The statoconial membrane of the guinea pig utricular macula. Scanning electron microscopic investigation combined with the freeze-fracturing technique

The superstructure of the guinea pig utricular macula was investigated using scanning electron microscopy combined with the freeze-fracturing technique. The statoconial membrane was composed of the otoconial layer, otolithic membrane and subcupular meshwork. The otolithic membrane consisted of closely arranged filaments with beaded appearance and densely packed globular matrix near the otoconial layer. The subcupular meshwork consisted of long branching filaments cross-bridged to one another. The filaments were continuous with those of the otolithic membrane on one side and with the surface of the epithelium on the other, which fills the space between the otolithic membrane and the macular surface. The otolithic membrane would function as a rigid plate and equally distribute the shear force of the otoconial layer to all the ciliary bundles. The subcupular meshwork may play an important role in transmitting the shear strain of the otolithic membrane to all the ciliary bundles and may also exert an additional dampening effect to prevent unwanted vibration.

Morphometry of otoliths in chicken macula lagena

The macula lagena located at the apical end of the cochlea in birds is characterized by the presence of numerous otoliths with unclear sensory functions. These otoliths are reported to be similar to those in the vestibular system but their detailed features in morphology are unknown. In the present study, we examined the number, size and shape of otoliths from the macula lagena in Chinese domestic chickens (Gallus Ling Nan) with a scanning electron microscope for morphometry. For chickens aged 10-15 post-hatch days, the otoliths in each macula lagena were counted to be 16,055 +/- 4038 (mean +/- S.D., n = 4). The average length and width were 12.98 +/- 3.70 microm and 5.10 +/- 1.48 microm (n = 526 otoliths), respectively. The ratio of length to width for the otolith was 2.58 +/- 0.39 (n = 526 otoliths) and remained relatively constant despite their variations in physical size. Almost all the otoliths were in regular shape and appeared like isolated cylinders with smooth facets at each end, but a few of them (0.025% of 64,221 otoliths screened) were found to be in odd shapes, such as T-shape and cross-shape. The results suggest that otoliths in the macula lagena and those in the vestibular system of bird's inner ear have similar physical properties and may play a similar role in sensing gravitational and acceleration signals.

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.

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.

No correlation between multilamellar bodies in the inner ear and further organs of mutant (backstroke, bks) and wildtype zebrafish embryos

The origin of the proteinacious matrix of the inner ear stones (otoliths) of vertebrates has not yet been clarified. Using the backstroke mutant (bks) of the zebrafish Danio rerio, which is characterized by a complete lack of otoliths, we searched for possibly missing or aberrant structural components within the macular epithelia of the inner ears of embryos on the ultrastructural level. Numerous multilamellar bodies (MLBs) were found. The MLBs were, however, not restricted to the inner ears of mutants but were also found in wildtype individuals and in further organs such as brain and liver. MLBs have hitherto never been described from the inner ear of fish and are generally estimated to be rare structures. Their occurrence in fish liver can, however, be induced by using particular chemical substances, which seem to effect adaptive compensatory processes on the cellular level. Such a chemical treatment also affects the ultrastructure of further organelles. Since the occurrence of MLBs in the liver of zebrafish was not accompanied by an alteration of the morphology of other organelles, their occurrence seems not to be due to environmental stress. The findings indicate that the MLBs cannot be correlated with bks-inherent features as well as with missing otolith development/growth. Since the occurrence of MLBs was independent from the developmental stage of a specimen and its overall tissue preservation, it can moreover be excluded that these MLBs merely represent fixation artifacts. Their presence more likely indicates cellular remodelling processes of hitherto unknown significance.

Ultrastructural analysis of [3H]thymidine-labeled cells in the rat utricular macula

Ototoxic drugs stimulate cell proliferation in adult rat vestibular sensory epithelia, as does the infusion of transforming growth factor alpha (TGFalpha) plus insulin. We sought to determine whether new hair cells can be regenerated by means of a mitotic pathway. Previously, studies have shown that the nuclei of some newly generated cells are located in the lumenal half of the sensory epithelium, suggesting that some may be newly generated sensory hair cells. The aim of this study was to examine the ultrastructural characteristics of newly proliferated cells after TGFalpha stimulation and/or aminoglycoside damage in the utricular sensory epithelium of the adult rat. The cell proliferation marker tritiated-thymidine was infused, with or without TGFalpha plus insulin, into the inner ears of normal or aminoglycoside-damaged rats for 3 or 7 days by means of osmotic pumps. Autoradiographic techniques and light microscopy were used to identify cells synthesizing DNA. Sections with labeled cells were re-embedded, processed for transmission electron microscopy, and the ultrastructural characteristics of the labeled cells were examined. The following five classes of tritiated-thymidine labeled cells were identified in the sensory epithelium: (1) labeled cells with synaptic specializations that appeared to be newly generated hair cells, (2) labeled supporting cells, (3) labeled leukocytes, (4) labeled cells that we have classified as "active cells" in that they are relatively nondescript but contain massive numbers of polyribosomes, and (5) labeled degenerating hair cells. These findings suggest that new hair cells can be generated in situ by means of a mitotic mechanism in the vestibular sensory epithelium of adult mammals.

