. The widespread expression of both ClC-K isoforms in the cochlea may help to explain the symptoms of Bartter's syndrome Type III, a mutation in the hClC-Kb gene (human homologue of ClC-K2), which results in renal salt wasting without deafness. These data support the hypothesis that both isoforms of ClC-K are co-expressed in some cell membranes and account for the preservation of hearing in the presence of a mutation in only one channel isoform.

Animals↗
PubMed1995

Normal variations of the temporal bone on high-resolution CT: their incidence and clinical significance.

We evaluated normal variations of the temporal bone on high-resolution computed tomograms (HR-CTs) and investigated their incidence. HR-CTs of the temporal bones of 325 patients were retrospectively reviewed. Six groups of variants, which were considered important for presurgical planning, were evaluated on HR-CTs. These included: (1) an incomplete bony covering of a high-positioned jugular bulb; (2) severe asymmetry of the jugular foramen; (3) an anteriorly located sigmoid sinus; (4) a deep sinus tympani; (5) a large internal auditory canal; and (6) a large cochlear aqueduct. The frequency of the variations were as follows: (1) 2.4%; (2) 4.0%; (3) 1.6%; (4) 5.9%; (5) 2.3%; and (6) 3.0%. Bilateral involvement with variation (4), (5) and (6) was frequently seen. Normal anatomical variations of the temporal bone are therefore not rare and awareness of the possible variants is necessary before surgery of the inner ear, middle ear and posterior fossa.

Adolescent↗
PubMed2000

Effects of endolymphatic-perilymphatic fistula on endolymphatic hydrops in guinea pig.

The endolymphatic-perilymphatic shunt operation between the scala media and scala tympani was performed in 22 guinea pigs with endolymphatic hydrops induced by the silver nitrate injection method 2 months before the operation. Two (n = 10), 7 (n = 6), and 28 (n = 6) days after the operation, the condition of the fistula and the inner ear pathology were studied histopathologically. In half of the animals, the fistula was open, whereas in the other half, the fistula was closed. In the group examined after 2 postoperative days, the animals with open fistulas showed a slight collapse of Reissner's membrane, which might indicate that excess endolymph escaped through the fistula into the scala tympani. In the groups examined later, especially in the group examined after 28 postoperative days, the animals showed distention of hydrops despite preservation of the fistula. Moderate-to-severe degenerative changes of the organ of Corti, macula sacculi, and stria vascularis were observed in the vicinity of the fistula. We conclude from this study that the endolymphatic-perilymphatic fistula created inside the cochlea apparently was not sufficient to reduce the hydrops. Rather, it caused degenerative changes of the inner ear structure. This type of surgery is not suitable for Meniere's disease.

Animals↗
PubMed1996

Receptors for glucocorticoids in the human inner ear.

Glucocorticoid receptors were detected in the human inner ear. The highest concentration of glucocorticoid receptor protein was measured by enzyme-linked immunosorbent assay in the spiral ligament tissues; the lowest concentration of glucocorticoid receptors was measured in the macula of the saccule. The demonstration of the presence of glucocorticoid receptors in human Inner ear tissues provides a basis to consider the direct effects of glucocorticoid action on select inner ear cells, rather than assuming a systemic antiinflammatory or immunosuppressive effect during the therapeutic treatment of patients with given inner ear disorders.

Acoustic Maculae↗
PubMed1980

Endocochlear potential and scala media oxygen tension during partial anoxia.

The endocochlear potential and the scala media oxygen tension were measured from the basal coil of the guinea pig cochlea during levels of partial respiratory anoxia. This was accomplished by simultaneously positioning two electrodes (for endocochlear potential and PO2) into the endolymph by a transbasilar membrane approach, in order to minimize blood capillary damage. The intracochlear pO2 was stable until the respiratory oxygen concentration was reduced to 16 per cent on the average. Thereafter, a further reduction of oxygen in the respiratory mix reduced both the endolymph pO2 and the endocochlear potential. The coupling between the endocochlear potential and endolymph pO2 is proportional over a 70 to 80 per cent reduction in endolymph pO2, and at this point the endocochlear potential is reduced by 50 per cent. For partial levels of respiratory oxygen that are not too low (e.g., > 12 per cent oxygen) it is possible to maintain partial levels of intracochlear pO2 for at least six minutes. During this time period the endocochlear potential is also held at a stable but reduced level.

Animals↗
PubMedDate not supplied

Possible functional roles of Na+,K+-ATPase in the inner ear and their relevance to Ménière's disease.

