The origin of the otoconia in the rat

Localization and functional studies of pendrin in the mouse inner ear provide insight about the etiology of deafness in pendred syndrome

Immunolocalization studies of mouse cochlea and vestibular end-organ were performed to study the expression pattern of pendrin, the protein encoded by the Pendred syndrome gene (PDS), in the inner ear. The protein was restricted to the areas composed of specialized epithelial cells thought to play a key role in regulating the composition and resorption of endolymph. In the cochlea, pendrin was abundant in the apical membrane of cells in the spiral prominence and outer sulcus cells (along with their root processes). In the vestibular end-organ, pendrin was found in the transitional cells of the cristae ampullaris, utriculi, and sacculi as well as in the apical membrane of cells in the endolymphatic sac. Pds-knockout (Pds-/-) mice were found to lack pendrin immunoreactivity in all of these locations. Histological studies revealed that the stria vascularis in Pds-/- mice was only two-thirds the thickness seen in wild-type mice, with the strial marginal cells showing irregular shapes and sizes. Functional studies were also performed to examine the role of pendrin in endolymph homeostasis. Using double-barreled electrodes placed in both the cochlea and the utricle, the endocochlear potential and endolymph potassium concentration were measured in wild-type and Pds-/- mice. Consistent with the altered strial morphology, the endocochlear potential in Pds-/- mice was near zero and did not change during anoxia. On the other hand, the endolymphatic potassium concentration in Pds-/- mice was near normal in the cochlea and utricle. Together, these results suggest that pendrin serves a key role in the functioning of the basal and/or intermediate cells of the stria vascularis to maintain the endocochlear potential, but not in the potassium secretory function of the marginal cells.

Zebrafish chaperone protein GP96 is required for otolith formation during ear development

Chaperone proteins are considered to be fairly ubiquitous proteins that promote the correct folding and assembly of multiple newly synthesized proteins. While performing an embryonic screen in zebrafish using morpholino phosphorodiamidate oligonucleotides (MPOs), we identified a role for an endoplasmic reticulum chaperone protein family member, zebrafish GP96. Knockdown of GP96 resulted in a specific otolith formation defect during early ear development. Otolith precursor particles did not adhere to the kinocilia of the tether cells in the GP96-MPO-injected embryos, aggregating instead into a single clump. Although otolith development was abnormal, the patterning of the ear and the differentiation of tether cells and macular sensory and support cells was not affected. We have isolated and sequenced the full open reading frame of zebrafish GP96 and characterized its expression pattern. GP96 is expressed both maternally and zygotically. GP96 RNA is localized within the floorplate, hatching gland, and in the cells of the otic placode and otic vesicle, consistent with the function of GP96 in ear development. We conclude that the GP96 chaperone protein is involved in the otolith formation during normal ear development. This is the first report of a specific function during organism development being attributed to a chaperone class molecule.

The crystalline pattern of calcium in different topographical regions of the otoconial membrane. An electron probe and spectroscopic diffraction study

Quantitative electron probe microanalysis and electron spectroscopic diffraction analysis was used to determine the gradient of distribution of calcium and its crystalline pattern at different levels (lower gelatinous membrane, upper gelatinous membrane and otoliths) in the otoconial membrane of adult OF1 mice. Our quantitative electron probe microanalytical data, obtained with scanning-transmission electron microscopy, indicated that there was a gradient in calcium concentration which increased from the vestibular surface towards the otoliths. Differences between the three regions of the otoconial membrane were statistically significant in both the utricle and saccule. Our results with electron spectroscopic diffraction revealed an increasing crystalline development from the lower gelatinous membrane towards the otoliths. Our findings with both techniques suggest that the gelatinous membrane is involved in the maturation and crystallization of the otoliths.

