[Ultrastructural study of ampulla of mouse inner ear]

Objective: To observe the ultrastructure of the ampulla, and analyze its physiological and pathological significance. Methods: In this study, 20 Kunming mice were used, and scanning electron microscopy was used to observe the ultrastructure of the ampulla of inner ear. Results: Otoconia was found among the cilia bundles of different haircell(intercilla otoconia of ampulla). The cupula was attached to the lateral wall of the ampulla, and easily to be separated; after separated, a kind of slender crystal(surface otoconia of ampulla) could be seen between the cupula and lateral wall of the ampulla, both sides of ampullary crest were covered with slender crystals too. On the canal side of the ampulla wall, there was more particulate matter attached to the wall near the bottom of ampullary crest, partially embedded in the wall, and less on the utricle side of the ampulla wall. Conclusions: The observation of the ultrastructure of the ampulla is helpful for better understanding the physiological functions of the semicircular canals and the ampulla, and better understanding the pathogenesis and solution of some vertigo diseases.

Plastin 1 widens stereocilia by transforming actin filament packing from hexagonal to liquid

With their essential role in inner ear function, stereocilia of sensory hair cells demonstrate the importance of cellular actin protrusions. Actin packing in stereocilia is mediated by cross-linkers of the plastin, fascin, and espin families. Although mice lacking espin (ESPN) have no vestibular or auditory function, we found that mice that either lacked plastin 1 (PLS1) or had nonfunctional fascin 2 (FSCN2) had reduced inner ear function, with double-mutant mice most strongly affected. Targeted mass spectrometry indicated that PLS1 was the most abundant cross-linker in vestibular stereocilia and the second most abundant protein overall; ESPN only accounted for ∼15% of the total cross-linkers in bundles. Mouse utricle stereocilia lacking PLS1 were shorter and thinner than wild-type stereocilia. Surprisingly, although wild-type stereocilia had random liquid packing of their actin filaments, stereocilia lacking PLS1 had orderly hexagonal packing. Although all three cross-linkers are required for stereocilia structure and function, PLS1 biases actin toward liquid packing, which allows stereocilia to grow to a greater diameter.

Closure of supporting cell scar formations requires dynamic actin mechanisms

In many vertebrate inner ear sensory epithelia, dying sensory hair cells are extruded, and the apices of surrounding supporting cells converge to re-seal the epithelial barrier between the electrochemically-distinct endolymph and perilymph. These cellular mechanisms remain poorly understood. Dynamic microtubular mechanisms have been proposed for hair cell extrusion; while contractile actomyosin-based mechanisms are required for cellular extrusion and closure in epithelial monolayers. The hypothesis that cytoskeletal mechanisms are required for hair cell extrusion and supporting cell scar formation was tested using bullfrog saccules incubated with gentamicin (6h), and allowed to recover (18h). Explants were then fixed, labeled for actin and cytokeratins, and viewed with confocal microscopy. To block dynamic cytoskeletal processes, disruption agents for microtubules (colchicine, paclitaxel) myosin (Y-27632, ML-9) or actin (cytochalasin D, latrunculin A) were added during treatment and recovery. Microtubule disruption agents had no effect on hair cell extrusion or supporting cell scar formation. Myosin disruption agents appeared to slow down scar formation but not hair cell extrusion. Actin disruption agents blocked scar formation, and largely prevented hair cell extrusion. These data suggest that actin-based cytoskeletal processes are required for hair cell extrusion and supporting cell scar formation in bullfrog saccules.

Control of hair cell excitability by vestibular primary sensory neurons

In the rat utricle, synaptic contacts between hair cells and the nerve fibers arising from the vestibular primary neurons form during the first week after birth. During that period, the sodium-based excitability that characterizes neonate utricle sensory cells is switched off. To investigate whether the establishment of synaptic contacts was responsible for the modulation of the hair cell excitability, we used an organotypic culture of rat utricle in which the setting of synapses was prevented. Under this condition, the voltage-gated sodium current and the underlying action potentials persisted in a large proportion of nonafferented hair cells. We then studied whether impairment of nerve terminals in the utricle of adult rats may also affect hair cell excitability. We induced selective and transient damages of afferent terminals using glutamate excitotoxicity in vivo. The efficiency of the excitotoxic injury was attested by selective swellings of the terminals and underlying altered vestibular behavior. Under this condition, the sodium-based excitability transiently recovered in hair cells. These results indicate that the modulation of hair cell excitability depends on the state of the afferent terminals. In adult utricle hair cells, this property may be essential to set the conditions required for restoration of the sensory network after damage. This is achieved via re-expression of a biological process that occurs during synaptogenesis.

Effect of Intratympanic Application of Aminoglycosides on Click-Evoked Myogenic Potentials in Guinea Pigs

OBJECTIVES

Although numerous studies have identified damage to the cochlear and vestibular end organs as the primary site of aminoglycoside-induced ototoxicity, the effect on the saccule remains poorly understood, possibly due to lack of monitoring saccular function in experimental animals. Therefore, this study applied three kinds of aminoglycosides into the tympanic space of guinea pigs to examine their toxic impact on the saccule by way of click-evoked myogenic potential test coupled with morphologic assessment.

DESIGN

Albino guinea pigs were treated with saline, gentamicin, tobramycin, or amikacin, with 10 animals assigned to each group. Each compound was injected directly overlying but not through the round window membrane on the left ear, with the right ear serving as a control. One week after injection, each animal underwent auditory brain stem response, caloric test, and click-evoked myogenic potential test. Animals were then killed for morphologic assessment through the use of light and electron microscopic examinations.

RESULTS

The animals treated with saline, gentamicin, tobramycin, or amikacin exhibited abnormal auditory brain stem response in 0%, 30%, 100%, and 30% of cases; abnormal caloric responses were found in 0%, 100%, 40%, and 40% of cases; absent click-evoked myogenic potentials were found in 0%, 100%, 30%, and 40% of cases, respectively. Gentamicin and other groups differed significantly in abnormal rates of caloric responses and click-evoked myogenic potentials. Morphologic study of the gentamicin-treated animals confirmed that the absence of click-evoked myogenic potential originated from the lesion in the saccular macula.

CONCLUSIONS

Gentamicin represents the dominant susceptibility of aminoglycoside-induced vestibulotoxicity for eliminating both semicircular canal and saccular functions. This study further confirms the findings of human studies in which the caloric and vestibular evoked myogenic potentials responses were monitored to assess the abolition of vestibular function in patients treated with intratympanic gentamicin injection.

Layer thickness and curvature effects on otoconial membrane deformation in the utricle of the red-ear slider turtle: static and modal analysis

Finite element models of otoconial membrane (OM) were developed to investigate the effects of three geometric variables on static and modal response of the OM: (1) curvature of the macular surface, (2) spatial variation in thicknesses of three OM layers, and (3) shape of the macular perimeter. A geometrically accurate model of a turtle utricle was constructed from confocal images. Modifying values for each variable formed variants of this model: modeling the macula surface as flat, OM layer thicknesses as spatially invariant, and the macular perimeter as a rectangle. Static tests were performed on each modified OM model, and the results were compared to determine the effects of each geometric variable on static mechanical gain (deflection per unit acceleration). Results indicate that all three geometric variables affect the magnitude and directional properties of OM static mechanical gain. In addition, through modal analysis, we determined the natural frequencies and displacement modes of each model, which illustrate the effects of the three geometric variables on OM dynamics. This study indicates the importance of considering three-dimensional OM geometry when attempting to understand responses of the OM and, therefore, the modulation of hair cell signals to accelerations during head movements.