Localization of the two constitutively expressed nitric oxide synthase isoforms (nNOS and eNOS) in the same cell types in the saccule maculae of the frog Rana pipiens by immunoelectron microscopy: evidence for a back-up system?

There is growing evidence for a nitric oxide/cyclic GMP pathway of signal transduction in the vestibular system. Recently, two isoforms of nitric oxide (NO) synthase (nNOS and eNOS) and NO itself have been identified at the light microscopic level in the vestibulocochlear system of mice using specific antibodies and a new fluorescence indicator. In order to acquire more information about signal transduction and tissue modulation in this neuroepithelium at the cellular and subcellular levels, ultrathin sections of London Resin White-embedded saccule maculae of the frog Rana pipiens were incubated with various concentrations of commercially available antibodies to nNOS and eNOS. The immunoreactivity was visualized by a gold-labelled secondary antibody and the amount of the immunoreactions per microm2 was quantified for the different cell types and subcellular regions. Significant eNOS immunoreactivity was identified in the hair bundles, cuticular plates and the rest of the cytoplasm of the hair cells as well as in different subcellular regions of the supporting cells. Gold-labelled anti-nNOS antibodies stained mainly stereovilli and cuticular structures of hair cells and supporting cells, whereas the number of the immunoreactions in the remaining cytoplasm of both cell types was near the background level. The spatial co-localization of the two NOS isotypes in the same cell regions of hair cells and supporting cells was confirmed in double-labelling experiments. The immunocytochemical findings are suggestive of a redundant system in which one NOS isoform can (partially) replace the other. The different subcellular localization of the NOS isoforms may allow for isoform specific regulation of NOS activity by different Ca2+ currents at the subcellular level, underlining the importance of NO-regulated processes in neuroepithelia of the inner ear.

Extracellular matrices associated with the apical surfaces of sensory epithelia in the inner ear: molecular and structural diversity

The ultrastructure and molecular composition of the extracellular matrices that are associated with the apical surfaces of the mechanosensory epithelia in the mouse inner ear are compared. A progressive increase in molecular and structural organization is observed, with the cupula being the simplest, the otoconial membrane exhibiting an intermediate degree of complexity, and the tectorial membrane being the most elaborate of the three matrices. These differences may reflect changes that occurred in the acellular membranes of the inner ear as a mammalian hearing organ arose during evolution from a simple equilibrium receptor. A comparison of the molecular composition of the acellular membranes in the chick inner ear suggests the auditory epithelium and the striolar region of the maculae are homologous, indicating the basilar papilla may have evolved from the striolar region of an otolithic organ. A comparison of the tectorial membranes in the chick cochlear duct and the mouse cochlea reveals differences in the structure of the noncollagenous matrix in the two species that may result from differences in the stochiometry of alpha- and beta-tectorin and/or differences in the post-translational modification of alpha-tectorin. This comparison also indicates that the appearance of collagen in the mammalian tectorial membrane may have been a major step in the evolution of an electromechanically tuned vertebrate hearing organ that operates over an extended frequency range.

Sciaenid inner ears: a study in diversity

Sciaenid fishes (Family Sciaenidae) could potentially serve as models for understanding the relationship between structure and function in the teleost auditory system, as they show a broad range of variation in not only the structure of the ear but also in the relationship between the ear and swim bladder. In this study, scanning electron microscopy (SEM) was used to investigate inner ear ultrastructure of the Atlantic croaker (Micropogonias undulatus), spotted seatrout (Cynoscion nebulosus), kingfish (Menticirrhus americanus) and spot (Leiostomus xanthurus). These species reflect the diversity of otolith and swim bladder morphology in sciaenids. The distribution of different hair cell bundle types, as well as hair cell orientation patterns on the saccular and lagenar maculae of these fishes were similar to one another. The rostral ends of the saccular sensory epithelia (maculae) were highly expanded in a dorsal-ventral direction in the Atlantic croaker and spotted seatrout as compared to the kingfish and spot. Also, ciliary bundles of the saccular maculae contained more stereocilia in the Atlantic croaker and spotted seatrout as compared with kingfish and spot. The shapes of the lagenar maculae were similar in all four species. In the Atlantic croaker and spotted seatrout lagenar maculae, the number of stereocilia per bundle was greater than those for the kingfish and spot. Given that saccular macula shape and numbers of stereocilia per bundle correlate with swim bladder proximity to the ear in the studied species, it is possible that inner ear ultrastructure could be indicative of auditory ability in fishes.

Complex vestibular macular anatomical relationships need a synthetic approach

Mammalian vestibular maculae are anatomically organized for complex parallel processing of linear acceleration information. Anatomical findings in rat maculae are provided in order to underscore this complexity, which is little understood functionally. This report emphasizes that a synthetic approach is critical to understanding how maculae function and the kind of information they conduct to the brain.

The inner ear macular sensory epithelia of the Daubenton's bat

The inner ear macular sensory epithelia of the Daubenton's bat were examined quantitatively to estimate the area and total number of hair cells. Ultrastructural examination of the sensory epithelium reveals two main types of hair cells: the chalice-innervated hair cell and the bouton-innervated hair cell. The existence of an intermediate type, with a nerve ending covering the lateral side of the hair cell, indicates that the chalice-innervated hair cells are derived from bouton-innervated hair cells. Thus, at least a part of the bouton-innervated hair cells forms a transitional stage. A number of immature as well as apoptotic hair cells were observed. It is suggested that a continuous production of new hair cells takes place in mature individuals, probably based on transdifferentiation of supporting cells.