Production of otoconia in the endolymphatic sac in the Japanese red-bellied newt, Cynops pyrrhogaster: light and transmission electron microscopic study

The formation of otoconia in the endolymphatic sac (ES) of the larval newt, Cynops pyrrhogaster, has been studied by light and transmission electron microscopy. Some of the epithelial cells of the ES contain an abundance of swollen vesicles, Golgi complexes, rough endoplasmic reticula and ribosomes at the late larval stages 50 and 51, approximately 26-30 days after eggs are laid. Five days later, at stage 52, crystals are present in the vacuoles between the epithelial cells. Serial sections indicate that these vacuoles actually form small canals which lie in the wall and join the lumen of the ES. Reconstruction of the ES shows that several canals are contained in the ES wall. At stage 56, about 72 days after eggs are laid, a large number of otoconia are present in the ES lumen, while the otoconia disappear from the canals. It appears that the otoconia are first produced in the canals and then released to the lumen. Some epithelial cells of the ES are thought to expel the organic and inorganic material to the canals to form the otoconia in situ. The process of formation of the otoconia in the ES is discussed.

A critical period of ear development controlled by distinct populations of ciliated cells in the zebrafish

The zebrafish (Danio rerio) is a useful model system for analyzing development of the inner ear. A number of mutations affecting the inner ear have been identified. Here we investigate the initial stages of otolith morphogenesis in wild-type embryos as well as in monolith (mnl) mutant embryos, which fail to form anterior otoliths but otherwise appear normal. Otolith growth is initiated at 18-18.5 h by localized accretion of free-moving precursor particles. This process, referred to as otolith seeding, is regulated by two classes of cilia: First, kinocilia of precociously forming hair cells (tether cells) bind seeding particles, thereby localizing otolith formation. Tether cells usually occur in pairs at the anterior and posterior ends of the ear. Despite the presence of functional kinocilia, tether cells initially appear immature and do not acquire the characteristics of mature hair cells until approximately 21.5 h. Second, beating cilia distributed throughout the ear agitate seeding particles, thereby inhibiting premature agglutination. Constraining particles with laser tweezers caused them to fuse into large untethered masses. Bringing such masses into contact with tethered otoliths caused them to fuse, greatly enhancing otolith growth. Selectively enhancing one otolith greatly inhibited growth of the second, creating an imbalance that persisted for many days. Seeding particles and beating cilia disappear soon after 24 h, and the rate of otolith growth decreases by nearly 90%. In mnl mutant embryos, tethers and beating cilia are distributed normally, but anterior otoliths fail to form in 80-85% of mutant ears. The binding properties of seeding particles appear normal, as shown by their ability to fuse when entrapped by laser tweezers and their binding to posterior tethers. We infer that anterior tethers have a weakened ability to bind seeding particles in mnl embryos. Immobilizing mnl embryos with the anterior end of the ear oriented downward effectively concentrated the dense seeding particles near the anterior tethers and permitted all to form anterior otoliths. However, immobilizing mnl embryos after 24 h when seeding particles were depleted did not facilitate anterior otolith formation. Together, these data demonstrate that the ability to initiate otolith formation is limited to a critical period, from 18.5 to 24 h, and that interfering with the functions of tether cell kinocilia or beating cilia impairs otolith seeding and subsequent otolith morphogenesis.

Growth and turnover of rat otoconia as revealed by labeling with tetracycline

BACKGROUND

The formation process of otoconia remains controversial, and the turnover rate of mammalian otoconia has not been determined.

METHODS

Tetracycline was administered as a tracer for calcium, and the growth and turnover of rat otoconia were examined by fluorescence microscopy.

RESULTS

Exposure of rats to tetracycline from 15.5 gestational day to 3-day postpartum resulted in incorporation of the drug into central portions of all otoconia, in both maculae sacculi and utriculi. When postnatal rats were injected subcutaneously with tetracycline, uptake of the drug into otoconia depended on the age of rats at injection. Apparent fluorescence was emitted from the periphery of all otoconia when tetracycline was injected into 7-day-old or younger rats. However, very little or no fluorescence was observed when this reagent was administered 10 days after birth. No fluorescence was detected when rats of 12 days of age or older were given this antibiotic. Otoconia that had been labeled with tetracycline during gestation were monitored during subsequent development, and it was found that all otoconia retained labeling in their central portions for at least 12 months. No otoconia that were not labeled with tetracycline were found.

