Cytoskeletal and muscle-like elements in cochlear hair cells

Microtubule and auditory function - an underestimated connection

The organ of Corti, located in the cochlea within the inner ear is the receptor organ for hearing. It converts auditory signals into neuronal action potentials that are transmitted to the brain for further processing. The mature organ of Corti consists of a variety of highly differentiated sensory cells that fulfil unique tasks in the processing of auditory signals. The actin and microtubule cytoskeleton play essential function in hearing, however so far, more attention has been paid to the role of actin. Microtubules play important roles in maintaining cellular structure and intracellular transport in virtually all eukaryotic cells. Their functions are controlled by interactions with a large variety of microtubule-associated proteins (MAPs) and molecular motors. Current advances show that tubulin posttranslational modifications, as well as tubulin isotypes could play key roles in modulating microtubule properties and functions in cells. These mechanisms could have various effects on the stability and functions of microtubules in the highly specialised cells of the cochlea. Here, we review the current understanding of the role of microtubule-regulating mechanisms in the function of the cochlea and their implications for hearing, which highlights the importance of microtubules in the field of hearing research.

Distortion-product otoacoustic emissions: a useful test for monitoring ototoxicity induced by pegylated interferon and ribavirin treatment in patients with chronic hepatitis C

Pegylated-interferon (peg-IFN) and ribavirin combination therapy for the treatment of hepatitis C virus (HCV) infection is well known to be associated with significant adverse effects. Several studies have investigated a possible auditory pathway involvement during IFN therapy, but a method to monitor the potential auditory involvement during treatment has not yet been described. The aim of this study is to evaluate possible modifications of the outer hair cell (OHC) function in HCV patients receiving peg-IFN and ribavirin combination therapy. Thirteen adult HCV patients (8 F/5 M, mean age 52∓12 years) treated with peg-IFN and ribavirin combination therapy underwent Pure Tone Audiogram and Distortion Product Otoacoustic Emission (DPOAE) tests. We compared mean auditory thresholds (PTA) and mean DPOAE amplitude before, at month 3 during, and at the end of treatment (T0, T3, and Tend, respectively), and 3 months after treatment discontinuation (Tfu). No significant differences were found in hearing levels at the different time points analyzed. During treatment, three patients developed tinnitus, which in 2 cases resolved spontaneously after the end of therapy. Compared to T0 (19.5±0.83), a statistically significant DPOAE increase at T3 (30±1,26) and Tend (28.6±2.16) was found (p<0.05 at both time points), while DPOAEs returned to pre-treatment levels at Tfu (19.3±1.3). In our group, none of the patients reported a permanent auditory impairment, excluding one patient with persistent tinnitus. Peg-IFN could produce an increase of motility of the OHCs by means of intracellular pathways. DPOAE test could be considered a new method for monitoring ototoxicity induced by IFN. On the basis of recent literature and our audiological results, physicians should be aware of the possible ototoxic effects of peg-IFN, requiring appropriate surveillance, and the patient should be informed of the potential side effects of IFN therapy on the auditory pathway.

Prestin gene expression in the rat cochlea following intense noise exposure

Noise-induced permanent loss of cochlear amplification was observed previously with the majority of outer hair cells (OHCs) still surviving in the cochlea and even with a normal OHC receptor potential, indicated by CM (cochlear microphonics) recording [Chen, G.D., Fechter, L.D., 2003. The relationship between noise-induced hearing loss and hair cell loss in rats. Hear. Res. 177(1-2), 81-90; Chen, G.D., Liu, Y., 2005. Mechanisms of noise-induced hearing loss potentiation by hypoxia. Hear. Res. 200, 1-9]. This study focused on effects of an intense noise exposure (10-20 kHz at a level of 110 dB SPL for 4 h) on the OHC motor protein (prestin) and structural proteins in the OHC membrane skeleton. The noise exposure significantly disrupted CM and CAP (cochlear compound action potential). The injured CM recovered after 1-week resting period. The impaired CAP at frequencies lower than the noise band also recovered. However, the CAP recovery at frequencies of the noise band stopped at a linear line one week after the noise exposure, indicating a permanent loss of cochlear amplification. Gene expression of prestin, beta-spectrin, and beta-actin was significantly up-regulated after the noise exposure. The elevated gene expression peaked at the 3rd post-exposure day and returned to baseline 4 weeks after the noise exposure. The up-regulated gene expression may be in response to injury of the proteins, which may be responsible for the loss of cochlear amplification.

Changes in MAP2 and tyrosinated alpha-tubulin expression in cochlear inner hair cells after amikacin treatment in the rat

The expression of MAP2 (microtubule-associated protein 2) and of tyrosinated alpha-tubulin was investigated immunocytochemically in the cochleas of normal and amikacin-treated rats. For MAP2, two different antibodies were used: anti-MAP2ab, against the high molecular weight forms, and anti-MAP2abc, additionally against the embryonic form c. In the cochlea of the normal rat, the outer (OHCs) and inner (IHCs) hair cells were labeled for MAP2abc. The labeling was weaker in IHCs than in OHCs. The hair cells were rarely labeled for MAPab. Both OHCs and IHCs were labeled for tyrosinated alpha-tubulin. In the cochlea of the amikacin-treated rat, aggregates of anti-MAP2abc and anti-tyrosinated alpha-tubulin antibodies were seen in the apical region of the IHCs as early as the end of the antibiotic treatment. In rats investigated during the following week, the cell body of most of the surviving IHCs were not labeled for MAP2abc and tyrosinated alpha-tubulin. Then, labeling for these two antibodies reappeared in the surviving IHCs, including their giant stereocilia. Fewer surviving IHCs were labeled for tyrosinated alpha-tubulin than for MAP2abc. The amikacin-poisoned IHCs were rarely labeled for MAP2ab. These results suggest that cochlear hair cells essentially express form c of MAP2. In the amikacin-damaged cochlea, the apical aggregation of MAP2c and tyrosinated alpha-tubulin within the poisoned IHCs could be implicated in a cell degenerative process. By contrast, the extinction and recovery of MAP2c and tyrosinated alpha-tubulin labeling in the remaining IHCs suggest the occurrence of a limited repair process. A possible role of MAP2 and tubulin in hair cell survival is discussed.