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.

Vesicular bodies in fish maculae are artifacts not contributing to otolith growth

The presence, morphology and possible origin of vesicle-like bodies (VBs) within the inner ear macular otolithic membrane of developmental stages of cichlid fish Oreochromis mossambicus and neonate (i.e. functionally fully developed except the reproductive organs) swordtail fish Xiphophorus helleri were analyzed by means of transmission and scanning electron microscopy (TEM and SEM, respectively) employing various fixation procedures. Some authors believe that these VBs are involved in the formation of the organic phase of inner ear otoliths (or statoliths in birds and mammals). Decreasing the osmolarity of the fixation medium from a value rather close to that of native fresh water fish tissue (i.e. 250 mOsm and 290--300 mOsm, respectively) to a value of fixatives mostly employed in TEM studies (ca. 190 mOsm), the amount of VBs increased and the components of sensory inner ear tissue increasingly dilated. Whilst a conventional prefixation with aldehydes followed by osmium tetroxide postfixation yielded numerous VBs, only few of them were observed when the tissue was fixed with aldehydes and osmium tetroxide simultaneously. Therefore, the results demonstrate that inner ear sensory epithelia are extremely sensitive to altering fixation media. On this background it must be concluded that VBs are fixative (i.e. glutaraldehyde) induced artificial structures, so-called membrane blisters. Thus, the protein matrix of otoliths (and possibly that of statoliths in higher vertebrates) is rather provided by secretion processes than by the release of vesicles.

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.

Ultrastructural localization of GABA-like immunoreactivity in the human utricular macula

In the vertebrate vestibular periphery, gamma-aminobutyric acid (GABA) has long been presumed to be a neurotransmitter candidate. However, experimental reports about the localization and function of GABA in the vestibular systems of vertebrates are contradictory. In addition, there is no information in the literature concerning the localization of GABA in the human vestibular periphery. The present study investigates the ultrastructural localization of GABA-like immunoreactivity in the human utricular macula. A modified pre-embedding immunostaining electron microscopy technique was applied using two different commercially available polyclonal antibodies to GABA. GABA-like immunoreactivity is confined to the vesiculated nerve fibers and terminals of the human vestibular neurosensory epithelia. The GABA-containing nerve terminals make asymmetrical axo-dendritic synapses with the afferent chalices surrounding the type I sensory hair cells. Type I and type II hair cells as well as afferent chalices are devoid of GABA-like immunoreactive staining. The present study demonstrates that GABA exists in the human vestibular periphery, and that GABA is a neurotransmitter candidate of the human efferent vestibular system.

Ultrastructural localization of ChAT-like immunoreactivity in the human vestibular periphery

Acetylcholine (ACh) has long been considered a neurotransmitter candidate in the efferent vestibular system of mammals. Recently, choline acetyltransferase (ChAT), the synthesizing enzyme for ACh, was immunocytochemically localized in all five end-organs of the rat vestibule (Kong et al. (1994) Hear. Res. 75, 192-200). However, there is little information in the literature concerning the cholinergic innervation in the vestibular periphery of man. In the present study the ultrastructural localization of the ChAT-like immunoreactivity in the human vestibular periphery was investigated in order to reveal the cholinergic innervation in the human vestibular end-organs. A modified method of pre-embedding immunoelectron microscopy was applied. It was found that the ChAT-like immunoreactivity was located in the bouton-type vesiculated nerve terminals in the vestibular neurosensory epithelia of man. These ChAT-like immunostained nerve terminals make synaptic contacts either with afferent chalices surrounding type I vestibular sensory hair cells, or with type II vestibular sensory hair cells. These results show that the ChAT-like immunoreactivity in the human vestibular periphery is confined to the efferent vestibular system. The ChAT-containing efferents innervate both type I hair cells and type II hair cells, making postsynaptic and presynaptic contacts, respectively. This study presents evidence that ACh is a neurotransmitter candidate in the efferent vestibular system of man.

Effect of cisplatin on the basement membrane anionic sites in the ampulla, macula, and stria vascularis of guinea pigs

Our objective was to compare changes in the basement membrane anionic sites (BMAs) in the ampulla, macula, and stria vascularis following the infusion of cisplatin (CDDP). After CDDP was administered to anesthetized Hartley guinea pigs, the bony labyrinth was immersed in a solution of polyethyleneimine (PEI). The size and distribution of PEI particles associated with BMAs in the stria vascularis and in the dark cell and sensory cell areas of the vestibular labyrinth were determined by electron microscopy. A significant reduction in the number and size of PEI particles was observed on CDDP-treated strial vessels. The number and size of PEI particles on the basement membranes of the vestibular labyrinth did not differ from those in the control. Our findings suggest that the BMAs of the vestibular labyrinth were not significantly affected by the administration of a single dose of CDDP.