Afferent synapses are present in utricular hair cells from otoconia-deficient mice

The head tilt mouse (het/het, abbr. het) is a naturally occurring mutant whose salient phenotypic traits include the complete absence of otoconia in both the utricle and saccule. Cursory histologic evaluation has indicated that the neuroepithelia exhibit a normal appearance. Though evidence exists indicating that utricular function is severely if not completely compromised in these animals, it is not yet known whether afferent synapses exist within utricular hair cells of otoconia-deficient mutants. The absence of synapses would be suggestive of a trophic relationship between stimulus-evoked hair cell activation and the afferent synapse. To address this question, we have conducted an ultrastructural survey of utricular sensory epithelia from confirmed het mice. The specific objective was to determine whether utricular hair cells made synaptic contact with afferent neurons. We found that both type I and II hair cells from utricles of het mice exhibited afferent synapses that were found at numerous sites distributed throughout the utricle. These results indicate that afferent synapses within vestibular hair cells do not critically depend upon stimulus-evoked activity.

Regeneration of vestibular otolith afferents after ototoxic damage

Regeneration of receptor cells and subsequent functional recovery after damage in the auditory and vestibular systems of many vertebrates is well known. Spontaneous regeneration of mammalian hair cells does not occur. However, recent approaches provide hope for similar restoration of hearing and balance in humans after loss. Newly regenerated hair cells receive afferent terminal contacts, yet nothing is known about how reinnervation progresses or whether regenerated afferents finally develop normal termination fields. We hypothesized that neural regeneration in the vestibular otolith system would recapitulate the topographic phenotype of afferent innervation so characteristic of normal development. We used an ototoxic agent to produce complete vestibular receptor cell loss and epithelial denervation, and then quantitatively examined afferent regeneration at discrete periods up to 1 year in otolith maculas. Here, we report that bouton, dimorph, and calyx afferents all regenerate slowly at different time epochs, through a progressive temporal sequence. Furthermore, our data suggest that both the hair cells and their innervating afferents transdifferentiate from an early form into more advanced forms during regeneration. Finally, we show that regeneration remarkably recapitulates the topographic organization of afferent macular innervation, comparable with that developed through normative morphogenesis. However, we also show that regenerated terminal morphologies were significantly less complex than normal fibers. Whether these structural fiber changes lead to alterations in afferent responsiveness is unknown. If true, adaptive plasticity in the central neural processing of motion information would be necessitated, because it is known that many vestibular-related behaviors fully recover during regeneration.

The inner ear ultrastructure from the paddlefish (Polyodon spathula) using transmission electron microscopy

The structure of the hair cells on each sensory macula from the inner ear of the paddlefish (Polyodon spathula) was studied using scanning and transmission electron microscopy, revealing the nucleated cell bodies and peripheral nerve fibres of the saccule utricle and lagena. Examination of the structures within the cell body revealed comparable features with those found in the inner ear hair cells from bony fish species, although in P. spathula the afferent cell body is almost twice the size. This is the first time that the inner ear hair cells from an Acipenseriform fish have been studied using transmission microscopy, thus providing benchmark anatomical information in relation to the cellular morphology of the afferent receptors from a 'healthy' P. spathula ear. Structural information is of assistance in the study of aquatic animal hearing for environmental monitoring purposes, as morphological data can be used to confirm if evidence of raised hearing thresholds from animals exposed to intense anthropogenic noise or other destructive agents (determined using electrophysiological or behavioural techniques) are a direct result of damage to the ultrastructure of the inner ear.

Otopetrin 1 is required for otolith formation in the zebrafish Danio rerio

Orientation with respect to gravity is essential for the survival of complex organisms. The gravity receptor is one of the phylogenetically oldest sensory systems, and special adaptations that enhance sensitivity to gravity are highly conserved. The fish inner ear contains three large extracellular biomineral particles, otoliths, which have evolved to transduce the force of gravity into neuronal signals. Mammalian ears contain thousands of small particles called otoconia that serve a similar function. Loss or displacement of these structures can be lethal for fish and is responsible for benign paroxysmal positional vertigo (BPPV) in humans. The distinct morphologies of otoconial particles and otoliths suggest divergent developmental mechanisms. Mutations in a novel gene Otopetrin 1 (Otop1), encoding multi-transmembrane domain protein, result in nonsyndromic otoconial agenesis and a severe balance disorder in mice. Here we show that the zebrafish, Danio rerio, contains a highly conserved gene, otop1, that is essential for otolith formation. Morpholino-mediated knockdown of zebrafish Otop1 leads to otolith agenesis without affecting the sensory epithelium or other structures within the inner ear. Despite lack of otoliths in early development, otolith formation partially recovers in some fish after 2 days. However, the otoliths are malformed, misplaced, lack an organic matrix, and often consist of inorganic calcite crystals. These studies demonstrate that Otop1 has an essential and conserved role in the timing of formation and the size and shape of the developing otolith.

The structure of tip links and kinocilial links in avian sensory hair bundles

Recent studies have indicated that the tip links and kinocilial links of sensory hair bundles in the inner ear have similar properties and share a common epitope, and that cadherin 23 may also be a component of each link type. Transmission electron microscopy was therefore used to study and compare the fine structure of the tip links and kinocilial links in avian sensory hair bundles. Tannic acid treatment revealed a thin strand, 150-200 nm long and 8-11 nm thick, present in both link types. Fourier analysis of link images showed that the strand of both link types is formed from two filaments coiled in a helix-like arrangement with an axial period of 20-25 nm, with each filament composed of globular structures that are approximately 4 nm in diameter. Differences in the radius and period of the helix-like structure may underlie the observed variation in the length of tip and kinocilial links. The similar helix-like structure of the tip links and kinocilial links is in accord with the presence of a common cell-surface antigen (TLA antigen) and similarities in the physical and chemical properties of the two link types. The spacing of the globular structures comprising each filament of the two link types is similar to the 4.3 nm center-to-center spacing reported for the globular cadherin repeat, and is consistent with the suggestion that cadherin 23 is the tip link.

Computational models of hair cell bundle mechanics: III. 3-D utricular bundles

Six utricular hair bundles from a red-eared turtle are modeled using 3-D finite element analysis. The mechanical model includes shear deformable stereocilia, realignment of all forces during force load increments, and tip and lateral link inter-stereocilia connections. Results show that there are two distinct bundle types that can be separated by mechanical bundle stiffness. The more compliant group has fewer total stereocilia and short stereocilia relative to kinocilium height; these cells are located in the medial and lateral extrastriola. The stiff group are located in the striola. They have more stereocilia and long stereocilia relative to kinocilia heights. Tip link tensions show parallel behavior in peripheral columns of the bundle and serial behavior in central columns when the tip link modulus is near or above that of collagen (1x10(9) N/m(2)). This analysis shows that lumped parameter models of single stereocilia columns can show some aspects of bundle mechanics; however, a distributed, 3-D model is needed to explore overall bundle behavior.