Effects of lidocaine on basilar membrane vibration in the guinea pig

The effects of lidocaine on basilar membrane (BM) vibration and compound action potential (CAP) were studied in guinea pigs in order to elucidate the site of lidocaine action in the cochlea. BM vibration was measured with a laser Doppler vibrometer through an opening made in the lateral bony wall of the scala tympani at the basal turn. Ten min after local administration of lidocaine (250 microg) into the scala tympani, the velocity of BM vibration and the CAP amplitude decreased significantly at around the characteristic frequency of the stimulus sound (p < 0.05). The maximum decreases were 4 dB in the velocity of the BM vibration and 40 dB in the CAP amplitude. In contrast, such changes were not observed after i.v. injection of lidocaine (1.5 mg/kg). These results suggest that when lidocaine is administered locally in the cochlea it acts not only on the cochlear nerve but also on the outer hair cells.

Application of a platinum replica method to the study of the cytoskeleton of isolated hair cells, supporting cells and whole mounts of the organ of Corti

We adapted a method of platinum replica to study the cytoskeleton of isolated cells of the guinea pig organ of Corti. This technique combined high image resolution with the ability to visualize the three-dimensional organization of the cytoskeleton of a whole cell. The procedure includes: isolation of hair cells and supporting cells using collagenase digestion, attachment of the cells to a coverslip, detergent extraction, chemical fixation, critical point drying, platinum/carbon coating, and transmission electron microscopy analysis. By using the method of platinum replica, we confirmed the existence of structural domains in the cortical lattice of outer hair cells. Based on the analysis of the partly destroyed cortical lattice, we propose that circumferential filaments are underlined with a thin flexible network. In addition, we established that the base of each stereocilium had a cone-like expansion of actin filaments and was surrounded by a thin bundle of filaments. We also produced replicas of the protrusion of the cuticular plate into the cytoplasm (infracuticular network) and the reticular lamina cytoskeleton. Our data indicated that the platinum replica method is useful for studying structural interactions among different cytoskeletal elements in the reticular lamina, as well as the cortex of outer hair cells and the cytoskeleton of supporting cells.

The incorporation and turnover of radiolabelled amino acids in developing stereocilia of the chick cochlea

Hair cell stereocilia are composed of packed actin filaments, oriented such that the preferred end for the addition of actin monomers is at the tips of the sterocilia. It has therefore been suggested that when stereocilia grow, they do so from their tips (Tilney and DeRosier, 1986, Dev. Biol. 116, 119-129). In order to test the hypothesis, radiolabelled amino acids were applied to the air-sac of chicken eggs at day 17 of incubation, i.e., at the beginning of a phase in which the stereocilia have achieved their mature width, but are growing rapidly in length. Incorporation of radiolabel was studied autoradiographically, followed by image analysis and averaging grain counts over many hair cells. In contrast to the position expected from the above hypothesis, there was no sign of preferential incorporation of label in the upper part of the stereociliary bundle. The greatest density of labelling was found in the lower part of the bundle, while the upper part of the bundle was under-represented in the autoradiographic averages. The turnover time (to fail to 1/e) was significantly greater in the bundle (16 days) than in the cuticular plate or in the rest of the cell (9 days). The results (i) give no support for the hypothesis that stereocilia grow from the tips, and (ii) suggest that during development at least some components of the stereocilia turn over with a relatively short time course.

Shape deformation of the organ of Corti associated with length changes of outer hair cell

Cochlear outer hair cells (OHC) are commonly assumed to function as mechanical effectors as well as sensory receptors in the organ of Corti (OC) of the inner ear. OHC in vitro and in organ explants exhibit mechanical responses to electrical, chemical or mechanical stimulation which may represent an aspect of their effector process that is expected in vivo. A detailed description, however, of an OHC effector operation in situ is still missing. Specifically, little is known as to how OHC movements influence the geometry of the OC in situ. Previous work has demonstrated that the motility of isolated OHCs in response to electrical stimulation and to K(+)-gluconate is probably under voltage control and causes depolarisation (shortening) and hyperpolarization (elongation). This work was undertaken to investigate if the movements that were observed in isolated OHC, and which are induced by ionic stimulation, could change the geometry of the OC. A synchronized depolarization of OHC was induced in guinea pig cochleae by exposing the entire OC to artificial endolymph (K+). Subsequent morphometry of mid-modiolar sections from these cochleae revealed that the distance between the basilar membrane (BM) and the reticular lamina (RL) had decreased considerably. Furthermore, in the three upper turns OHC had significantly shortened in all rows. The results suggest that OHC can change their length in the organ of Corti (OC) thus deforming the geometry of the OC. The experiments reveal a tonic force generation within the OC that may change the position of RL and/or BM, contribute to damping, modulate the BM-RL-distance and control the operating points of RL and sensory hair bundles. Thus, the results suggest active self-adjustments of cochlear mechanics by slow OHC length changes. Such mechanical adjustments have recently been postulated to correspond to timing elements of animal communication, speech or music.

Fimbrin expression in the developing rat cochlea

The expression of fimbrin in the developing rat cochlea was analyzed using an immunohistochemical technique with fimbrin antibody. The cochlea displayed temporal and lateral-longitudinal gradients for fimbrin expression during development. Fimbrin immunoreactivity first appeared in the inner hair cell stereocilia of the basal turn on the first gestational day studied (day 18). At birth, both inner (IHC) and outer hair cell (OHC) stereocilia of the basal turn showed positive labeling with fimbrin antibody. The progression of appearance was always from IHCs to OHCs and fimbrin immunostaining appeared in the apical hair cells by postnatal day 6. Immunostaining was restricted to stereocilia and the cuticular plate, and no immunoreactivity was observed in neighboring structures of the epithelium. Double labeling using both fimbrin antibody and phalloidin binding revealed similar chronological expression from the earliest stage studied. Increasing fimbrin immunoreactivity was observed in hair cells until late postnatal and adult stages. This study suggests that fimbrin is expressed with F-actin during development and fimbrin together with actin may constitute the two basic molecules that participate in stereocilia formation. We speculate that fimbrin may help maintain the parallel growth of actin filaments within the stereocilia. These data additionally support previous findings that hair cell maturation occurs from the base to the apex and from IHCs to OHCs.