This article reviews the functions of the enzyme Na+,K+-ATPase in epithelial tissues and discusses early and recent biochemical, physiologic and morphologic studies of the enzyme in the inner ear. The purpose of the investigation was to learn whether a relationship between perturbations in activity of the enzyme and Ménière's disease is possible. It is concluded that the preponderance of the evidence indicates that Na+,K+-ATPase plays a role in regulating ion transport into the scala media, but that the significance of the distribution of the enzyme along only one cell type (the marginal) in the functional chains of cells of the outer cochlear wall needs further study. The possible vasoconstrictive effects of ouabain perfusions employed by some investigators must also be taken into account. Recent cytochemical and autoradiographic studies have demonstrated high levels of Na+, K+-ATPase on cochlear nerve fibers, especially near the foramina nervosa and within the organ of Corti. Thus, perturbations in Na+,K+-ATPase activity in the inner ear not only could affect certain aspects of fluid balance, but also could account for the sensory disturbances experienced by patients who have Ménière's disease.

Animals↗
PubMedDate not supplied

The vascular anatomy of the gerbil cochlea.

The vascular pattern of the cochlea was studied in 12 adult gerbils (Meriones unguiculatus) using the Berlin blue (Prussian blue) contrast injection technique. The capillary areas of the gerbil cochlea are similar to those of other mammals studied. As in some other mammals, the vessel of the basilar membrane was inconsistently identified in the basal turn. It was usually replaced by a larger uninjected channel, supporting the suggestion that the vessel of the basilar membrane may be more functionally important in fetal life than in adulthood. The vascular pattern of the external cochlear wall is well maintained from base to apex, although a simplification of vasculature can be observed apically.

Animals↗
PubMed2003

Endolymphatic sodium homeostasis by Reissner's membrane.

Cochlear sensory transduction depends on active extrusion of sodium ion (Na(+)) from the luminal fluid, endolymph. Reissner's membrane epithelium forms much of the barrier between cochlear endolymph and perilymph and we hypothesized that Reissner's membrane might be responsible for this function. We found that Reissner's membrane isolated from gerbil produced a short circuit current (I(sc)) directed into the apical side, consistent with cation absorption and/or anion secretion. I(sc) was inhibited by amiloride analogs in the potency sequence benzamil>amiloride>>ethylisopropylamiloride, consistent with Na(+) absorption through an epithelial sodium channel in the apical cell membrane. I(sc) was also inhibited by an inhibitor of Na(+),K(+)-ATPase, ouabain, and by the K(+) channel blockers Ba(2+), 4-aminopyridine and quinine but not tetraethylammonium nor glibenclamide, consistent with the presence of a voltage-activated K(+) channel. Bumetanide, an inhibitor of the Na(+),2Cl(-),K(+)-cotransporter, had no effect on I(sc). Contrary to previous hypotheses, no evidence was found for electrogenic secretion of Cl(-) under control of cAMP since neither forskolin nor genistein affected I(sc) when Na(+) absorption was blocked. These results provide the first direct evidence that Reissner's membrane contributes to normal cochlear function by absorption of Na(+) from endolymph.

Amiloride↗
PubMed2000

Structural evidence for ion transport and tectorial membrane maintenance in the gerbil limbus.

Cells medial to the tunnel of Corti were examined to assess fine structural features relevant to their proposed role in cochlear K(+) homeostasis. A dense network of canaliculi referred to as canalicular reticulum (CR) resided in the foot body of inner pillar cells, where it bordered and could resorb ions released from inner radial and spiral nerves. Lateral interdental cells (IDCs) formed columns which connected the inner sulcus epithelium with the base of the tectorial membrane's (TM) middle zone. A spout-like neck in cells at the top of lateral IDC columns housed a dense concentration of CR which resembled that characteristic of ion transporting epithelia and appeared to be located here for transporting ions and fluid toward the TM. Clustered IDCs in the center of the limbus connected underlying limbal stroma with the TM's limbal zone and appeared capable of transporting ions from stroma to TM. Abundant CR in limbal stellate fibrocytes evidenced their capacity to transport ions and fluid, presumably from inner sulcus epithelium toward central IDCs. The most medial IDCs possibly function as the terminus of an ion cycling path from scala vestibuli to endolymph. Light fibrocytes situated between supralimbal fibrocytes and medial IDCs appeared to serve as a link in this pathway. The limbal zone of the TM overlying central IDCs consisted of three distinct regions which offered a structural basis for transformation of an amorphous matrix supplied by central IDCs into the protofibrils of the membrane's middle zone.