Compartmentalized vesicular traffic around the hair cell cuticular plate

Through thin-section and freeze-fracture electron microscopy, we identify structural correlates of an intense vesicular traffic in a narrow band of cytoplasm around the cuticular plate of the bullfrog vestibular hair cells. Myriads of coated and uncoated vesicles associated with longitudinally oriented microtubules populate the narrow cytoplasmic region between the cuticular plate and the actin network of the apical junctional belt. If microtubules in the sensory hair cells, like those in axons, are pathways for organelle transport, then the characteristic distribution of microtubules around the cuticular plate represents transport pathways across the apical region of the hair cells. This compartmentalized membrane traffic system appears to support an intense vesicular release and uptake along a band of apical plasma membrane near the cell border. Functions of this transport system may include membrane recycling as well as exocytotic and endocytotic exchange between the hair cell cytoplasm and the endolymphatic compartment.

Morphological evidence for local microcircuits in rat vestibular maculae

Previous studies suggested that intramacular, unmyelinated segments of vestibular afferent nerve fibers and their large afferent endings (calyces) on type I hair cells branch. Many of the branches (processes) contain vesicles and are presynaptic to type II hair cells, other processes, intramacular nerve fibers, and calyces. This study used serial section transmission electron microscopy and three-dimensional reconstruction methods to document the origins and distributions of presynaptic processes of afferents in the medial part of the adult rat utricular macula. The ultrastructural research focused on presynaptic processes whose origin and termination could be observed in a single micrograph. Results showed that calyces had 1) vesiculated, spine-like processes that invaginated type I cells and 2) other, elongate processes that ended on type II cells pre- as well as postsynaptically. Intramacular, unmyelinated segments of afferent nerve fibers gave origin to branches that were presynaptic to type II cells, calyces, calyceal processes, and other nerve fibers in the macula. Synapses with type II cells occurred opposite subsynaptic cisternae (C synapses); all other synapses were asymmetric. Vesicles were pleomorphic but were differentially distributed according to process origin. Small, clear-centered vesicles, approximately 40-60 nm in diameter, predominated in processes originating from afferent nerve fibers and basal parts of calyces. Larger vesicles approximately 70-120 nm in diameter having approximately 40-80 nm electron-opaque cores were dominant in processes originating from the necks of calyces. Results are interpreted to indicate the existence of a complex system of intrinsic feedforward (postsynaptic)-feedback (presynaptic) connections in a network of direct and local microcircuits. The morphological findings support the concept that maculae dynamically preprocess linear acceleratory information before its transmission to the central nervous system.

[Ultrastructure and peculiarities of the otolith lagena in pigeons]

Rosenhall reported the polarization of kinocilium of otolith organs in the avian inner ear by LM. However, the otolith lagena, which is called the third otolith organ, is not well known, especially in terms of the 3-dimensional relationship between each maculae (utricular maculae, saccular maculae, maculae of the otolith lagena), the details of the striola and otoconial layers, and so on. Therefore, the author conducted a study to clarify these points using 20 Columba Domestica pigeons (40 ears), under the rules for animal experiments established by Nihon University School of Medicine. The pigeons were divided into 4 groups, 1. observations of the membranous labyrinth with binocular microscopes, 2. histological examination of serial sections of inner ear, 3. observations of the otolith lagena by SEM, 4. computer-aided 3-dimensional reconstruction of the membranous labyrinth. The following results were obtained. 1. the mean angle between the utricular maculae and maculae of the otolith lagena was 31 degrees (n = 3), the mean angle between the saccular maculae and maculae of the otolith lagena was 45 degrees (n = 3). 2. striola of the otolith lagena demonstrated a C form and the kinocilium exhibited an orientation identical to that of the striola of the outer saccular maculae, 3. the otolithic membrane of the otolith lagena demonstrated a mesh form and the otoconial layer was observed to be thin above the striola. 4. the surface area of the maculae of the otolith lagena was 0.98mm2 (n = 3) and the number of sensory cells was 16,800 (n = 3). The author also considered the functions of the otolith lagena.

Comparative cytoskeletal analyses of the inner ear in man and the squirrel monkey

Serially sectioned human and squirrel monkey labyrinths were analyzed with high-resolution light microscopy after using 25 different monoclonal antibodies (mAbs) identifying all three main classes of cytoskeletal proteins. A high degree of similarity was found in labyrinths from man and squirrel monkey. Only 1 of 25 mAbs stained differently between the two species. In the squirrel monkey but not in the human the mAbs identifying S-100 proteins stained subpopulations of type I vestibular hair cells in the striola of the two macula and the summit of the cristae as compared to the same type of hair cells in the periphery of vestibular organs. Such an establishment of subpopulations of hair cells with the same ultrastructure has previously not been described in higher vertebrates. In contrast to the species differences in the distribution of neuroactive substances, the cytoskeletal architecture seems to be relatively unchanged and stable during evolution. Since each species has its own hearing and equilibrium function, neurotransmitters (neuropeptides, amino acids, etc.) could contribute to such species-specific functions.

Scanning electron microscopic evaluation of age-related changes in the rat vestibular epithelium

An ultrastructural study was performed to assess age-related changes in the vestibular end organs of Fischer 344 rats. The surfaces of the maculae and cristae from 3-, 12-, and 24-month-old Fischer 344 rats were observed by use of scanning electron microscopy. Age-related changes in the morphology of the vestibular neuroepithelium included a substantial loss of hair cells, as well as a reduction in the number of kinocilia and stereocilia on those that remained. These changes were greatest in the central upper regions of the three ampullae. In aged animals a greater area of the neuroepithelial surface was covered with microvilli, and in some instances, giant cilia were found among the microvillous surfaces. In contrast, there were few differences among the three age groups in the number and condition of hair cells in the saccules and utricles. The changes observed in the cristae may contribute to the age-associated impairment of vestibular function. If similar changes occur in human beings, these could in part account for the presbycusis observed in the elderly.