Localization of F-actin and fodrin along the organ of Corti in the chinchilla

The distribution of the two cytoskeletal proteins, filamentous actin (F-actin) and fodrin, was investigated along the organ of Corti of the chinchilla using laser scanning confocal fluorescence microscopy. High intensity labeling of F-actin was seen in outer and inner hair cells, including the stereocilia. High intensity staining was also seen for fodrin in outer and inner hair cells, but not in their stereocilia. Staining intensity of both proteins along the lateral cell wall of the outer hair cells appeared to be greater in the middle and basal cochlear turns than in the apical turn. Pillars and Deiters cells also exhibited high intensity labeling of F-actin. The lack of significant differences in the distribution of fodrin between outer and inner hair cells makes the role of this protein in the active processes still unclear. Comparison of the distribution of F-actin and fodrin in the chinchilla with those reported in the guinea pigs suggest possible species differences.

The changing microtubule arrangements in developing hair cells of the chick cochlea

It has been suggested that microtubules in auditory hair cells might be involved in directing the morphological and hence functional polarisation of the sensory hair bundles. The distribution of microtubules was studied in hair cells of the chick cochlea, during the developmental stages when the stereocilia and cuticular plate were being formed. Cochleae were immunofluorescently labelled with antibodies to tubulin at specific stages in development, and hair cell ultrastructure was observed by electron microscopy. We found that the microtubule array changed from a simple symmetrical apical plate with a central kinocilium before the cuticular plate forms, to a ring with the kinocilium to one side when the cuticular plate begins to form, through to a cup-like arrangement below the cuticular plate once the plate has formed. In the earliest stages, no asymmetries were observed in the distribution of the microtubules, suggesting that structures other than the microtubules set up the functional polarisation of the stereociliary bundle.

Structural basis for mechanical transduction in the frog vestibular sensory apparatus: II. The role of microtubules in the organization of the cuticular plate

The actin matrix of the cuticular plate, which supports the sensory stereocilia bundle, is coupled to the axial cytoskeleton of the hair cell through a well defined microtubule columnar framework. A collection of axial microtubules in a columnar organization penetrate deep into the dense actin matrix of the cuticular plate. Each microtubule displays at the end a 300-500 nm long fuzzy cap that enmeshes with the actin matrix of the cuticular plate. The microtubule associated proteins MAP-1A and MAP-1B were localized by confocal immunofluorescence to the point of microtubule insertion in the cuticular plate. These proteins are likely components of the microtubule capping structure and may mediate the interaction of the microtubules with the actin matrix. The structural interaction of the microtubules with the cuticular plate provides important mechanical coupling of the transduction apparatus to the axial cytoskeleton of the hair cell.

F-actin, tubulin and spectrin in the organ of Corti: comparative distribution in different cell types and mammalian species

Laser scanning confocal microscopy was used to determine the distribution of actin, spectrin and tubulin in whole mounts of the organ of Corti of guinea pig, monkey, rat and chinchilla. Actin, spectrin and tubulin were localized in all cell types in the auditory epithelium. No specialized cytoskeletal organization of tubulin was detected in the cytoplasmic domain of hair cells. The only specialized organization of actin and spectrin in the cytoplasmic domain was the infra-cuticular network, found exclusively in apical guinea pig outer hair cells. In contrast, the lateral wall of inner and outer hair cells contained a homogeneous distribution of label specific for actin and spectrin. The label intensity was similar in the base and the apex of the cochlea. These results indicate that the distribution of spectrin and actin in the auditory epithelium is similar to that in other epithelial cells, suggesting that actin and spectrin participate in the formation of cellular shape and possibly in docking molecules to the membrane.

ATP-induced cytoplasmic [Ca2+] increases in isolated cochlear outer hair cells. Involved receptor and channel mechanisms

Outer hair cells (OHC) of the mammalian cochlea are thought to preprocess the sound signal by active movements, which can be induced by electrical or chemical stimulation, e.g. depolarization evoked by high [K+] or increased cytoplasmic [Ca2+]. Extracellular ATP has been found to induce cytoplasmic [Ca2+] increases in OHC but involved mechanisms have not been elucidated. Cytoplasmic [Ca2+] was measured in non-enzymatically isolated single OHC using Fura-2 microspectrometry. Results, using ATP/derivatives and other P2-purinergic receptor (P2R) ligands, as well as Ca(2+)-channel blockers and pertussis toxin, revealed several signal transduction pathways that increase cytoplasmic [Ca2+] in OHC: a P2-purinergic receptor (P2R)--G-protein--effector (phospholipase C or an ion channel) system and a voltage-dependent Ca2+ channel. Agonist potency studies denote a pattern analogous to that found in skeletal muscle, i.e. ATP-alpha-S > ATP = 2-methyl-S-ATP >> ADP > alpha,beta-methylene-ATP, but no activation by ADP beta F or UTP, leaving a choice of P2y or P2zR subtypes. The latter possibility gained strength from calculations showing that up to 8% of ATP may have formed the P2zR agonist ATP4- in the experimental medium. Experiments in Ca(2+)-free medium and with pertussis toxin revealed that the main Ca2+ source was intracellular. Pertussis toxin did not affect [Ca2+] increase induced by carbachol. Acetylcholine, administered a few seconds before ATP, did not affect total cytoplasmic [Ca2+] increases. Induced cytoplasmic [Ca2+] increases were high enough (> 500 nM at 50 microM ATP/derivatives) to hyperpolarize the OHC membrane by opening K(+)-channels and decreased little with time.(ABSTRACT TRUNCATED AT 250 WORDS)

An analysis of actin isoforms expressed in hair-cell enriched fractions of the chick basilar papilla by the polymerase chain reaction technique

Actin mRNA was characterised in hair-cell enriched fractions of the chick basilar papilla, by means of the reverse-transcription polymerase chain reaction technique. Primers were directed against the 3' untranslated portions of the actin mRNAs. Evidence for beta-cytoplasmic and gamma-cytoplasmic actin mRNA was found; no evidence was found for alpha-skeletal, alpha-cardiac or type 5 cytoplasmic actin mRNAs. Since beta-actin is known to form bundles of filaments whereas gamma-actin does not, this suggests that the hair-cell stereocilia are composed of beta-actin.