CONCLUSIONS

These findings indicate that: (1) all otoconia grow synchronously during late gestation and the neonatal period (up to about 10 days after birth) by accretion, (2) no new otoconia are formed subsequently, and (3) essentially no turnover of otoconia occurs and probably no turnover of calcium takes place under normal conditions once otoconia have formed.

Molecular cloning and characterization of an inner ear-specific structural protein

Molecular biological studies of the mammalian inner ear have been limited by the relatively small size of the sensory endorgans contained within. The saccular otolithic organ in teleostian fish is structurally similar to its mammalian counterpart but can contain an order of magnitude more sensory cells. The prospect of the evolutionary conservation of proteins utilized in the vertebrate inner ear and the relative abundance of teleostian saccular sensory tissue made this an attractive system for molecular biological studies. A complementary DNA obtained by differential screening of a saccular complementary DNA library was identified that encodes an inner ear-specific collagen molecule.

Immunocytochemical analysis of a novel carbohydrate differentiation antigen (CDA-3C2) associated with olfactory and otic systems during embryogenesis in the rat

Carbohydrate differentiation antigens are known to display specific patterns of expression during mammalian development and are thought to participate in significant morphogenetic events. In the present study, two monoclonal antibodies that react with a novel carbohydrate differentiation antigen (CDA-3C2) were used to analyze, by light microscopy, the spatiotemporal distribution of this unique high molecular weight antigen during embryogenesis in the rat. Correlative analysis of the development of peripheral neural structures, in which CDA-3C2 was expressed, was carried out with an anti-neurofilament antibody. Enzymatic digestion, combined with Western blots, reveal that the CDA-3C2 epitope is a carbohydrate which is carried on a high molecular weight glycoprotein with a mass of greater than 1 million Daltons. Characteristic of carbohydrate antigens, immunoreactivity was found in several distinct cellular patterns: only along the apical border of cells, along lateral and basal membranes of cells, and extracellular-like staining in the mesenchyme. During neurulation, CDA-3C2 showed differential staining in the ectoderm, distinguishing lateral from neural regions. Following closure of the neural tube, there was a striking specificity of expression of CDA-3C2 in the periphery, found almost exclusively in olfactory and otic epithelial structures. While CDA-3C2 is found in placode-derived tissues that subserve sensory transduction, it appears to be primarily associated with the supportive cells (and their secretions) in both otic and olfactory regions and less so with the sensory cells. The data suggest that a unique carbohydrate antigen on a large macromolecule may play a role in neurulation and/or morphogenesis of the placode-derived otic and olfactory structures.

Utricular otoconia of some amphibians have calcitic morphology

This report concerns the morphological features of otoconia removed from the inner ear of four amphibian species. Results from scanning electron microscopic examination are compared based on the site of origin. These results show that utricular otoconia have a mineral structure that mimics calcite, rather than the widely accepted idea that they are mineralized by calcium carbonate of the aragonite polymorph.

The fine structure of the developing otolithic organs of the rat

The fine structure of the otolithic organs in fetal and neonatal rats were studied by light and transmission electron microscopy. The otoconia were found at the 16.5 gestational day while the otolithic membrane appeared much later. Types II and I hair cells were first observed at the 16.5 and 18.5 gestational days, respectively. Secretory granules in the supporting cells seemed to release an organic material which was destined to be incorporated into the organic matrix of the otoconia and/or the otolithic membrane. Protrusions of apical cytoplasm, cytoplasmic globules and dilated rough endoplasmic reticulum (rER) were observed in epithelial cells of both sensory and non-sensory regions, in particular, in the transitional cells. By contrast, otoconia were mainly located above the neuroepithelial area. The functional roles of protrusions and cytoplasmic globules in the genesis of otoconia remain to be clarified. In addition, the pathway for transport of calcium remains obscure.