Recovery mechanism of postural disturbance after vestibular neurectomy

Sectioning of the unilateral vestibular nerve of the bull frog resulted in tilting of the head and the body. The angle of head tilt was periodically measured. The angle returned to 0 degree in an average of 7.3 weeks. A morphological study demonstrated normal structure of the sensory epithelia and a well-regenerated nerve bundle. Also, discrete action potentials could be recorded from the nerve centrally to the cut site of the vestibular nerve. These results indicate that recovery from the tilting involves two mechanisms, one being central compensation and the other reactivation of the sensory cell-nerve unit.

Immunoelectron microscopy of the human inner ear

The immediate fixation required for satisfactory morphologic preservation of the human inner ear has not only limited the ultrastructural study of this region, but has also limited the application of immunohistochemistry. The technique of postembedding protein A-colloidal gold immunoelectron microscopy was used on human inner ear tissue taken at operation and on celloidin-embedded temporal bone sections from a traditional temporal bone bank. We describe the utility of postembedding immunoelectron microscopy for studying the localization of a wide variety of antigens including type I collagen, S-100, and calcitonin gene-related peptide. The use of this method in material available in temporal bone banks or that has been routinely processed for transmission electron microscopy provides the potential for broad application to collections of otologic material.

High-voltage electron microscopic observations on the suprastructure of the macula utricle

Thick sections of the suprastructure of the utricular macula of the guinea pig were observed by high-voltage electron microscope. Whole views and the relationship of the otoconia, the otoconial membrane and the sensory ciliary bundles became very clear. In addition, stereo micrographs were even more helpful for simultaneously recognizing this three-dimensional relationship.

Carbohydrates of the guinea pig vestibular supporting cells

Ultrastructural localization of glycoconjugates with special reference to the synthesizing process was studied in the guinea pig vestibular supporting cells using the tannic acid and ruthenium red staining technique. The extracellular glycoconjugate (glycocalyx) was well stained by the ruthenium red which may be synthesized by GERL complex. The supporting cell has a numerous number of granules which also contain tannic acid positive glycoconjugate. These granules were closely related to the Golgi complex as well as the apical cell surface. It may be indicated that the glycoconjugate of the supporting cells is glycosylated at the Golgi complex, stored in the granules, transported, secreted, and form otoconial membrane and the cupula.

Functional aspects of carbohydrate complex of the statoconial membrane of the guinea pig utricular macula

A functional aspect of the carbohydrate complex of the statoconial membrane of the guinea pig utricular macula was observed by the use of the lectins as well as the ruthenium red staining techniques. The ruthenium red staining technique visualized the whole structure of the statoconial membrane as well as the contact zone between the sensory epithelium and the statoconial membrane. The statoconial membrane is composed of the otoconial layer, gelatinous layer, and subcupular meshwork. The otoconial layer was interconnected with sensory epithelium by the gelatinous layer and subcupular meshwork which formed honeycomb structure and housed the sensory hair bundle. This may allow the free movement of the sensory hair bundle. The gelatinous layer and the subcupular meshwork contain N-acetylglucosamine, mannose, and galactose which form acidic mucopolysaccharide and glycoprotein. It has been indicated that these carbohydrate complex may play an important role for the mechanical coupling not only between the statoconial membrane and the sensory epithelium but also between the sensory hair bundle and the statoconial membrane.

Morphological evidence for parallel processing of information in rat macula

Study of montages, tracings and reconstructions prepared from a series of 570 consecutive ultrathin sections shows that rat maculas are morphologically organized for parallel processing of linear acceleratory information. Type II cells of one terminal field distribute information to neighboring terminals as well. The findings are examined in light of physiological data which indicate that macular receptor fields have a preferred directional vector, and are interpreted by analogy to a computer technology known as an information network.

Morphological changes in the vestibular epithelia and ganglion induced by ototoxic drug

The morphological changes of the vestibular sensory epithelia and the vestibular ganglions induced by Gentamicin(GM) were investigated using scanning electron microscope, transmission electron microscope and light microscope. The guinea pigs were injected with a single application of 4 mg (0.1ml) of GM into the middle ear through the tympanic membrane. The vestibular organs and the ganglions were observed up to 6 months after the treatment. Four days after the injection, fused, ballooned and missing cilia were observed in the vestibular sensory epithelia. These changes progressed and extended toward the periphery of the crista and the macula. The changes of the vestibular ganglions were first observed one month after the treatment. The degenerative process started from destruction of the mitochondrial cristae and vacuolization of the cytoplasm in the Schwann cell. The next step of the change was dissociation of the myelin sheath around the ganglion cell. The cytoplasmic organelles in the ganglion cell gradually deteriorated. At the later stage, the myelin sheath around the ganglion cell disappeared and the number of the cell reduced. Furthermore, the myelin sheath of the nerve fiber was dissociated. In this study the signs of the vestibular ganglion damage were later than that of the vestibular organ. However, we thought the changes in the ganglion are probably due to direct influence of GM, since the degeneration was found to develop in a relatively short period.