Immunohistochemical localization of intracellular Ca-ATPase in outer hair cells, neurons and fibrocytes in the adult and developing inner ear

Intracellular isoforms of the enzyme Ca-ATPase were identified in the inner ear by immunostaining paraffin sections with a polyclonal antiserum against rabbit cardiac muscle Ca-ATPase. In the adult cochlea, intense staining was present at the lateral border of outer hair cells in regions corresponding with the distribution of the subsurface cisternal system. Other cell types containing high levels of Ca-ATPase were skeletal muscle fibers in the tensor tympani, vascular smooth muscle, spiral ganglion neurons and subpopulations of fibrocytes in the limbus, spiral ligament and underlying vestibular neurosensory epithelium. In neonatal gerbils, staining of tensor tympani muscle fibers was observed at 4 days after birth and approached adult levels by 8 days after birth. Ca-ATPase was first detected in other cell types between postnatal days 12 and 14 but immunostaining still remained well below the intensity seen in adults at 20 days after birth. The demonstration of abundant calcium pumps in the subsurface cisternae confirms the role of this organelle as an intracellular reservoir for Ca2+ in outer hair cells. The presence of high levels of Ca-ATPase in spiral ganglion neurons and in fibrocytes specialized for ion transport points to a role for the enzyme in regulating the activity of other cell types of importance to normal hearing.

Distribution of F-actin and fodrin in the hair cells of the guinea pig cochlea as revealed by confocal fluorescence microscopy

We double-stained paraformaldehyde fixed guinea pig cochleas with rhodaminated phalloidin to detect F-actin and with a monoclonal antibody against non-erythroid spectrin (fodrin). The hair cells were studied in surface specimens of the organ of Corti with confocal fluorescence microscopy. In serial optical sections, phalloidin stained the stereocilia, cuticular plate, and a circumferential ring beneath it in the inner and outer hair cells (IHCs and OHCs). The cytoplasm of the IHCs and the OHCs was unlabelled, but the infracuticular network of the OHCs in the upper turns showed a strong reaction. The lateral plasma membrane was unreactive with phalloidin in the IHCs and OHCs, except in the basal turn, where a moderate reaction, probably representing actin of Deiter's cups, was seen along the lateral walls of the basal pole of the OHCs. Fodrin was similarly seen in the cuticular plate, in a circumferential ring beneath it, and in the infracuticular network of the apical OHCs. The most interesting finding was the fodrin-specific distinct labelling of the lateral cell surface in the OHCs of the basal cochlear turn. This staining diminished towards the apex and was practically absent in the OHCs located above the level of 15 mm from the round window. The lateral cell surface of IHCs showed moderate fodrin labelling in all cochlear turns. This staining was much weaker than that seen in the basal OHCs. Fodrin labelling revealed deformation from the regular cylindrical shape in midportion of the OHC bodies in the basal turn of the cochlea.

Actin-binding and microtubule-associated proteins in the organ of Corti

Actin-binding and microtubule-associated proteins regulate microfilament and microtubule number, length, organization and location in cells. In freeze-dried preparations of the guinea pig cochlea, both actin and tubulin are found in the sensory and supporting cells of the organ of Corti. Fodrin (brain spectrin) co-localized with actin in the cuticular plates of both inner and outer hair cells and along the lateral wall of the outer hair cells. Alpha-actinin co-localized with actin in the cuticular plates of the hair cells and in the head and foot plates of the supporting cells. It was also found in the junctional regions between hair cells and supporting cells. Profilin co-localized with actin in the cuticular plates of the sensory hair cells. Myosin was detected only in the cuticular plates of the outer hair cells and in the supporting cells in the region facing endolymph. Gelsolin was found in the region of the nerve fibers. Tubulin is found in microtubules in all cells of the organ of Corti. In supporting cells, microtubules are bundled together with actin microfilaments and tropomyosin, as well as being present as individual microtubules arranged in networks. An intensely stained network of microtubules is found in both outer and inner sensory hair cells. The microtubules in the outer hair cells appear to course throughout the entire length of the cells, and based on their staining with antibodies to the tyrosinated form of tubulin they appear to be more dynamic structures than the microtubules in the supporting cells. The microtubule-associated protein MAP-2 is present only in outer hair cells within the organ of Corti and co-localizes with tubulin in these cells. No other MAPs (1,3,4,5) are present. Tau is found in the nerve fibers below both inner and outer hair cells and in the osseous spiral lamina. It is clear that the actin-binding and microtubule-associated proteins present in the cochlea co-localize with actin and tubulin and that they modulate microfilament and microtubule structure and function in a manner similar to that seen in other cell types. The location of some of these proteins in outer hair cells suggests a role for microfilaments and microtubules in outer hair cell motility.

Monoclonal antibodies to inner ear antigens: II. Antigens expressed in sensory cell stereocilia

To develop biological reagents for investigating structure-function relationships in the organ of Corti, we have raised monoclonal antibodies, (MAb) to inner ear tissues. Our first series of antibodies prepared after intrasplenic immunization of mice with guinea pig tissues, identified antigens restricted to supporting cell structures, but no hair cell specific antibodies were developed [Zajic et al., Hear. Res. 52, 59-72, 1991]. In this report we describe the isolation, binding specificity and initial characterization of the stereocilia-binding monoclonal antibodies, KHRI-4, and KHRI-5. Mice were immunized with avian, amphibian and mammalian sensory hair cell-containing tissues and antibodies were screened for selective binding to cochlear extracts in ELISA. In the inner ear, KHRI-4 and KHRI-5 bind specifically to stereocilia in both avian and mammalian cochlear and vestibular tissue preparations using immunofluorescence and immunoperoxidase assays. In other tissues only certain cells of mesothelial origin, such as smooth muscle in gut and the arteriolar vasculature, were stained by KHRI-4 indicating that the antigenic structure defined by this antibody has limited distribution. KHRI-5 binding could be detected in other tissues only at high antibody concentrations suggesting that the gene product identified by this antibody is also weakly expressed in other cell lineages. Western blot analysis showed that KHRI-4 and -5 detect different protein complexes. KHRI-4 identifies an antigenic structure common to gut, cochlea, vestibular tissue and cultured fibroblasts consisting of a approximately 195 and a 230 kDa heterodimer designated p195/230. KHRI-5 binds to a prominent approximately 200-210 kDa band in Western blots of cochlear tissues, gut and fibroblasts. In immunoprecipitation experiments, KHRI-5 precipitated three proteins of Mr approximately 200-210, 230 and 260 kDa indicating that the approximately 200-210 kDa protein carrying the epitope for this antibody is a member of a heterotrimer complex. Our results show that these protein complexes are structural components of stereocilia and that the same proteins are arrayed in conjunction with the actin stress fibers of cultured mesothelial cells. Thus, they are likely to be important for maintaining the actin structure of stereocilia essential to transduction in sensory hair cells.