Endolymphatic calcium supply for fish otolith growth takes place via the proximal portion of the otocyst

The presence of calcium within the utricle of larval cichlid fish Oreochromis mossambicus was analysed by means of energy-filtering transmission electron microscopy. Electron-spectroscopic imaging and electron energy loss spectra revealed discrete calcium precipitations that were more numerous in the proximal endolymph than in the distal endolymph, clearly indicating a decreasing proximo-distal gradient. This decreasing proximo-distal gradient was also present within the proximal endolymph between the sensory epithelium and the otolith. Further calcium particles covered the peripheral proteinaceous layer of the otolith. They were especially pronounced at the proximal surface of the otolith indicating that otolithic calcium incorporation takes place here. Other calcium precipitates accumulated at the macular junctions clearly supporting an earlier assumption according to which the endolymph is supplied with calcium via a paracellular pathway. The present results clearly show that the apical region of the macular epithelium is involved in the release of calcium and that the calcium supply of the otoliths takes place via the proximal endolymph.

Quantitative analysis of stereociliary arrays on vestibular hair cells

We have developed a method for quantifying the number, spacing, and distribution of stereocilia on the apical surface of hair cells using spatial autocorrelation analysis and statistics for directional data. Here, we illustrate the method using idealized hair bundles, and we apply it to scanning micrographs of turtle hair cells from the utricle and posterior canal, and to freeze-fracture preparations of bullfrog saccule. The analysis suggests three common features of stereociliary bundles. First, bundle geometries form a continuum from 'loose' to 'tight' rather than two distinct groups. Second, interciliary spacing along the three hexagon axes is not equal; spacing is usually widest along the hexagon axis closest to the bundle's axis of bilateral symmetry (the presumptive excitatory axis). Third, spacing between stereocilia changes with distance from the kinocilium. All three features will influence predictions of the tip link tensions that accompany bundle deflection.

Morphometry of otoconia in the utricle and saccule of developing Japanese quail

The development of otoconia in the utricular and saccular maculae from initial embryonic formation to adult stages was examined in Japanese quails. Both the morphology and size of the otoconia were quantified at different developmental stages. It was observed that the otoconia were initially formed on embryologic stage E5 in the saccule and E6 in the utricle. Otolith mass areas increased in a sigmoidal growth pattern, with saccular otolith areas being smaller than the utricular mass areas. Saccular otolith masses reached adult values at embryonic stage E12 and utricular areas reached adult values at post-hatch day 7. Mature individual otoconia were characterized by a barrel shape with two trihedral faceted ends. However, initial formation of otoconia at E5 (saccular) and E6 (utricular) maculae was characterized by a double fluted morphology that consisted of an hourglass shape with extended fins forming trihedral angles of 120 degrees. Double fluted otoconia rapidly filled, so that by embryonic day 8 mature otoconia dominated the maculae for the remainder of development through adulthood. Thus, a progression from double fluted to mature forms was noted. Mature utricular otoconia in adult quails averaged 11 microm in length and 5 microm in width, with length/width ratios of approximately 2.5:1, for all size ranges. Saccular otoconia were smaller, having about 70% the size of utricular otoconia in both length and width. During development, the average size and range of individual otoconia increased nearly linearly for both otolith organs. In the utricular macula, large otoconia were concentrated in the lateral regions of the epithelium. In contrast, otoconia of various sizes were distributed uniformly across the surface of the saccular macula.

Extremely elongated mitochondria in ionocytes of the saccular epithelium of a teleost, Oreochromis mossambicus (Cichlidae)

Unusually large mitochondria are a rather scarce feature in normal biological tissue and string-like giant mitochondria have hitherto not been reported in animals. Investigating the role of inner ear ionocytes for otolith growth, large ionocytes of the saccular epithelium of the cichlid fish Oreochromis mossambicus were analyzed by imaging of thick sections with energy-filtering transmission electron microscopy. We report here that ionocytes do not contain numerous small-sized mitochondria as has been suggested earlier but rather few, extremely elongated megamitochondria. Since the particular mitochondrial structure is important for normal cell function, such megamitochondria possibly reflect a functional advantage in the context of the presumed role of teleostean ionocytes in regulating the composition of the endolymphatic fluid.

Calcium gradients in the fish inner ear sensory epithelium and otolithic membrane visualized by energy filtering transmission electron microscopy (EFTEM)

Inner ear otolith formation in fish is supposed to be performed by the molecular release of proteinacious precursor material from the sensory epithelia, followed by an undirected and diffuse precipitation of calcium carbonate (which is mainly responsible for the functionally important weight of otoliths). The pathway of calcium into the endolymph, however, still remains obscure. Therefore, the presence of calcium within the utricle of larval cichlid fish Oreochromis mossambicus was analyzed by means of energy filtering transmission electron microscopy (EFTEM). Electron spectroscopic imaging (ESI) and electron energy loss spectra (EELS) revealed discrete calcium precipitations, which were especially numerous in the proximal endolymph as compared to the distal endolymph. A decreasing proximo-distal gradient was also present within the proximal endolymph between the sensory epithelium and the otolith. Further calcium particles covered the peripheral proteinacious layer of the otolith. They were especially pronounced at the proximal surface of the otolith. Other calcium precipitates were found to be accumulated at the macular junctions. These results strongly suggest that the apical region of the macular epithelium is involved in the release of calcium and that calcium supply of the otoliths takes place in the proximal endolymph.

Caspase inhibitors promote vestibular hair cell survival and function after aminoglycoside treatment in vivo

The sensory hair cells of the inner ear undergo apoptosis after acoustic trauma or aminoglycoside antibiotic treatment, causing permanent auditory and vestibular deficits in humans. Previous studies have demonstrated a role for caspase activation in hair cell death and ototoxic injury that can be reduced by concurrent treatment with caspase inhibitors in vitro. In this study, we examined the protective effects of caspase inhibition on hair cell death in vivo after systemic injections of aminoglycosides. In one series of experiments, chickens were implanted with osmotic pumps that administrated the pan-caspase inhibitor z-Val-Ala-Asp(Ome)-fluoromethylketone (zVAD) into inner ear fluids. One day after the surgery, the animals received a 5 d course of treatment with streptomycin, a vestibulotoxic aminoglycoside. Direct infusion of zVAD into the vestibule significantly increased hair cell survival after streptomycin treatment. A second series of experiments determined whether rescued hair cells could function as sensory receptors. Animals treated with streptomycin displayed vestibular system impairment as measured by a greatly reduced vestibulo-ocular response (VOR). In contrast, animals that received concurrent systemic administration of zVAD with streptomycin had both significantly greater hair cell survival and significantly increased VOR responses, as compared with animals treated with streptomycin alone. These findings suggest that inhibiting the activation of caspases promotes the survival of hair cells and protects against vestibular function deficits after aminoglycoside treatment.