Animals↗
PubMed2000

Effects of AC and DC stimulation on chinchilla SOAE amplitude and frequency.

The effects of AC and DC current on spontaneous otoacoustic emissions (SOAEs) were studied in normal chinchillas and chinchillas with selective inner hair cell (IHC) loss. Electrical stimulation was delivered through an electrode on the round window or through an electrode in scala media. SOAE frequencies ranged from 4 to 11 kHz and amplitudes ranged from 13 to 51 dB SPL. AC simulation suppressed SOAE amplitude. The suppression contours had a narrowly tuned, low-threshold tip located above the frequency of the SOAE. AC suppression contours were similar to acoustic suppression contours except that the AC suppression contours lacked a high-threshold, low frequency tail. The lowest threshold of the AC suppression contour was 3.9 microA rms whereas the lowest acoustic suppression threshold was 19 dB SPL. AC stimulation, which induced an electrically evoked otoacoustic emission, interacted with the SOAE to generate distortion product otoacoustic emissions (DPOAEs) of up to 26 dB SPL at 2f(S)-f(AC) (f(S)=SOAE). DPOAE amplitude increased with AC current, but saturated at high levels. DC current steps affected both SOAE frequency and amplitude. Positive current at the round window decreased SOAE amplitude and frequency whereas negative current increased SOAE frequency, but had little effect on amplitude. The effects of AC and DC current on SOAEs in animals with IHC loss were similar to those in normal chinchillas.

Animals↗
PubMed2002

Spiral ligament pathology in quiet-aged gerbils.

The ultrastructure of the spiral ligament was compared in aged and young gerbils to assess the involvement of connective tissues in the lateral wall and particularly the fibrocytes in development of presbyacusis. Pathologic features in fibrocytes of senescent gerbils spanned a wide range reflecting different stages of lateral wall involution. All of the type II, IV and V fibrocytes selectively developed cytosolic vacuoles in an early degenerative phase showing minimal strial involvement. Clear spaces indicative of interstitial edema separated the vacuolated cell bodies and their plasmalemmal processes. As a presumed intermediate phase, profiles of amorphous substance apparently derived from apoptosis/necrosis of type II fibrocytes infiltrated the type II fibrocyte area among nearly normal appearing cells. In cochlear turns with advanced strial degeneration, type II fibrocytes disappeared from the spiral prominence area leaving only type I-like fibrocytes occasionally accompanied by a collagen infiltrate. Type V fibrocytes disappeared similarly from the suprastrial area. The extent of atrophy in type II fibrocytes corresponded in general with that in the neighboring stria vascularis. Age-dependent atrophy in the lateral wall largely spared type I fibrocytes except that they often enclosed discrete amorphous foci lacking organelles. The involution thus affected principally the Na,K-ATPase-positive fibrocytes functioning in active uptake rather than passive conductance of K(+). The vacuolization and degeneration exclusive to ATPase-rich fibrocytes and the associated intercellular edema are interpreted as secondary responses, possibly as a result of impaired diffusion of K(+) through downstream marginal cells.

Aging↗
PubMed2003

Migration of cochlear lateral wall cells.

The role of apoptosis and proliferation in maintenance of cochlear lateral wall cells was examined. The methods employed for detection of apoptosis were the Hoechst fluorescence stain and TUNEL (TdT-mediated dUTP-biotin nick-end-labeling) assay, and proliferations were 5-bromo-2'-deoxyuridine (BrdU) incorporation and presence of the proliferating cell nuclear antigen. The incidence of apoptosis in the strial marginal cell was 50% greater (32.9+/-3.7%) than strial intermediate and basal cells but similar to spiral ligament cells. Although division of marginal strial cells was rarely detected, a significant number of proliferating cells in the remaining stria vascularis and spiral ligament were observed. These data implied that replacement of marginal cells arose elsewhere and could be followed by a BrdU-deoxythymidine pulse-chase study. At 2 h post injection, nuclear BrdU in marginal cells was not detected; however, by 24 h post injection, 20-25% of marginal cell nuclei were BrdU-positive. These observations are consistent with the hypothesis that marginal cells were replaced by underlying cells. Cell migration appears to be an important mechanism for preserving the function and structure of the stria vascularis.

Animals↗
PubMed2003

Time course of inner ear degeneration and deafness in mice lacking the Kir4.1 potassium channel subunit.