Spatiotemporal development of cochlear innervation and hair cell differentiation in the rat

The apical cytoskeleton of cochlear hair cells is largely comprised of actin microfilaments and actin-associated proteins, of which fodrin is one of the most prominent. We studied the development of this mechanosensory apical portion of cochlear hair cells of the rat by fluorescence microscopy using rhodamine conjugated phalloidin to detect F-actin and an antibody against alpha-fodrin. An antibody against the 160 kDa neurofilament polypeptide was used for tracing nerve fibers. The first sign of differentiation of the mechanosensory region, actin-containing stereocilia, was observed on the 19th gestational day in the inner hair cells of the basal coil. The appearance of expression of cytoskeletal actin in the cochlear hair cells proceeded gradientally from basal to apical coil and from inner to outer hair cells. Corresponding maturation sequences were observed in the development of fodrin immunoreactivity in the cuticular plates, but the first evidence of this reactivity was found one day later than the appearance of stereocilia in the hair cells at the same location. Also the penetration of neurofilament-positive neurites into the sensory epithelium followed the same kind of longitudinal and radial maturation gradients throughout the cochlea. Fibers were revealed beneath the sensory cells shortly before the first appearance of differentiation of their mechanosensory region. The results suggest that ingrowing nerve fibers may influence the timing of the apical cytoskeleton differentiation in cochlear hair cells or that both these processes could be controlled by the same external signals that are gradientally expressed throughout the cochlea.

Organization of microtubules in cochlear hair cells

The organization of microtubules in hair cells of the guinea-pig cochlea has been investigated using transmission electron microscopy and correlated with the location of tubulin-associated immunofluorescence in surface preparations of the organ of Corti. Results from both techniques reveal consistent distributions of microtubules in inner and outer hair cells. In the inner hair cells, microtubules are most concentrated in the apex. Reconstruction from serial sections shows three main groups: firstly, in channels through the cuticular plate and in a discontinuous belt around its upper perimeter; secondly, forming a ring inside a rim extending down from the lower perimeter of the plate; and thirdly, in a meshwork underlying the main body of the plate. In the cell body, microtubules line the inner face of the subsurface cistern and extend longitudinally through a tubulo-vesicular track between the apex and base. In outer hair cells, the pattern of microtubules associated with the cuticular plate is similar, although there are fewer present than in inner hair cells. In outer hair cells from the apex of the cochlea, microtubules occur around an infracuticular protrusion of cuticular plate material. In the cell body, many more microtubules occur in the region below the nucleus compared with inner hair cells. The possible functions of microtubules in hair cells are discussed by comparison with those found in other systems. These include morphogenesis and maintenance of cell shape; intracellular transport, e.g., of neurotransmitter vesicles; providing a possible substrate for motility; mechanical support of structures associated with sensory transduction.

Outer hair cell electromotility and otoacoustic emissions

Outer hair cell electromotility is a rapid, force generating, length change in response to electrical stimulation. DC electrical pulses either elongate or shorten the cell and sinusoidal electrical stimulation results in mechanical oscillations at acoustic frequencies. The mechanism underlying outer hair cell electromotility is thought to be the origin of spontaneous otoacoustic emissions. The ability of the cell to change its length requires that it be mechanically flexible. At the same time the structural integrity of the organ of Corti requires that the cell possess considerable compressive rigidity along its major axis. Evolution appears to have arrived at novel solutions to the mechanical requirements imposed on the outer hair cell. Segregation of cytoskeletal elements in specific intracellular domains facilitates the rapid movements. Compressive strength is provided by a unique hydraulic skeleton in which a positive hydrostatic pressure in the cytoplasm stabilizes a flexible elastic cortex with circumferential tensile strength. Cell turgor is required in order that the pressure gradients associated with the electromotile response can be communicated to the ends of the cell. A loss in turgor leads to loss of outer hair cell electromotility. Concentrations of salicylate equivalent to those that abolish spontaneous otoacoustic emissions in patients weaken the outer hair cell's hydraulic skeleton. There is a significant diminution in the electromotile response associated with the loss in cell turgor. Aspirin's effect on outer hair cell electromotility attests to the role of the outer hair cell in generating otoacoustic emissions and demonstrates how their physiology can influence the propagation of otoacoustic emissions.

Nonerythroid spectrin (fodrin) is a prominent component of the cochlear hair cells

We studied the distribution of nonerythroid spectrin, fodrin, in surface preparations and cryosections of the cochlear hair cells as well as isolated hair cells of the guinea pig by using a monoclonal antibody (Mab) reacting with Mr 240 kD alpha-fodrin polypeptide. The Mab gave a strong reaction with the cuticular plate of both the inner and outer hair cells (IHCs and OHCs). Stereocilia were nonreactive and only a weak cell surface reaction was seen in the supporting cells. In the outer hair cells the upper turns of the cochlea, fodrin was observed in a cytoplasmic spiralling structure extending from the cuticular plate towards the cell nucleus. Some labelling was also seen along the cell surface membrane and in the synaptic region. The results suggest that fodrin may be an important constituent in the active processes of hair cells such as cell motility and/or signal transduction.

Tubulin and microtubules in cochlear hair cells: comparative immunocytochemistry and ultrastructure

The distribution of tubulin has been investigated in surface preparations of the guinea pig organ of Corti using indirect immunofluorescence microscopy. Two different monoclonal antibodies to tubulin produce similar distinct patterns of labelling in hair cells. Labelling is greater in inner hair cells than outer hair cells. It occurs in rings around the cell apex, and in a meshwork below and channels through, the cuticular plate. In outer hair cells from the apical region of the cochlea, labelling occurs around the location of a basalward protrusion of the cuticular plate. These patterns correlate with the location of microtubules observed using transmission electron microscopy. A large patch of labelling occurs on the strial side of the cell corresponding to the largest channel through the cuticular plate and the kinociliary basal body. Strands of labelling are seen running parallel to the long axis of the cell between the subcuticular and synaptic region. Many more of these strands are seen in the inner hair cell than the outer hair cell and may correspond to tracks of microtubules transporting neurotransmitter vesicles or other organelles. In outer hair cells, intense labelling and many microtubules are seen in the subnuclear region. The possible roles of the different microtubule arrangements are discussed.