The morphological changes in the vestibular sensory epithelia following electrical stimulation

The morphological changes of the vestibular sensory epithelia of the guinea pig following electrical stimulation were investigated using scanning electron microscope. Positive and negative square wave pulse stimulation was given through a silver ball electrode placed on the round window membrane for one hour. The current intensities used were 100, 200 and 300 microA. While the direct current stimulation at intensities of 100 or 200 microA did not cause any significant changes, severe damage of the utricular macula and the ampullar crista of the lateral semicircular canal was observed at 300 microA. The degenerative changes such as fusion of sensory hairs, protrusion of the cuticular plate and loss of sensory cells were found on both the utricle and the semicircular canal. In the most severely damaged area, the sensory epithelial surface was badly torn apart. In the clinical application of direct current to the inner ear for relieving tinnitus, special attention should be paid to the vestibular organ.

Sensory and nonsensory ciliated cells in the ear of the sea lamprey, Petromyzon marinus

The inner ear of the sea lamprey, Petromyzon marinus, was examined using scanning and transmission electron microscopy. Many of the nonsensory surfaces of the ear chamber are lined by numerous, noninnervated, multiciliated epithelial cells. Each multiciliated epithelial cell has 43-66 true cilia projecting from its apical surface into the lumen of the ear. Although the cilia leave the cell individually, all of the cilia from a single cell come together just above the apical cell surface and are held together by a cross-network of fibrillar material. The cell bodies of the multiciliated cells sit upon a basal lamina which overlies a collagen-filled matrix. Petromyzon has typical vertebrate sensory hair cells on the cristae of the two semicircular canals as well as on the main sensory epithelium, the macula communis. Cell bodies of the sensory hair cells are similar to hair cells of other vertebrates. However, unlike other fishes, the sensory hair cells in Petromyzon have striated organelles between the nucleus and the apical cell membrane. The hair cells are innervated by afferent and efferent nerve fibers.

Electron-microscopic observations of the gravity receptor epithelia of normal and spinner juvenile Octopus maya

Light and electron microscopy of the gravity receptor epithelia (maculae) of statocysts of normal and "spinner" juvenile Octopus maya showed differences between the structures of the hair cells, supporting cells, and afferent neurons of these cephalopods. The maculae of spinner animals were approximately 30% smaller in their surface area and had 40% fewer hair cells. Moreover, the average distance between randomly-chosen hair bundles in scanning electron micrographs of maculae of normal animals was significantly greater (4.33 +/- 6.47 microns) than those of maculae of spinner animals (3.38 +/- 4.90 microns; P less than 0.0001). The sectional area of the supporting cell's microvilli in spinner maculae was larger (0.16 +/- 0.18 microns) than those of normal (0.10 +/- 0.10 micron; P less than 0.0001) O. maya. The morphological differences observed between certain structural components of the maculae of normal and spinner O. maya may be related to the absence and/or malformation of the neuroepithelial suprastructures in spinners. This may have direct or indirect effects to their inability to orient to gravity with these organs.

Three-dimensional structures of the vestibular sensory epithelia

Vestibular neurosensory epithelia of the guinea pig and the bull frog were investigated by scanning electron microscopy. The crista ampullaris or macula were freeze fractured followed by maceration with 0.1% OsO4 solution for 24-60 h (osmium-DMSO-osmium method). Following this, three-dimensional intracellular structures were observed. The mitochondria which exist in the nerve chalice surrounding the type I cell were various in shape, globular, long and slender. Golgi apparatus, endoplasmic reticulum and thin fibrous structures of the sensory cells or supporting cells were clearly demonstrated. Concerning nervous system, nerve fibre, afferent and efferent nerve endings, further synaptic structures were also observed stereoscopically.

Development of the utricular macula in the mouse

There have been many studies reporting on the development of the utricular macula using transmission electron microscopy. This study demonstrates the morphological development of the utricular macula in the mouse using scanning electron microscopy. Utricular maculae from CBA/CBA mice were studied from the 11.5th gestational day to 2 weeks after birth. Between the 11.5th and 12.5th gestational day there are numerous microvilli and a primary cilium on the surface of the otocyst. Sensory hairs appear on the 13.5th gestational day. The kinocilium and stereocilia are short and the latter of almost equal in length. The polarity of the kinocilium is first noticed after the 16.5th gestational day. The formation of otoconia starts on the 14.5th gestational day. They are shaped like a dumb-bell and are oval and columnar. Otoconia with a hexagonal form are recognized after the 16.5th gestational day.

[Structural organization and orientation of hair cells of the macula of the saccule of the membranous labyrinth of Rana temporaria]

Morphological peculiarities of the hair cells in the saccular macula of the internal ear have been studied by means of scanning electron microscopy in the Rana temporaria. Two main types of the hair cells are described: central and marginal. Cineciliary poles of the hair cells are oriented in two mutually opposite directions, in consequence, the macula is divided into two orientation zones.