Histological preparation of developing vestibular otoconia for scanning electron microscopy

The unique nature of vestibular otoconia as calcium carbonate biominerals makes them particularly susceptible to chemical deformation during histological processing. We fixed and stored otoconia from all three otolith endorgans of embryonic, hatchling and adult Japanese quail in glutaraldehyde containing either phosphate or non-phosphate buffers for varying lengths of time and processed them for scanning electron microscopy. Otoconia from all age groups and otolith endorgans processed in 0.1 M phosphate buffer (pH 7.4) showed abnormal surface morphology when compared to acetone fixed controls. Otoconia processed in 0.1 M sodium cacodylate or HEPES buffered artificial endolymph (pH 7.4) showed normal morphology that was similar to controls. The degree of otoconial deformation was directly related to the time exposed to phosphate buffer. Short duration exposure produced particulate deformations while longer exposures resulted in fused otoconia that formed solid sheets. Otoconial surface deformation and fusing was independent of the glutaraldehyde component of the histological processing. These findings should help vestibular researchers to develop appropriate histological processing protocols in future studies of otoconia.

Localization of glucocorticoid receptors in the murine inner ear

We performed an immunohistochemical investigation of the distribution of glucocorticoid receptors (GRs) in the murine inner ear and found that GRs were expressed extensively, but with various degrees of immunoreactivity in different regions. We observed the strongest GR expression in the type III fibrocytes of the spiral ligament. Although the immunoreactivity of the cochlear hair cells and of the vestibular sensory epithelia was weak, the neighboring cochlear supporting cells and the subepithelial regions of the vestibular sensory epithelia were immunostained. Staining for GRs was also positive in the spiral ganglia and vestibular ganglia, as well as in the endolymphatic sac. The role of GRs in the inner ear is discussed.

Depolarization redistributes synaptic membrane and creates a gradient of vesicles on the synaptic body at a ribbon synapse

We used electron tomography of frog saccular hair cells to reconstruct presynaptic ultrastructure at synapses specialized for sustained transmitter release. Synaptic vesicles at inhibited synapses were abundant in the cytoplasm and covered the synaptic body at high density. Continuous maximal stimulation depleted 73% of the vesicles within 800 nm of the synapse, with a concomitant increase in surface area of intracellular cisterns and plasmalemmal infoldings. Docked vesicles were depleted 60%-80% regardless of their distance from the active zone. Vesicles on the synaptic body were depleted primarily in the hemisphere facing the plasmalemma, creating a gradient of vesicles on its surface. We conclude that formation of new synaptic vesicles from cisterns is rate limiting in the vesicle cycle.

The role of gravity in the phylogeny of structure and function in animal sensors of spatial orientation, and their predicted action in weightlessness

The evolution of the structural, functional and cytochemical organization of the gravity receptor which determines a body position in the gravitational field of the earth by means of muscular regulation was traced both invertebrates and vertebrates, using electron microscopic and histochemical methods. In the course of evolution of vertebrates, the specialized gravity receptor-statocyst which, as a rule, consists of primary sensory cells and supplies otoliths, is formed. In vertebrates, there exists a vestibular apparatus made up of secondary sensory cells and also having otoliths. The receptor cells, both of statocysts and the vestibular apparatus, are supplied with special antennas (kinocilia and stereocilia). Deviation of the antennas stimulated by displacement of the otoliths resulting from locomotor activity of animals leads to excitation of the receptor cells. When exposed to a modified gravitational field (linear accelerations of 10 g, for 3 min), the receptor cells of the vestibular apparatus, in all classes of vertebrates, show progressive changes in RNA content and protein synthesis (increase followed by decrease) which return to normal only after 12 days. Thus, immediate transfer of animals and man from acceleration to weightlessness appears to be a reason for movement disease. The above consideration showed the need for an experiment in which an animal (with its vestibular apparatus) which had not undergone previous accelerations, would be exposed to weightlessness. Frog embryos, Rana temporaria, at the stage preceding the organogenesis, when the vestibular apparatus and other organs were lacking, were chosen as a suitable subject. Frog embryos at the stage of an early gastrula were placed in a special container Emkon aboard the Soyuz 10 spacecraft. After short accelerations, they were exposed to weightlessness for 44 hours. The embryos were allowed to continue to develop to the stage of early tail bud. The experimental embryos showed normally developed acoustic vesicles and vestibular ganglia. Clear differentiation of the receptor cells with antennas (kinocilia and stereocilia) was found in the acoustic vesicles. Thus, in weightlessness, vestibular apparatus develops just as well as in the gravitational field of the earth. However, only a much longer stay in weightlessness conditions will indicate whether there are any changes in the structural, cytochemical and functional organization of vestibular apparatus. The similarity in the structural, functional and cytochemical organization of the gravity receptor in vertebrates and invertebrates appears to allow the prediction of the behaviour of the gravity receptor as a whole, and of its receptor elements, both in normal and changed gravitational fields. The first attempts were carried out only on the vestibular apparatus of vertebrates.

Subcellular immunolocalization of NMDA receptor subunit NR1, 2A, 2B in the rat vestibular periphery

The immunohistochemical localization of the NMDA glutamate receptor subunits NR1, NR2A, and NR2B was investigated in the rat vestibular periphery at the light and electron microscopy level using specific antipeptide antibodies. The afferent calyceal terminals and nerve fibers innervating type I vestibular hair cells were strongly NR1, NR2A, and NR2B immunoreactive. Under electron microscopy, the basolateral type I hair cell membrane was NR1 immunoreactive. The type II hair cell and its afferent boutons were NR1, NR2A, and NR2B non-immunoreactive. Nearly all of Scarpa's ganglion neurons were NR1 immunoreactive, but there was a subset of NR2A non-immunoreactive neurons. Additionally, the larger sized Scarpa's ganglia neurons were NR2B immunoreactive, while the smaller neurons were non-immunoreactive. These findings are strong evidence for functional NMDA receptor mediation or modulation of afferent excitatory neurotransmission from type I but not type II vestibular hair cells to the primary afferent nerve. The receptor subtype(s) may be a combination of NR1/NR2A, NR1/NR2B, and/or NR1/NR2A/NR2B.

Effects of neurotrophin and neurotrophin receptor disruption on the afferent inner ear innervation

Two neurotrophins and their two receptors appear to regulate the survival of vestibular and cochlear neurons in the developing ear. Mice lacking either brain derived neurotrophic factor (BDNF) or its associated receptor, Trk B, show a severe reduction in the number of vestibular neurons and a loss of all innervation to the semicircular canals. Mice lacking NT-3 or its receptor, Trk C, show a severe reduction of spiral neurons in the basal turn of the cochlea. Mice lacking both BDNF and NT-3 or Trk B and Trk C, reportedly lose all innervation to the inner ear. These two neurotrophins and their associated receptors are necessary for the normal afferent innervation of the inner ear.

Gravity receptors: an ultrastructural basis for peripheral sensory processing

Our ultrastructural study of serial sections has shown that type II hair cells of the anterior part of the utricular macula are integrated into the afferent neural circuitry of type I cells, which are arranged in clusters. Additionally, there exists a complex system of intramacularly originating efferent-type nerve fibers and terminals. The findings taken together suggest that, on morphological grounds, complex processing of sensory information occurs in gravity receptors. Asymmetry of such a complex system may contribute to motion and Space-motion sickness.