Functional role of the olivo-cochlear bundle: a motor unit control system in the mammalian cochlea

A fiber optic lever is applied to the measurement of the motion of the basilar membrane motion in guinea pigs. In response to intense tones from either ear, the motion includes a substantial summating shift in the mean position in addition to a travelling wave originally described by von Békésy. His stroboscopic technique and most techniques used since have been concentrated upon measuring vibrations of the basilar membrane synchronous with the stimulus and have been insensitive to variations in the baseline position such as a summating component of motion analogous to the extracellular summating potential. In addition to the role of the outer hair cells in providing normal hearing sensitivity, they evidently play a role in regulating the mean position of the basilar membrane. For a fixed frequency, the polarity of the mean position varies systematically with sound level and place and summates with time since onset. Since these cells are the target cells for the olivocochlear bundle, homeostasis in the cochlea would appear to be linked efferent function and involve cochlear mechanics. The negative damping hypothesis asserts that hair cell activity is necessary for low thresholds. The results presented here demonstrate that OHC activity exists independent of neural thresholds. The discussion develops the concept that threshold losses are due to a mismatch of opposing tonic forces which normally maintain the mean position of the basilar membrane. Structure is examined in relation to function and the group of outer hair cells innervated by a single medial efferent neuron is identified as a motor unit. Implications of central control of individual motor units include peripheral involvement in selective attention tasks.

Cholecalcin (28-kDa CaBP) in the rat cochlea. Development in normal and hypothyroid animals. An immunocytochemical study

The distribution of cholecalcin (28-kDa calcium-binding protein) in the cochlea of developing rats was followed by immunocytochemistry. In normal animals, the protein first appeared in utero in the cells of Kölliker's organ, a structure involved in the secretion of the tectorial membrane. The inner hair cells, followed by the outer hair cells, then became immunoreactive from the base of the cochlea to the apex. Their cuticular plate, the anchoring structure for stereocilia, was particularly labeled. The cells of Kölliker's organ lost their immunoreactivity after the first postnatal week, the time when they lose their secretory activity. During the same period, when the tunnel of Corti and the space of Nuel open, labeling appeared in the supporting cells. The neurons of the spiral ganglion were stained from the second postnatal week and the fibers of the cochlear nerve after the end of the first month. No difference was induced by thyroid deficiency until the second postnatal week. Thereafter, Kölliker's organ did not transform and its cholecalcin immunoreactivity persisted, together with the secretory activity. As a result, the tectorial membrane was greatly distorted and the mechanical properties of the organ of Corti were dramatically impaired. The supporting cells were labeled although the tunnel of Corti and the space of Nuel did not open. Staining of the spiral ganglion neurons was delayed. All the nervous structures of the cochlea were, however, stained at the end of the first postnatal month, as in normal animals, despite the irreversible complete deafness. Cholecalcin is thus present during development of the cochlea in both non-neuronal and neuronal structures, and is probably involved in very different processes in various types of cells.

Tropomyosin co-localizes with actin microfilaments and microtubules within supporting cells of the inner ear

The distribution of tropomyosin, actin and tubulin in the supporting cells of the organ of Corti was studied by immunofluorescent localization of antibodies to these proteins. Tropomyosin colocalizes with actin and tubulin in the regions of the tunnel pillar and Deiters cells where actin microfilaments and microtubules had previously been observed ultrastructurally. Despite the implications of the presence of antiparallel actin filaments in the supporting cells, the presence of tropomyosin and the absence of myosin suggest that the role of tropomyosin may be to confer rigidity to the actin filaments. Thus the primary function of the cytoskeletal proteins in the supporting cells may be structural.

Differences in the distribution of F-actin in outer hair cells along the organ of Corti

There is evidence of differences in the structure, innervation and physiological responses between outer hair cells (OHCs) of the basal and apical turns of the mammalian cochlea. In this study we have used rhodamine-labelled phalloidin to investigate the differential distribution of F-actin in OHCs along the organ of Corti of the guinea pig. Isolated OHCs and surface preparations and cryosections of the organ of Corti were studied. F-actin was observed in stereocilia and the cuticular plate of all OHCs. In addition, some OHCs had a network of F-actin extending from the cuticular plate towards the nucleus. This infracuticular network was observed in most OHCs of the apical cochlear turns but was not seen in any OHCs of the basal turn. These microstructural differences between OHCs of the base and apex could be related to differences in OHC function between the apical and basal portions of the cochlea.

Three-dimensional visualizations of the inner ear hair cell of the guinea pig. A rapid-freeze, deep-etch study of filamentous and membranous organelles

The fine structure of the filamentous and membranous organelles in the stereocilia and in the cuticular plate of sensory hair cells from the guinea pig was examined using a rapid-freeze, deep-etch method. In fixed and unfixed tissue the outer surface of the plasma membrane of the stereocilia had numerous surface protrusions of various sizes and shapes, while the protoplasmic fractured face of the membrane had rather sparse intramembrane particles. Many tiny cross links were present between the adjacent actin filaments and between actin filaments and the plasma membrane of the stereocilia. Numerous fibrils radiating from the hair rootlet were attached to the peripheral actin filaments in the cuticular plate. The radiating fibrils differed from the tiny cross links which interconnected the adjacent, randomly-oriented actin filaments in the cuticular plate. These complex structures consisting of actin filaments in the hair rootlets, radiating fibrils, and peripheral actin filaments may play an important role in regulating stereociliary bending.

Early morphological and chemical changes induced by cisplatin in the guinea pig organ of Corti

Guinea pigs were treated with a single dose of Cisplatin (5 mg IP). After 2-4 days the cochleas were prepared for morphological analysis by scanning electron microscopy and chemical analysis by X-ray dispersive microanalysis. Following Cisplatin, the bundles of stereocilia on the hair cells were found to be rough, disarrayed, fused, and finally absorbed. Significant increases were found in the levels of calcium, sulphur, and phosphorus in the abnormal hair cells. It is suggested that the high calcium levels might be due to the inhibition of enzymes which normally keep cytoplasmic calcium low, and that some of the changes in the stereocilia might be secondary to this.

Inhibition of myosin light-chain kinase activity in the organ of Corti by 0.3-5 kilodalton substances of the otosclerotic perilymph

The guinea pig organ of Corti contains myosin light-chain kinase (MLCK) activity. The upper and lower most parts of the cochlea do not show significantly different activities of the enzyme, which is Ca2+ and calmodulin-dependent. Short-term noise exposure does not cause a significant change. 0.3-5 Kilodalton substances of the otosclerotic perilymph, separated by SG-25 column chromatography, inhibit the MLCK activity in in vitro organ of Corti preparations. This inhibitory action of the perilymph substances can also be observed with the purified MLCK of turkey gizzard. The activity of the enzyme can be specifically inhibited by cyclic AMP-dependent protein kinase.