Malformation and degeneration in the inner ear of mos transgenic mice

Transgenic mice carrying the mos proto-oncogene linked to a retroviral transcriptional control sequence display behavioral abnormalities including circling, hyperactivity, head tilt, and bobbing. Axonal degeneration, neuronal chromatolysis, spongiform encephalopathy, gliosis, and inflammatory infiltrates are reportedly found in the central nervous systems of all mutants with the behavioral traits. Hearing was tested by means of broadband free-field rarefaction clicks with auditory brain stem response recorded between vertex and mouth electrodes. No detectable auditory response was elicited in transgenic animals, in contrast to five distinct positive peaks observed in littermate control animals. Light microscopic survey of temporal bone histopathology in mutants revealed extensive degeneration of the organ of Corti with loss of hair cells in all cochlear turns and loss of supporting cells and atrophy of spiral ganglion cells. The spiral limbus was deformed, with replacement of the usual convexity of the superior surface by a flattened trough configuration. Hair cells of the vestibular end organs appeared normal. Pathologic alteration in levels of mos transgene RNA appears to have a direct effect on the structural integrity of the inner ear.

Distribution of expression of 2AR (osteopontin) in the embryonic mouse inner ear revealed by in situ hybridisation

Using in situ hybridisation we have determined the distribution of expression of 2ar (also known as osteopontin, bone sialoprotein 1 or 44-kDa bone phosphoprotein) in the developing mouse inner ear. We have identified several discrete sites, both osteogenic and non-osteogenic, that express 2ar from embryonic day 16.5 (E16.5). In addition to the regions of developing bone of the calvaria and temporal bone, we have found 2ar expression in the epithelium of the sensory maculae (but not in the organ of Corti), in the vestibular and auditory ganglia and nerves (but not in the nerves that innervate the whiskers in the snout), in the epithelium that lines the endolymphatic sac (but not in the neighbouring and contiguous endolymphatic duct) and also in the epithelium that lines the semicircular canals. We found also individual cells scattered throughout the brain, loose mesenchyme and blood vessels of the head that were expressing 2ar. Several of the sites in the inner ear, for example the maculae and the endolymphatic sac, are known to be involved in the production of calcified matrix. The results extend the range of tissue types known to express the protein and demonstrate that tissues of histologically similar appearance can nonetheless differ in their gene expression.

Effects of dinocap on otolith development: evaluation of mouse and hamster fetuses at term

The morphology of otoliths in CD-1 mouse and Syrian hamster fetuses exposed to the fungicide dinocap were evaluated at the end of gestation. Pregnant mice were dosed by gavage with 0, 10, 15, 30, or 60 mg/kg/day dinocap in corn oil on days 7-16 of gestation. Pregnant hamsters were dosed by the same route with 0, 50, 100, or 200 mg/kg/day on days 7-14 of gestation. At the end of gestation (day 18 in mice, day 15 in hamsters) dams were killed and all fetuses were removed and fixed overnight in 70% ethanol. Fetal heads were then removed, left in 70% ethanol for at least 3 days, and then dehydrated in a graded ethanol series and cleared with methyl salicylate. Otoliths were examined by darkfield microscopy, and each otolith was scored for morphological completeness on a scale of 0 to 3. Otolith development was complete by day 18 of gestation in control mouse fetuses. Otolith development was complete in many, but not all, of the hamster fetuses by day 15 of gestation. In the mouse, dinocap exposure inhibited fetal otolith formation in a dose-related manner, with a significant effect on total otolith score occurring at 10 mg/kg/day and above. Dinocap affected otolith formation in the hamster only at 100 mg/kg/day (200 mg/kg/day was embryolethal), concomitant with severe maternotoxicity and fetotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Appearance and distribution of neuron-specific enolase and calbindin (CaBP 28 kDa) in the developing human inner ear