The Kir4.1 gene (KCNJ10) encodes an inwardly rectifying K(+) channel subunit abundantly expressed in the CNS. Its expression in the mammalian inner ear has been suggested but its function in vivo in the inner ear is unknown. Because diverse human hereditary deafness syndromes are associated with mutations in K(+) channels, we examined auditory function and inner ear structure in mice with a genetically inactivated Kir4.1 K(+) channel subunit. Startle response experiments suggest that Kir4.1-/- mice are profoundly deaf, whereas Kir4.1+/- mice react like wild-type mice to acoustic stimuli. In Kir4.1-/- mice, the Reissner membrane is collapsed, the tectorial membrane is swollen, and type I hair cells and spiral ganglion neurons as well as their central processes degenerate over the first postnatal weeks. In the vestibular ganglia, neuronal cell death with apoptotic features is also observed. Immunostaining reveals that Kir4.1 is strongly expressed in stria vascularis of wild-type but not Kir4.1-/- mice. Within the spiral ganglion, Kir4.1 labeling was detected on satellite cells surrounding spiral ganglion neurons and axons. We conclude that Kir4.1 is crucial for normal development of the cochlea and hearing, via two distinct aspects of extracellular K(+) homeostasis: (1). in stria vascularis, Kir4.1 helps to generate the cochlear endolymph; and (2). in spiral and vestibular ganglia, Kir4.1 in surrounding glial cells helps to support the spiral and vestibular ganglion neurons and their projecting axons.

Animals↗
PubMed1997

Stretch-activated nonselective cation, Cl- and K+ channels in apical membrane of epithelial cells of Reissner's membrane.

Ion channels on the apical membrane of epithelial cells (the surface facing the endolymph) of acutely isolated Reissner's membrane from guinea-pig cochlea were investigated by using patch-clamp technique in cell-attached and inside-out configurations. Three types of ion channel were identified: namely, a stretch-activated nonselective cation, a chloride and a potassium channel. When the pipette was filled with high-K+ endolymph-like solution, the most significant channel activity was nonselective cation channels (85/110, 77% patches). The current versus voltage relationship was linear with a unitary conductance of 22.1 +/- 0.4 pS and reversal potential (Vr) of 2.3 +/- 0.8 mV (n = 18). The channel exhibited a lower conductance (14.0 +/- 0.6 pS, n = 8) to Ca2+. The open probability was low (NPo approximately 0.1) in cell-attached configuration under +60 mV pipette potential and increased when the membrane was stretched with negative pressure. The channel was blocked by 10 microM extracellular Gd3+. The two other types of channels were a small voltage-sensitive Cl- channel (6.0 +/- 0.3 pS; 91/99, 92% patches) and a K+ channel (approximately 30 pS; 29/191, 15% patches). These channels might play roles in the regulation of cell volume, in balancing the hydrostatic pressure across Reissner's membrane and in maintaining the electrochemical composition of endolymph.

Animals↗
PubMed1997

Fixation-induced shrinkage of Reissner's membrane and its potential influence on the assessment of endolymph volume.

The quantification of endolymph volume by histological techniques or by magnetic resonance (MR) microscopy requires the inner ear to be first treated with chemical fixatives. If the fixative induces soft-tissue shrinkage, it would tend to return a distended Reissner's membrane towards a straight position, since this membrane is anchored to bone at its medial and lateral edges. The goal of this study was to determine the degree of Reissner's membrane shrinkage induced by different fixation protocols to establish methods which minimize tissue shrinkage. Fragments of fresh Reissner's membrane were dissected from isolated cochleae in an artificial perilymph. Specimens were viewed with an inverted microscope during infusion of fixatives, and changes recorded on video tape. Size changes of the specimen were quantified, usually over a 20 min period. Heidenhain-Susa, a fixative which is widely used in histological studies of hydropic cochleae, caused substantial shrinkage of Reissner's membrane, decreasing the length of specimens by an average of 15.1%. Other fixation procedures induced far less shrinkage. The use of 3.1% glutaraldehyde in Hanks' balanced salt solution produced a mean length decrease of only 0.3%. The inclusion in the fixation medium of 4.5% mercuric chloride, corresponding to the concentration which is present in Heidenhain-Susa and which acts to increase the contrast of Reissner's membrane in MR microscopy, contributes significantly to specimen shrinkage. We can conclude that the degree of endolymphatic hydrops may be underestimated in specimens fixed with media containing high levels of mercuric chloride.

Animals↗