Hearing loss in facioscapulohumeral dystrophy

Bilateral sloping high frequency hearing loss of 20-90 dB was found in six out of ten patients with infantile or adolescent onset FSHD. In all cases the basic defect could be traced to the cochlea. The outer hair cells of the basal turn are predominantly affected. In 20 patients with various other forms of muscular dystrophy or neuromuscular disorders with an FSH distribution, no sensorineural hearing loss was found. Myopathology of FSHD patients extended from mild to severe, often showing inflammatory infiltrates and type I fibre atrophy, without unequivocal differences between the two groups with and without hearing loss. It is concluded that cochlear dysfunction is a specific and frequent phenomenon of early onset FSHD.

Effects of noise and ototoxic drugs at the cellular level in the cochlea: a review

Currently available information concerning the cellular mechanisms involved in acoustic trauma and aminoglycoside ototoxicity is reviewed to shed some new light on the cellular events that may be related to functional impairment of the auditory organ. Based on the available data, the following postulations can be made concerning the cellular mechanisms involved. 1) The macromolecular disruption of the stereocilia and cuticular plates is the initial cellular event in acoustic trauma. This disruption would affect the micromechanics of the transduction process, leading to temporary threshold shift. Further cellular impairment would involve basic cellular functions such as the protein, lipid, and glucose synthesis needed for cell repair and survival, and such impairment would result in permanent cell injury or cell death, leading to permanent threshold shift. 2) It can be postulated that the cellular mechanisms involved in aminoglycoside ototoxicity include two events. The early event is the reversible blockage of the transduction channels from the endolymph side of the hair cells. The later event is the interference in such cellular functions as protein and/or phospholipid synthesis because of binding of aminoglycoside to the phospholipids and/or protein, leading to cell death. The latter event may be facilitated by penetration or membrane-mediated internalization of the aminoglycoside from the perilymph side of the hair cell.

Functional structure of the organ of Corti: a review

The mammalian auditory organs have a dual sensory system (inner vs. outer hair cells) with distinctly different cellular organizations and innervation patterns. However, the inner (IHCs) and outer (OHCs) hair cells are mechanoreceptors sharing similar general characteristics such as organization of stereocilia (including linkage system) and a gradation of stereociliary height along the length of the cochlea. This gradation of stereociliary height may be the single most important anatomic feature in the tuning capability of the sensory cell. Several lines of evidence suggest that the stereociliary stiffness may be modulated by the sensory cells themselves, most likely via the cuticular plate-rootlet complex. The stereociliary bundles of both types of hair cell are organized in a 'W' formation with a steplike arrangement. In the OHCs, the 'W' formation is sharply angulated and slanted toward the apex, coinciding with the slanted fiber arrangement of the overlying tectorial membrane, which is firmly coupled to the tips of the tallest row of the stereociliary bundles. However, in the IHCs, the 'W' formation is wide and its long axis is linear and arranged at a right angle to the radial axis of the organ of Corti; also, the ciliary bundles are freestanding (with a few exceptions in the basal turn). This arrangement in the IHCs would be best suited for deflection by the radial flow of the endolymph. Present evidence suggests that the subtectorial fluid space exists, is filled with endolymph, and freely communicates with endolymph. Because of the discovery of the phenomenon of 'cochlear emission', the possible motility of the sensory cells, particularly of the OHCs, has drawn intense interest in recent years. Recent investigations with dissociated sensory cells (OHCs) indicate some motile capability under various experimental conditions, although it has not been established that this motility is present in vivo. For this reason, the specialized cellular organization for motility and localization of contractile and cytoskeletal proteins have been investigated. These results support the possibility that the OHCs may have cellular facilities for this function.(ABSTRACT TRUNCATED AT 400 WORDS)

The responses of inner and outer hair cells in the basal turn of the guinea-pig cochlea and in the mouse cochlea grown in vitro

Until recently the responses of the mechanosensitive hair cells of the cochlea have been inferred from their morphology, morphological relationships with other structures in the cochlea, and by indirect electrophysiological measurements. With the advent of techniques for making intracellular recordings from hair cells in the cochleas of anaesthetised mammals it has been possible to measure the responses of hair cells to acoustic stimulation and to assess their roles in sensory transduction in the cochlea. Intracellular recordings of the responses of inner and outer hair cells in the basal turn of the guinea-pig cochlea show that they differ considerably from each other. The receptor potentials of inner hair cells are larger, predominantly depolarizing to low frequency tones and at their best frequencies (16-20 kHz) they generate depolarizing dc receptor potentials. Outer hair cells generate predominantly hyperpolarizing potentials to low frequency tones. They do not produce significant voltage responses at high frequencies except at high intensities when they generate slowly rising depolarizing potentials which are associated with loss of cochlear sensitivity. At low frequencies the receptor potentials of the inner hair cells phase lead those of the outer hair cell. Measurements of their frequency selectivity show that inner and outer hair cells are both sharply tuned. It is proposed that the responses of inner and outer hair cells are consistent with sensory and motor roles respectively in mechanoelectric transduction and that the outer hair cells are the site of an active mechanical process responsible for the frequency selectivity and sensitivity of the cochlea. Intracellular recordings from hair cells in the mouse cochlea maintained in vivo have provided a direct measure of the mechanosensitivity of cochlear hair cells (approximately 30 mV per degree of displacement of their stereociliary bundles) and indirect evidence that the transfer characteristics of the outer hair cells in vivo may be due to their mechanoelectrical interaction with the tectorial membrane. This is because the transfer characteristics of the inner and outer hair cells are similar in vitro in the absence of a tectorial membrane. Considerable importance is attributed to the shape of the transfer characteristics of the inner and outer hair cells. Changes in these characteristics during anoxia and following exposure to intense tones are associated with depolarization of the outer hair cells and loss of cochlear sensitivity and frequency selectivity. Current-voltage studies of hair cells in vivo show the inner and outer hair cells to be electrically nonlinear.(ABSTRACT TRUNCATED AT 400 WORDS)

Reversible contraction of isolated mammalian cochlear hair cells

Outer hair cells were isolated from the guinea pig cochlea using a micromechanical non-enzymatic procedure. Depolarization of outer hair cells in the presence of 25-125 mM K+ was accompanied by a longitudinal contraction of the isolated cells. A decrease of [K+] to 5.4 mM interrupted contraction and induced a relaxation. Individual hair cells were able to undergo as many as 5 cycles of contraction and relaxation. External Ca2+ was required for relaxation of the contracted hair cells. The contractile event led to the production of a visible cytoplasmic network between the supranuclear area and the cuticular plate.