Otx1 null mutant mice show partial segregation of sensory epithelia comparable to lamprey ears

We investigated the development of inner ear innervation in Otx1 null mutants, which lack a horizontal canal, between embryonic day 12 (E12) and postnatal day 7 (P7) with DiI and immunostaining for acetylated tubulin. Comparable to control animals, horizontal crista-like fibers were found to cross over the utricle in Otx1 null mice. In mutants these fibers extend toward an area near the endolymphatic duct, not to a horizontal crista. Most Otx1 null mutants had a small patch of sensory hair cells at this position. Measurement of the area of the utricular macula suggested it to be enlarged in Otx1 null mutants. We suggest that parts of the horizontal canal crista remain incorporated in the utricular sensory epithelium in Otx1 null mutants. Other parts of the horizontal crista appear to be variably segregated to form the isolated patch of hair cells identifiable by the unique fiber trajectory as representing the horizontal canal crista. Comparison with lamprey ear innervation reveals similarities in the pattern of innervation with the dorsal macula, a sensory patch of unknown function. SEM data confirm that all foramina are less constricted in Otx1 null mutants. We propose that Otx1 is not directly involved in sensory hair cell formation of the horizontal canal but affects the segregation of the horizontal canal crista from the utricle. It also affects constriction of the two main foramina in the ear, but not their initial formation. Otx1 is thus causally related to horizontal canal morphogenesis as well as morphogenesis of these foramina.

Lectin from Griffonia simplicifolia identifies an immature-appearing subpopulation of sensory hair cells in the avian utricle

The sensory hair cells of the inner ear are coated with a variety of glycoproteins and glycolipids which can be identified by the binding of specific lectins. The present study examined the binding patterns of three lectins-Wheat Germ Agglutinin, Peanut Agglutinin, and lectin from Griffonia simplicifolia (Isoform B(4))-in the avian utricle. Each of the lectins exhibited a distinct pattern of hair cell labeling. Wheat Germ Agglutinin (WGA) appeared to label the ciliary bundles of all sensory hair cells. In contrast, the binding of Peanut Agglutinin (PNA) was mainly confined to the ciliary bundles of extrastriolar hair cells. Finally, lectin from Griffonia simplicifolia (GS-IB(4)) labeled a subpopulation of hair cells in all regions of the chick utricle. Those bundles were much smaller than the majority of ciliary bundles labeled by either WGA or PNA, and the density of GS-IB(4)-labeled bundles in the normal mature utricle was relatively low. Increased densities of GS-IB(4)-labeled hair cells were observed in the embryonic utricle and during the process of hair cell regeneration. The observations suggest that GS-IB(4) labels a glycoprotein that is expressed preferentially on the ciliary bundles of immature hair cells.

High-resolution structure of hair-cell tip links

Transduction-channel gating by hair cells apparently requires a gating spring, an elastic element that transmits force to the channels. To determine whether the gating spring is the tip link, a filament interconnecting two stereocilia along the axis of mechanical sensitivity, we examined the tip link's structure at high resolution by using rapid-freeze, deep-etch electron microscopy. We found that the tip link is a right-handed, coiled double filament that usually forks into two branches before contacting a taller stereocilium; at the other end, several short filaments extend to the tip link from the shorter stereocilium. The structure of the tip link suggests that it is either a helical polymer or a braided pair of filamentous macromolecules and is thus likely to be relatively stiff and inextensible. Such behavior is incompatible with the measured elasticity of the gating spring, suggesting that the gating spring instead lies in series with the helical segment of the tip link.

Cytoskeletal protein mRNA expression in the chick utricle after treatment in vitro with aminoglycoside antibiotics: effects of insulin, iron chelators and cyclic nucleotides

In birds, spontaneous recovery of the hair cells of the inner ear can occur after damage induced by aminoglycoside antibiotics. The factors that influence this recovery and the process of hair cell regeneration itself have until recently been investigated largely by morphological and histological methods. The aim of this work was to use a molecular biological approach to the analysis of hair cell regeneration by measuring the changes that occur in expression of mRNA for hair cell-specific cytoskeletal proteins fimbrin and class III beta-tubulin, along with that for beta-actin, in the utricle of chicks after hair cell damage both in vitro and in vivo. Utricles were removed from 1-day-old chicks and incubated with the aminoglycoside antibiotics gentamicin or neomycin (both 1 mM), or chicks were injected intraperitoneally with 100 mg/kg gentamicin or neomycin for 4 consecutive days. At the end of the treatment periods, total RNA was extracted from single utricles, reverse transcribed to cDNA and the cDNA amplified by PCR with primers for beta-actin, fimbrin and class III beta-tubulin. Co-amplification allowed quantitative comparison of mRNA between fimbrin, or class III beta-tubulin and beta-actin from the same utricle. Both aminoglycosides, either after 48 h in vitro or immediately after treatment in vivo, caused a significant decrease in the expression of fimbrin mRNA and class III beta-tubulin mRNA, relative to beta-actin mRNA, which itself increased. Light and electron microscopy confirmed that this corresponded to loss of, and damage to, hair cells. The relative expression of fimbrin and class III beta-tubulin mRNAs was partly restored almost to control levels 4 days after cessation of treatment in vivo and fully normalised by 4 weeks, by which time hair cells appeared normal. However, their relative expression remained depressed 4 days after removal of antibiotic in vitro. The iron chelator desferrioxamine (100 microM) in vitro prevented the aminoglycoside-induced reduction in relative expression of mRNA for both fimbrin and class III beta-tubulin. Neither insulin (5 microM) nor a combination of dibutyryl cyclic AMP (5 mM) and the phosphodiesterase inhibitor IBMX (0.5 mM) prevented the decrease in relative expression of the mRNAs for the hair cell-specific proteins, but both treatments allowed their partial recovery in vitro during the 4-day-period after removal of aminoglycoside. It is concluded that the cells of the sensory epithelium of the chick utricle subjected to aminoglycoside-induced damage undergo a process in which mRNA expression is switched away from the production of functional proteins and towards proteins necessary for structural re-organisation. The restoration of mRNA expression to a normal pattern is promoted by the growth factor insulin and by cyclic AMP.

Otoliths developed in microgravity

Little is known about mechanisms that regulate the development of the otoliths in the gravity-sensing organs. Several reported experiments suggest that the growth of the otoliths is adjusted to produce a test mass of the appropriate weight. If this is the case, larger than normal otoliths would be expected in animals reared in reduced gravity and a reduced mass, relative to 1-g controls, would be expected in animals reared at elevated g. In gastropod mollusks, the gravity-sensing organ is the statocyst, a spherical organ whose wall is made largely of sensory receptor cells with motile cilia facing the lumen. Dense statoconia in the cyst lumen interact with cilia of receptor cells at the bottom of the cyst and action potentials in their axons carry information on direction and magnitude of gravity and linear acceleration. In the marine mollusk, Aplysia californica, larvae reared at 2 to 5-g, the volume of statoconia was reduced in a graded manner, compared to 1-g control animals. In the statocyst of the fresh-water pond snail, Biomphalaria glabrata, reared in space in the Closed Equilibrated Biological Aquatic System (CEBAS), the number and total volume of statoconia was increased approximately 50%, relative to ground-reared controls. Lychakov found the utricular otolith to be 30% larger in space-reared Xenopus, whereas we found the saccular otolith to be significantly larger in newt larvae reared in space. In cichlid fish reared on a centrifuge, the saccular otolith was smaller than in 1-g controls. Here, we demonstrate that the otoliths of late-stage embryos of the swordtail fish, Xiphophorus helleri, reared in space on STS-89 and STS-90 (Neurolab) were significantly larger than those of ground-controls reared in functionally identical hardware.