The human vestibular organs observed by scanning electron microscopy

In many of the reports dealing with disorders of the human inner ear the histology comes from temporal bone dissection. In the past decade the scanning electron microscope has changed, to a degree, the approach to inner ear investigation. However, many problems remain with adequate preservation of specimens from human temporal bones. Ordinarily, human material is obtained from cadavers at autopsy as soon as possible after death, and postmortem changes are evident. At the Otologic Medical Group, Inc., in Los Angeles, many translabyrinthine operations for acoustic neuromas are performed. This report covers findings from vestibular specimens obtained during surgery for removal of acoustic neuromas (four cases) and during eighth nerve surgery (one case).

A study of the development of utricular and saccular maculae in man and in rat

The ultrastructural development of utricular and saccular maculae in man and rat is described. In the human fetus with a crown-rump length (CRL) of 50 mm, the utricular and saccular epithelia consist of a pseudostratified layer of dark and light cells without nerve endings. In human fetuses of 110 and 220 mm CRL, as well as in 17- and 18-day-old rat embryos, it is already possible to identify sensory and supporting cells in various stages of differentiation. Nerve endings can also be found. Ciliogenesis is present in both sensory and supporting cells of the 17- and 18-day-old rat embryos. During epithelial differentiation a heavy calcium deposition occurs on vestibular cells of both man and rat. Otoconial mineralization increases gradually as epithelial maturation advances. Phagocytosis of otoconia by free macrophages occurs in 50-mm CRL human fetuses. Vestibular nerve myelination is asynchronous and is present in 110-mm CRL human fetuses and in 30-minute postpartum rats.

The aging vestibular hair cell

The ultrastructure of aging vestibular hair cells of the guinea pig can be abnormal even though they appear morphologically normal by light microscopy. In only a few of the hair cells studied did nonspecific changes occur. In general, the age-related changes involved specific structures in the hair cells: aggregations of lipofuscin pigments, multivesiculated bodies, disintegration of the cuticular plate, and rod-shaped inclusions from the cuticle area into the hair cell. Furthermore, sensory hairs are generally present when the cuticular plate shows an advanced disintegration. Nerve calyces can be ultrastructurally changed without morphologic changes in the adjacent hair cell. Type I hair cells show age-related morphologic changes more often than do type II hair cells.

The surface structures of the human vestibular apparatus

The surface of the human vestibular apparatus has been examined in the scanning electron microscope following early post mortem fixation. Preservation of the fine structures of the maculae is good but of the cristae of the semi-circular canals is only fair. The surface features of the maculae do not differ from those seen in experimental animals. There appears to be an age related loss of ciliated sensory cells from the cristae. This might explain some of the age related changes in vestibular function.

Ototoxic effect of quaternary ammonium compounds

In earlier investigations by the author it has been shown that chlorhexidine, when introduced into the middle ear of guinea pigs, caused serious damage to the inner ear. The present investigation was performed in order to study if the quaternary ammonium compounds benzethonium chloride and benzalkonium chloride, frequently used for skin disinfection, can also induce inner ear damage when introduced into the tympanic cavity of guinea pigs. The disinfectants in question, at a concentration of 0.1% and in a solution of aqua dest. or 70% alcohol, were introduced into the animals' middle ear for exposure times of 10, 30, or 60 min. The animals were sacrificed 2 or 9 weeks after the exposure and the organ of Corti and vestibular neuroepithelia were studied as surface preparations with phase contrast microscopy. It was found that most of the ears exposed to the disinfectants had suffered damage, affecting both the vestibular and cochlear parts of the inner ear. The extent of the damage was related both to the duration of exposure and to the length of the animals' survival after the exposure. Furthermore it was found that the tympanic cavity and the perilymphatic spaces of vestibulum and cochlea were pathologically changed.

Vaterite otoconia in two cases of otoconial membrane dysplasia

Scanning electron microscopy, electron microprobe analysis, and x-ray powder diffraction were used to study temporal bone specimens obtained at autopsy from an infant with Potter syndrome and from a second trimester fetus, which was the product of an elective abortion. The mothers of both the infant and fetus were juvenile-onset rheumatoid arthritis patients who took prostaglandin inhibitors during pregnancy. The infant's external ears were low set and the left ear canal was stenotic. The vestibular maculae on the left were covered by aberrant otoconia composed of vaterite. In the right inner ear, otoconia were entirely absent, although the gelatinous otoconial membranes were intact. Only the left saccule and right utricle from the fetus were studied; both contained vaterite crystals similar to those in the infant. In addition, apatite was present in the fetal utricle, apparently lying on the macula beneath the vaterite otoconia.

Fine structure of the afferent synapse and gap junctions on the sensory hair cell in the saccular macula of goldfish: a freeze-fracture study

Membrane specializations at the active zone of the afferent synapse in the saccular macula of goldfish are described. Those of the presynaptic membrane consist of three to six elongated aggregates of intramembrane particles separated by particle-free furrows on the concave part of the P face and its complementary figure, that is, an alternate arrangement of elongated aggregates of pits and smooth ridges on the convex part of the E face. The size of the specialized area is about 0.5 micrometer x 0.3 micrometer. Vesicle fusion sites are situated at the margin of the particle-free furrow and ridge of the presynaptic active zone. Round pores about 30-50 nm in diameter are seen on the P face around the active zone. They are probably openings of the anastomosing tubules or coated pits. A focal aggregate of intramembrane particles is observed on the E face of the postsynaptic membrane apposing the presynaptic active zone. The P face of the postsynaptic active zone shows pits and particles. Small gap junctions are found between hair cells and adjacent supporting cells. They are frequently associated with desmosomes. The possible functional significance of these gap junctions is discussed.