The onset and development of neuron-specific enolase (NSE) and calbindin immunoreactivities were studied in the inner ear of human fetuses aged from 6-7 to 14 weeks of gestation. NSE occurred very early in ganglion neurons. Its appearance in vestibular sensory cells at 8 weeks coincided with the formation of the first afferent synapses, and showed an apex/base gradient in the cristae. Calbindin was found in vestibular ganglion neurons at 6-7 weeks and in the cochlear ganglion neurons at 8-9 weeks. Vestibular sensory cells and the whole ventral wall of the cochlear duct were stained from 8-9 weeks. At 14 weeks, calbindin staining occurred only in the sensory cells of the cochlear neuroepithelium. Non-neuronal secretory structures, i.e. Kölliker's organ and some cells of the transitional zone of the utricle, were also reactive. Staining appeared in Kölliker's organ with a base to apex gradient and disappeared from it with an internal to external gradient. Calbindin appeared in vestibular sensory cells later than NSE staining, synapse formation and sensory hair bundle differentiation. By contrast in the cochlea, calbindin staining appeared in the neuroepithelium before sensory cell differentiation, but remained only in the hair cells after they had differentiated and been contacted by the afferent fibers.

Light- and electron-microscopic study of the endolymphatic sac of the tree frog, Hyla arborea japonica

The endolymphatic sac of the tree frog and its crystals were observed by light- and electron microscopy. Scanning electron microscopy revealed that the crystals have a faceted body and two pointed ends. Light- and transmission electron microscopy revealed that the endolymphatic sac is composed of many small chambers. In their lumina, numerous "ghosts" of crystals that resulted from decalcifications were observed. The ghosts were demarcated by a linear dense material or embedded in a flocculent substance. The epithelium of the endolymphatic sac is simple squamous or cuboidal and peculiar cytoplasmic granules are found in most cells. The granules are surrounded by a limiting membrane and have varying electron density. Some granules contain a core and/or tubular structures. Vacuoles containing large ghosts are also found in the epithelial cells. These ghosts were quite similar to those in the lumen and sometimes coexist with cell debris. The fine structure of the endolymphatic sac and its crystals is discussed.

Electron-microscopic study of the vestibular dark cells in the crista ampullaris of the guinea pig

The ultrastructure of the vestibular dark cells in the crista ampullaris of the guinea pig was observed using both transmission and scanning electron microscopy. The dark cells had numerous vacuoles of varying size and electron density, and also characteristic well-developed basal infoldings. These findings strongly suggest that the dark cells play an important role in fluid transport. Unique meshwork structures were observed on the luminal surface of the dark cells. Otoconia showing varying degree of degeneration were occasionally recognized on and near these structures. Electron microscopy revealed that the meshwork was comprised of cytoplasmic processes in a reticular arrangement. They seem to be engaged in the metabolism of otoconia, and perhaps also in fluid transport.

Mineralization in the newborn rat statoconia. An EDAX study

Several types of statoconia, differing in size and shape, have been observed in macula utriculi, the smaller ones in the macular periphery. The larger statoconia seem to be formed by the association of smaller units and calcium presence is comparatively higher: In some parts of the marginal zone there exists a granular material in which the presence of calcium is significant.

A scanning electron microscopy study on human otoconia genesis

The genesis of the otoconia was studied with SEM in several human fetuses. The results of the present paper together with previous reports (1, 2, 13) support the hypothesis that otoconia genesis may develop according the following stages: (1) calcification of some sensory and supporting cells; (2) extrusion of the calcified cellular material and some organic substances in "sponge-like bodies"; (3) deposition of diverse core seeds in these areas giving rise to a random calcium carbonate crystallization and incorporation of matrix organic material; (4) growth of otoconia developing linearly with a gradual change in shape from spherical to ovoid, rhombohedral, and cylindrical forms; (5) inhibition of crystal growth occurring once a critical unsaturated calcium endolymph state and an adequate size of statoconial membranous chambers are reached.