Structure and function in the saccule of the goldfish (Carassius auratus): a model of diversity in the non-amniote ear

The vertebrate inner ear is comprised of a remarkable diversity of cell types, including several types of sensory hair cells. In amniotes (reptiles, birds, and mammals), the morphological and physiological characteristics that distinguish these cell types have been well documented, while cellular variation in the ears of non-amniotes (all other vertebrate groups) has remained underrecognized. Since non-amniotes have become increasingly popular models for developmental and genetic research, a more comprehensive understanding of structure and function in the inner ears of these species is warranted. This paper first reviews the large body of data describing the morphology and physiology of hair cells and afferent neurons in the inner ear of the goldfish (Carassius auratus). In particular, we examine the structure of the goldfish saccule, an endorgan that has been the subject of numerous investigations on audition. New data on the structural variation of synaptic bodies in saccular hair cells are also presented, and the functional implications of these data are discussed. Finally, we conclude that hair cell structure varies along the length of the goldfish saccule in a manner consistent with known physiological characteristics of the endorgan. The saccule provides an excellent model for investigating structure-function relationships in the vertebrate inner ear, as well as the development of auditory and vestibular sensory epithelia.

Electronmicroscopic investigations on the role of vesicle-like bodies in inner ear maculae for fish otolith growth

The presence, morphology and possible origin of vesicle-like bodies (VBs) within the inner ear otolithic membrane of developmental stages of cichlid fish Oreochromis mossambicus and adult swordtail fish Xiphophorus helleri was analysed by means of transmission and scanning electron microscopy (TEM and SEM, respectively) employing various fixation procedures. The VBs are believed to be involved in the formation of the otolith (or statolith in birds and mammals) regarding the supply of the otolith's organic material. Increasing the osmolarity of the fixation medium decreased the number of VBs seen. Decalcification ended up in a complete disappearance of the VBs. Whilst a fixation with glutaraldehyde followed by OSO4 fixation yielded numerous VBs, only few of them were observed when the tissue was fixed with glutaraldehyde and OSO4 simultaneously. Therefore, the results strongly suggest that the VBs are fixative (i.e., glutaraldehyde) induced artifacts, so-called blisters. With this, the supply of an oto- or statolith's organic material remains obscure. Possibly, it is provided by secretion from the supporting cells as has been hypothesized earlier.

Establishment of hair bundle polarity and orientation in the developing vestibular system of the mouse

The morphological development of the vestibular maculae in the mouse was studied in order to identify elements that may determine how hair-bundle polarity is established. Utricles and saccules develop in parallel. Hair-bundles first appear at embryonic day (E) 13.5. They are initially not polarised and have a kinocilium located at the centre of the cell surface surrounded by stereocilia. Polarisation is rapidly established as the kinocilium becomes eccentrically positioned. The orientation of these polarised bundles is initially not random. It varies systematically across the maculae and the general orientation in utricles is the opposite of that in saccules. At E15.5, in both maculae, hair-bundle orientation angles fall into two populations that differ by approximately 180 degrees defining a line of orientation reversal, the position of which varies little during subsequent maturation. Many more immature hair bundles appear at E15.5 suggesting a second wave of hair cell differentiation is initiated. Otoconial membrane is produced simultaneously across the entire width of both maculae, indicating directional growth of the overlying extracellular matrix is unlikely to influence hair-bundle orientation. Growth of both maculae occurs asymmetrically, essentially outwards from the striola, but it is most pronounced after orientation is defined. Microtubules are prominent in hair cells at the earliest stages of their differentiation, but are oriented parallel to the long axis of the cell and, thus, may not have a role in directing hair-bundle polarity. Microfilament assemblies that are aligned parallel to the apical surface and connect to the adherens junctions in supporting cells could provide a "framework" for hair-bundle orientation. The striated rootlets of ciliary centrioles that are aligned parallel to the cell surface with their tips associated with microfilament assemblies at adherens junctions were the only structural asymmetry identified that might influence the development of hair-bundle polarity.

Distribution and time course of hair cell regeneration in the pigeon utricle

Vestibular and cochlear regeneration following ototoxic insult from aminoglycoside antibiotics has been well documented, particularly in birds. In the present study, intraotic application of a 2 mg streptomycin paste was used to achieve complete vestibular hair cell destruction in pigeons (Columba livia) while preserving regenerative ability. Scanning electron microscopy was used to quantify hair cell density longitudinally during regeneration in three different utricular macula locations, including the striola, central and peripheral regions. The utricular epithelium was void of stereocilia (indicating hair cell loss) at 4 days after intraotic treatment with streptomycin. At 2 weeks the stereocilia began to appear randomly and mostly in an immature form. However, when present most kinocilia were polarized toward the developing striola. Initially, regeneration occurred more rapidly in the central and peripheral regions of the utricle as compared to the striola. As regeneration proceeded from 2 to 12 weeks, hair cell density in the striola region equaled the density noted in the central and peripheral regions. At 24 weeks, hair cell density of the central and peripheral regions was equal to normal values, however the striola region had a slightly greater hair cell density than that observed for normal animals.

Math1: an essential gene for the generation of inner ear hair cells

The mammalian inner ear contains the cochlea and vestibular organs, which are responsible for hearing and balance, respectively. The epithelia of these sensory organs contain hair cells that function as mechanoreceptors to transduce sound and head motion. The molecular mechanisms underlying hair cell development and differentiation are poorly understood. Math1, a mouse homolog of the Drosophila proneural gene atonal, is expressed in inner ear sensory epithelia. Embryonic Math1-null mice failed to generate cochlear and vestibular hair cells. This gene is thus required for the genesis of hair cells.

[Traverse connectivity of innervation in the vestibular sensory epithelium]

OBJECTIVE

To further understand the traverse connections of nerve components in the vestibular sensory epithelium.

METHODS

The nerve innervations in the vestibular sensory epithelia and infraepithelia of ten healthy guinea pigs (16 ears) were observed by transmission electron microscopy.

RESULTS

Type I sensory cells were intricately connected with type II cells. Afferent and efferent fibers as well as their endings had also complicated connections with nerve trunks and branches. The traverse connections appeared among nerve components in the vestibular sensory epithelium were extremely complicated, forming multilevel connections. They stemmed from and end in the crista ampullaris and the maculae.

CONCLUSION

These complicated and multilevel connections are the basis of nerve activity in the vestibular apparatus.

Localization of myosin-Ibeta near both ends of tip links in frog saccular hair cells

Current evidence suggests that the adaptation motor of mechanoelectrical transduction in vertebrate hair cells is myosin-Ibeta. Previously, confocal and electron microscopy of bullfrog saccular hair cells using an anti-myosin-Ibeta antibody labeled the tips of stereocilia. We have now done quantitative immunoelectron microscopy to test whether myosin-Ibeta is enriched at or near the side plaques of tip links, the proposed sites of adaptation, using hair bundles that were serially sectioned parallel to the macular surface. The highest particle density occurred at stereocilia bases, close to the cuticular plate. Also, stereocilia of differing lengths had approximately the same number of total particles, suggesting equal targeting of myosin-Ibeta to all stereocilia. Finally, particles tended to clump in clusters of two to five particles in the distal two-thirds of stereocilia, suggesting a tendency for self-assembly of myosin-Ibeta. As expected from fluorescence microscopy, particle density was high in the distal 1 micrometer of stereocilia. If myosin-Ibeta is the adaptation motor, a difference should exist in particle density between regions containing the side plaque and those excluding it. Averaging of particle distributions revealed two regions with approximately twice the average density: at the upper ends of tip links in a 700-nm-long region centered approximately 100 nm above the side plaque, and at the lower ends of tip links within the tip plaques. Controls demonstrated no such increase. The shortest stereocilia, which lack side plaques, showed no concentration rise on their sides. Thus, the specific localization of myosin-Ibeta at both ends of tip links supports its role as the adaptation motor.