Gap junctions between sensory and supporting cells of the utricular and saccular maculae in Anolis carolinensis examined by transmission electron microscopy

Gap junctions, related to projections of sensory cells into supporting cells, occur from supranuclear to basal levels on hair cells, in both saccular and utricular maculae of the lizard, Anolis carolinensis. The larger numbers of junctional projections observed at some levels may indicate a zonular distribution within the neuroepithelia, but only on supracalyceal parts of hair cells in the utricular striola was a series of gap junctions found to be closely associated with the reticular lamina. The dimensions of the junctions, as characterized by the distance between opposite extremities of the arciform membrane appositions, averaged 0.14, 0.19, and 0.28 microgram, respectively, on noncalyceal utricular and saccular hair cells, and calyceal cells of the utricular striola. A notable number of the junctional profiles were adjacent or continuous to open, coated, putatively endocytotic vesicles. These findings are discussed in respect to phylogenetic, developmental, and functional considerations.

Streptomycin-induced defects of the otoconial membrane

Damage to the neuro-epithelium caused by the vestibulotoxic drugs has been well described in previous reports; however, little is known about the effect of such drugs on the otoconia. In this investigation, streptomycin sulfate was found to create a circumscribed defect in the crystalline layer of the otoconial membrane in the utricle of the guinea pig. The defect was secondary to a sharply outlined lesion of the neuroepithelium which corresponded in size and location to the otoconial defect. Many of the otoconia along the margin of the defect showed signs of degeneration and appeared similar to those observed in the saccule of man during aging. In advanced stages of decay, the otoconia were reduced to hollow shells consisting of longitudinal interconnecting strands. All the epithelial lesions occurred with remarkable consistency in the same region of the macula utriculi at the posterior end of the striola.

Otoconial formation in the fetal rat

The development of otoconia in the fetal rat was investigated by scanning and transmission electron microscopy and by x-ray elemental analysis. The transmission electron microscopical results indicate that primitive otoconia are highly organic appearing but are trigonal in cross section, indicating that they already possess a three-fold axis of symmetry and a complement of calcite. These otoconia develop into spindle-shaped units which accrue fibrous, organic material at an angle to their surfaces. Dumbbell-shaped otoconia, with distinct central cores and peripheral zones, result. These otoconia then mature to the adult crystal configuration having a more cylindrical body and pointed ends. The existence of trigonal, spindle- and dumbbell-shaped otoconia was verified by scanning electron microscopy of fresh-frozen material. Tissues prepared for transmission electron microscopy proved (by elemental analysis) to have been decalcified inadvertently, fortuitously revealing the arrangement of the organic material. Subsequent transmission electron microscopy of dumbbell-shaped otoconia not exposed to fluids during embedment showed that calcite deposits mimicked the arrangement of the organic material. X-ray elemental analysis demonstrated that calcium was present in lower quantities in the central core than peripherally. Findings are interpreted to indicate that organic material is essential to otoconial seeding and directs otoconial growth.

Scanning electron microscopic observations on the inner ear of the skate, Raja ocellata

The inner ear of the skate, Raja ocellata, was examined by scanning electron microscopy. The otolithic membranes have a gelatinous component and an endogenous class of otoconia. Cupulae are reticulate in form. The morphology and polarization of sensory cell hair bundles are described for the various regions of the labyrinth, and are compared with published observations on other species. In the otolithic maculae, the more centrally located receptor cells generally have longer sterecolia than the peripheral cells. The hair bundles of the lacinia are similar to those of the central portion of the sacculus and differed from those of the rest of the utricular macula. Hair bundles in the peripheral regions of all maculae and cristae are similar. The polarization pattern of the utriculus is similar to that of teleosts, while that of the lagena is less clearly dichotomized. The receptor cells of most of the sacculus are oriented in a bivertical direction, with cells in the anterior portion, and a few in the posterior region, being aligned longitudinally. The significance of morphology and polarization with respect to the functions of the otolithic organs is discussed. The relationship of cell processes of the ampullary receptors to the cupula is briefly considered.

Surface structures of vestibular end organs in surgical specimens

Ampullary cristae and utricular maculae of 5 patients who underwent translabyrinthine acoustic tumor removal were studied under the scanning electron microscope. The surfaces of maculae and cristae showed different distribution of microvilli. Most of the sensory cells in all specimens were devoid of stereocilia, but usually the roots of the hairs were left in the tops of the cells. This indicates a tearing artifact during removal rather than degenerative change. The supporting cells of the maculae displayed kinocilia.

Absence of otoconia in a human infant

Temporal bone specimens were obtained at autopsy from a six-week-old victim of sudden infant death syndrome. The inner ears were microdissected and studied by a combination of light and scanning electron microscopy. Otoconia were completely absent from the sensory organs of the saccule and utricle on both sides. However, the gelatinous otoconial membranes, neuroepithelia, and innervation were normal. In parallel with unusually light pigmentation of the skin in this Negro infant, relatively few melanocytes were found in the membranous walls of the saccule and utricle. No abnormalities were present in other inner ear structures. This anomaly, which we believe was congenital in nature, is strikingly similar to a genetically determined inner ear defect which has been extensively studied in experimental animals.