Immunoelectron microscopic and immunofluorescent localization of cytoskeletal and muscle-like contractile proteins in inner ear sensory hair cells

The distribution of actin, alpha-actinin, fimbrin, tropomyosin and tubulin in the apical region of inner and outer hair cells was studied by immunofluorescent localization of antibodies to these proteins. The macromolecular distribution of actin and alpha-actinin was studied using post-embedding immunoelectron microscopic techniques. Actin is present in the stereocilia and cuticular plate of both inner and outer hair cells. Antibodies to actin were localized with fluorescence and colloidal gold. Colloidal gold particles were distributed uniformly over the stereocilia, stereocilia rootlets and cuticular plate. Fimbrin is present in the stereocilia and the cuticular plate. Immunofluorescent label was more intense over the cuticular plate of outer hair cells than over the cuticular plate of inner hair cells. Alpha-actinin is present in the cuticular plate only. At the ultrastructural level, antibodies to alpha-actinin were labeled throughout the cuticular plate, with larger accumulations of colloidal gold over the electron dense bodies in the cuticular plate, as well as over the electron dense region at the junctional complex. There was no label over the electron dense portion of the stereocilia rootlets. Tropomyosin is observed in the area of the stereocilia rootlets by immunofluorescent techniques, but like fimbrin, the antigenic sites of tropomyosin did not withstand processing for ultrastructural localization. Tubulin is not present in the apical region of inner or outer hair cells, although its presence could be documented in the hair cell body and in the supporting cells.

Microtubules in the cochlea of the hypothyroid developing rat

In order to study the effects of hypothyroidism on the development of microtubules in the cochlea, rat pups were rendered hypothyroid by daily administration of propylthiouracil. Microtubules were studied by immunofluorescence and electron microscopy. The absence of immunostaining of pillar cells with antimicrotubule or antitubulin antibodies was correlated with a retarded morphological development of microtubules within these same structures. The above alterations induced an abnormal development of pillar cells, non-appearance of the tunnel of Corti, and stunted epithelial growth. In contrast, a distinct immunoreaction was observed under the outer hair cells. This was attributed to abnormal persistence of afferent dendrites containing microtubules. The results suggest that, while the effect of thyroid hormone on microtubules in afferent cochlear dendrites could not be demonstrated, thyroid hormone is necessary for the normal development of microtubules in epithelial structures.

Immunocytochemical detection of calcium-binding protein in the cochlear and vestibular hair cells of the rat

Specific antibodies raised against human cerebellar calcium-binding protein (CaBP) intensely labelled the cochlear hair cells of the rat. The vestibular hair cells also stained weakly. In both inner and outer cochlear hair cells, the cuticular plate was the most stained area. These results suggest that CaBP may prevent excessive concentrations of intracellular calcium and thus modulate some Ca2+-mediated biochemical processes, especially at the level of the cuticular plate and stereocilia; CaBP could be involved in the mechanochemical coupling of hearing or vestibular function.

Distribution and polarity of actin in inner ear supporting cells

Actin is present in the supporting cells of the chinchilla cochlea. Actin filaments which decorate with myosin subfragment S1 are found in both inner and outer pillar cells and in the Deiters' cells which surround outer hair cells. The opposing polarities of the S1 decorated actin filaments suggest that the supporting cells may play an active role in the structural support of the organ of Corti.

Head and neck carcinoma models. In vivo reproduction in athymic mice and in vitro culture

683 tumour fragments from 63 head and neck carcinoma patients were cultured in vitro. Two laryngeal carcinomas and two salivary gland carcinomas were established into permanent cell lines. Malignancy of these cultured cells was proved by cloning, by chromosomal analysis and by transplantation into athymic (nu/nu) mice. Experiments demonstrating preservation of histological, biochemical and antigenic properties in the tumour models counter the objection that tumour-specific characteristics may be lost.

Gravity and the cells of gravity receptors in mammals

Two new findings, that crystals located in the inner ear gravity receptors of mammals have the internal organization requisite for the piezoelectric property, and that sensory hair cells of these same receptors possess contractile-appearing striated organelles, have prompted the author to model mammalian gravity receptors in the ear on the principles of piezoelectricity and bioenergetics. This model is presented and a brief discussion of its implications for the possible effects of weightlessness follows.

Preservation and visualization of actin-containing filaments in the apical zone of cochlear sensory cells

The fine filamentous structure in the apical zone of cochlear sensory cells of the guinea pig was investigated under transmission electron microscopy (TEM) using various fixation methods. The true form of this structure, which is that of a dense core of sensory hairs and cuticular plates containing hair rootlets, has been hitherto unknown because of the selectively destructive effect of ordinary fixatives. We revealed the fine filamentous structure in great detail by fixing the specimens in tannic acid or by the modified glutaraldehyde--osmium fixation method, which can preserve action filaments during the procedures required to prepare the specimen for TEM. The filamentous structure gives the impression of a negatively stained image when prepared in this way. Filaments were packed regularly and tightly into dense cores which projected down deep into the cuticular plate as hair rootlets. Cross-striations were seen at intervals of 360 +/- 28 A along the packed filaments, a distance which is comparable to the periodicity of an actin paracrystal. The overall diameter of each filament was 83 A. In fact, the structure of dense cores and hair rootlets proved to be composed of actin paracrystals, probably containing some regulatory proteins. Cross-sectioned actin filaments in the paracrystal were arranged in an extremely regular hexagonal pattern. The characteristic filamentous texture in the cuticular plate was best seen in tissues that were pretreated with EDTA, and then fixed by tannic acid. It is probable that the greater part of the cuticular plate is composed of actin filaments and actin monomers, both containing Ca2+-dependent regulatory proteins. Utilizing the above ultrastructural findings, some functional models of this zone are proposed.

Immunofluorescence localizations of proteins in semithin 0.2--1 micron frozen sections of the ear. A report of improved techniques including gelatin encapsulation and cryoultramicrotomy

The present work describes a high resolution technique for locating proteins in frozen sections of the inner ear by immunofluorescence. Dissected organs are encapsulated in gelatin, and sections 0.1--1 micron thick are cut at --100 degrees C in a cryoultramicrotome. These are labelled with antibodies against two cytoskeletal proteins, actin and tubulin. Actin, which had previously only been described in the sensory cells, is found in the supporting cells as well. Tubulin is identified in the supporting cells and in outer spiral nerve fibres.