Macrophage and microglia-like cells in the avian inner ear

Recent studies suggest that macrophages may influence early stages of the process of hair cell regeneration in lateral line neuromasts; numbers of macrophages were observed to increase prior to increases in hair cell progenitor proliferation, and macrophages have the potential to secrete mitogenic growth factors. We examined whether increases in the number of leukocytes present in the in vivo avian inner ear precede the proliferation of hair cell precursors following aminoglycoside insult. Bromodeoxyuridine (BrdU) immunohistochemistry was used to identify proliferating cells in chicken auditory and vestibular sensory receptor epithelia. LT40, an antibody to the avian homologue of common leukocyte antigen CD45, was used to label leukocytes within the receptor epithelia. Macrophages and, surprisingly, microglia-like cells are present in normal auditory and vestibular sensory epithelia. After hair cell loss caused by treatment with aminoglycosides, numbers of macrophage and microglia-like cells increase in the sensory epithelium. The increase in macrophage and microglia-like cell numbers precedes a significant increase in sensory epithelial cell proliferation. The results suggest that macrophage and microglia-like cells may play a role in releasing early signals for cell cycle progression in damaged inner ear sensory epithelium.

Vestibular function in Belgian Waterslager canaries (Serinus canarius)

The purpose of this study was to measure vestibular function in Belgian Waterslager canaries using short latency vestibular evoked potentials (VsEPs) elicited by linear acceleration stimuli. Responses were recorded with vertex to mastoid leads using traditional signal averaging. Response thresholds, latencies, and amplitudes were quantified and compared to non-Waterslager controls. Cochlear and vestibular organs were also processed for scanning electron microscopy. Results indicated that vestibular response thresholds were slightly, but significantly, higher for Belgian Waterslager canaries and response amplitudes at 0 dBre: 1.0 g/ms were significantly reduced compared to non-Waterslagers. Response peak latencies were not significantly different. The most striking morphological finding was that the stereociliary bundles of Waterslager saccular hair cells showed no common orientation. Previous reports have also found significantly less hair cells in Waterslager saccules (Weisleder and Park, Hear. Res. 80 (1994) 64-70); however, the present study did not confirm this finding. The utricle and ampullae appeared normal. The present results indicate that vestibular neural function, as measured with VsEPs, is affected in Belgian Waterslager canaries. The results also suggest that one structural correlate of the functional loss is the disordered stereociliary bundles in the sacculus.

Immunocytochemical localization of calmodulin in the vestibular end-organs of the gerbil

This study demonstrates the presence of calmodulin in the vestibular end-organs of the gerbil by use of immunocytochemistry. Using fluorescence microscopy, calmodulin was localized to the cytoplasm, cuticular plate, and stereocilia of both type I and type II hair cells in the sensory epithelia of the utricle and cristae ampullaris; no label was found in the supporting cells, the dark cells, or the nerve fibers. There was no immunoreactive distinction between the labeling of type I and type II hair cells in the striolar or extrastriolar regions. Thus, immunocytochemical labeling for calmodulin provides a good marker for hair cells in gerbil vestibular epithelium. The presence of calmodulin in the stereocilia was confirmed by immunoelectron microscopy using secondary antibodies coupled to colloidal gold.

The effect of explantation and neomycin on hair cells and supporting cells in organotypic cultures of the adult guinea-pig utricle

Recent reports suggest that immature hair bundles are observed following aminoglycoside-induced hair-cell loss in the mammalian utricle in vitro as well as in vivo. It is therefore important to document the initial morphological changes associated with both culturing and aminoglycoside application so that degeneration can be clearly distinguished from regeneration. In this study, utricles from adult guinea pigs were maintained in culture for either 3 or 8 days, half being exposed to neomycin for days 2 and 3. They were then processed for microscopical examination and compared with control utricles from animals of the same age. The numbers of hair-cell and supporting-cell nuclei were counted and hair-cell morphology assessed. Bundles were classified as having either stepped (SHB) or unstepped (UHB) stereocilia, and their density determined. The numbers of hair-cell, but not supporting-cell, nuclei declined significantly compared with controls in both untreated and treated explants, the greatest reduction occurring 5 days after neomycin administration. The density of SHBs also declined but there was no significant change in UHB density, resulting in a residual population of hair bundles of more immature appearance in both untreated and treated utricles in vitro than in vivo. Although degenerative events such as hair-cell ejection from, or retraction into, the sensory epithelium were observed, no evidence of regeneration was found.

Ultrastructural aspects of otoliths and sensory epithelia of fish inner ear exposed to hypergravity

The present electron microscopical investigations were directed to the question, whether alterations in the gravitational force might induce structural changes in the morphology of otoliths or/and inner ear sensory epithelia of developing and adult swordtail fish (Xiphophorus helleri) that had been kept either under long-term moderate hypergravity (8 days; 3g) or under short-time extreme hypergravity (10 minutes up to 9g). The otoliths of adult and neonate swordtail fish were investigated by means of scanning electron microscopy (SEM). Macular epithelia of adult fish were examined both by SEM and transmission electron microscopy (TEM). The saccular otoliths (sagittae) of normally hatched adult fish revealed an enormous inter- (and even intra-; i.e. left vs. right) individual diversity in shape and size, whereas the otoliths of utricles (lapilli) and lagenae (asterisci) seemed to be more constant regarding morphological parameters. The structural diversity of juvenile otoliths was found to be less prominent as compared to the adults, differing from the latter regarding their peculiar crystalline morphology. Qualitative differences in the fine structure (SEM) of otoliths taken from adult and larval animals kept under 3g in comparison to 1g controls could not be observed. The SEM and TEM investigations of sensory epithelia also did not reveal any effects due to 3g stimulation. Even extreme hypergravity (more than 7g) for 10 minutes did not result in distinct pathological changes.

Meshwork arrangement of mitochondria-rich, Na+,K+-ATPase-rich cells in the saccular epithelium of rainbow trout (Oncorhynchus mykiss) inner ear

BACKGROUND

Electrolyte composition of the teleost fish inner ear endolymph is characterized by a high potassium concentration. From the ultrastructural characteristics, the mitochondria-rich cells (MRCs) in the inner ear epithelium are suggested to regulate the ionic composition of the endolymph.

METHODS

In the present study, the ultrastructure of MRCs in the saccular epithelium of the rainbow trout (Onchorhynchus mykiss) was studied, and the immunocytochemical detection of Na+,K+-ATPase, the key enzyme of the ion-transport, in the saccular epithelium was conducted. Electrolyte composition of the saccular endolymph was also determined.

RESULTS

Electron-microscopic observations revealed that MRCs located at the periphery of the sensory macula have numerous elongated mitochondria and a well-developed tubular system. Immunocytochemical detection of Na+,K+-ATPase on paraffin sections showed that immunoreactive (ir-) cells were distributed specifically around the sensory macula. Judging from their shape, size, and localization, the Na+,K+-ATPase ir-cells corresponded to the MRCs. The whole-mount immunocytochemistry using Na+,K+-ATPase as a marker for the MRC revealed that MRCs were connected with one another by extended cellular processes, and thus forming a dense meshwork structure around the macula. In the endolymph, potassium levels were 13 times higher than those in plasma, chloride levels were slightly higher whereas sodium, calcium, magnesium, and phosphate levels were lower.

CONCLUSIONS

Thus, the saccular MRCs abundant in Na+,K+-ATPase are distributed around the sensory macula forming a dense meshwork structure, with the suggested function to regulate the electrolyte composition of the saccular endolymph.

[Changes in morphology and calcium content of otoconia in rats after 120 d tail-suspension]

The morphology and calcium content of otoconia in rats after long term (120 d) tail-suspension were studied using scanning electron microscopy (SEM) and X-ray microanalysis, respectively. The results showed that after 120 d simulated weightlessness otoconia were round, irregularly shaped, or with fissures, and there were rough and fine granular or small globular substances on the surface. X-ray microanalysis showed that the calcium content in the otoconia of both utricule and saccule was significantly decreased in the 120 d tail-suspended rats than that in control (P < 0.01). These results suggest that a long term(120 d) simulation of the headward distribution of blood volume and hindlimb underloading effect induced by weightlessness may cause the morphological changes and lower calcium content of the otoconia. Finally, the possible mechanism and the physiologic meaning of these findings are discussed.

Morphological evidence for supporting cell to hair cell conversion in the mammalian utricular macula

The possible origin of the immature hair cells that appear in the utricular maculae of guinea pigs following gentamicin-induced hair cell death was investigated. Guinea pigs were continuously infused with bromodeoxyuridine, to label proliferating cells and their progeny, for 2 weeks after inducing damage to the inner ear on one side with gentamicin. The opposite ear in each animal served as control. Serial sections were cut through the entire utricular maculae of both ears of each animal and the number of labelled cells in the epithelium and underlying connective tissue was counted. Label was present in cells in the sensory epithelium in the utricles from the drug exposed ears but not in the controls. The nuclei of cells in the underlying connective tissue were also labelled in both ears. Some of the labelled nuclei in the epithelium were at the level normally occupied by hair cells, but most were at the level of supporting cell nuclei. However, the total number of labelled nuclei in the sensory epithelium was small; the maximum was 12 in one animal. The number of labelled nuclei in the connective tissue of the treated ears was significantly greater than the number in the untreated ear. This confirms that cell proliferation is stimulated in the mature mammalian utricular macula after hair cell loss, but the extent to which it occurs appears to be insufficient to explain the recovery in hair cell numbers which is observed. Detailed thin section studies of the utricular maculae of gentamicin-treated animals over a prolonged post-treatment period were also performed. In utricles which had suffered damage, there were cells which, like supporting cells but unlike hair cells, were resting on basement membrane, but which possessed at their apical ends organized bundles of microvilli similar to immature hair cell stereocilia. Other cells with more obvious stereocilia remained in contact with the basement membrane via and a small feet process. In still other cells, where a stereociliary bundle was obvious and almost mature in appearance, there was a foot process extending towards the basement membrane but not quite in contact, suggesting it had just detached. All these cells were contacted by nerve endings and specialization of the membranes were apparent at the site of cell-neurone contact. The morphological characteristics of these cells are consistent with phenotypic conversion of supporting cells into hair cells and this may account for some of the hair cell production in the mature mammalian vestibular sensory epithelia after hair cell death.

Expression of cytochrome oxidase in hair cells of the teleost utricle

Ultrastructural variation in some cytoplasmic organelles and synaptic structures is one characteristic distinguishing the types of hair cells in the teleost ear. In this study, we explored differences in mitochondria by analyzing mitochondrial reactivity for cytochrome oxidase (COX) in hair cells of the teleost utricle. The reactivity for COX within mitochondria in the subcuticular compartment directly beneath the cuticular plate differentiated among hair cells in utricles of three teleost species, Carassius auratus, Pantodon buchholzi, and Astronotus ocellatus. Mitochondria in the subcuticular region of hair cells in the striola reacted intensely. Within juxtastriola and extrastriolar hair cells near the striola, mitochondria reacted at a lowered intensity than in striolar hair cells. Subcuticular mitochondria of extrastriolar hair cells located distant from the striola reacted negligibly. The reactivity of mitochondria in other cytoplasmic compartments did not provide similar evidence for distinguishing among teleost hair cells. Mitochondria within intraepithelial branches of the eighth nerve terminals in the different utricular regions reacted to COX histochemistry commensurate with their respective presynaptic hair cells. Branches of sensory afferent neurons innervating striolar hair cells displayed a dense COX reaction. Sensory afferents innervating the extrastriolar hair cells did not display many mitochondria at synapses nor, when present, was the staining as dense. The presynaptic side of the hair cell-afferent nerve synapse usually, but not always, contained reactive mitochondria. The presynaptic side of the efferent nerve-hair cell synapse did not necessarily contain mitochondria. Mitochondria filling the cytoplasm in a type of juxtamacula cell revealed uniformly dense COX reactivity.

Hair cell precursors are ultrastructurally indistinguishable from mature support cells in the ear of a postembryonic fish

The ultrastructure of S-phase cells in the postembryonic fish ear was compared with that of mature support cells. S-phase cells were identified by injecting animals with [3H]thymidine and sacrificing 3 h later. Sensory epithelia (saccules, utricles, and canals) were processed for light-level autoradiography. Sections containing thymidine-labeled cells were re-embedded and re-examined using transmission electron microscopy. The results indicate that S-phase cells differ from mature support cells only in nuclear position and shape. Otherwise their cytoplasmic characteristics are indistinguishable. Both cell types, on the other hand, are readily distinguishable from hair cells. These data provide ultrastructural evidence for the ability of mature support cells to enter the cell cycle in postembryonic vertebrates.

Cell proliferation and hair cell addition in the ear of the goldfish, Carassius auratus

Cell proliferation and hair cell addition have not been studied in the ears of otophysan fish, a group of species who have specialized hearing capabilities. In this study we used the mitotic S-phase marker bromodeoxyuridine (BrdU) to identify proliferating cells in the ear of one otophysan species, Carassius auratus (the goldfish). Animals were sacrificed at 3 h or 5 days postinjection with BrdU and processed for immunocytochemistry. The results of the study show that cell proliferation occurs in all of the otic endorgans and results in the addition of new hair cells. BrdU-labeled cells were distributed throughout all epithelia, including the primary auditory endorgan (saccule), where hair cell phenotypes vary considerably along the rostrocaudal axis. This study lays the groundwork for our transmission electron microscopy study of proliferative cells in the goldfish ear (Presson et al., Hearing Research 100 (1996) 10-20) as well as future studies of hair cell development in this species. The ability to predict, based on epithelial location, the future phenotype of developing hair cells in the saccule of the goldfish make that endorgan a particularly powerful model system for the investigation of early hair cell differentiation.