Mutation of the Na-K-Cl co-transporter gene Slc12a2 results in deafness in mice

Hearing impairment is a common human condition, but we know little about the molecular basis of cochlear function. Shaker-with-syndactylism (sy) is a classic deaf mouse mutant and we show here that a second allele, sy(ns), is associated with abnormal production of endolymph, the fluid bathing sensory hair cells. Using a positional candidate approach, we demonstrate that mutations in the gene encoding the basolateral Na-K-Cl co-transporter Slc12a2 (Nkcc1, mBSC2) cause the deafness observed in sy and sy(ns) mice. This finding provides the molecular basis of another link in the chain of K+recycling in the cochlea, a process essential for normal cochlear function.

Lectin binding patterns in nonsensory regions of rat cochlea during postnatal development

The distribution of glycoconjugates was examined in the nonsensory regions of the rat cochlea during postnatal development using biotin-conjugated lectins. Temporal bones of rats at postnatal d 1 and at wk 2, 4 and 6 were fixed in 4% paraformaldehyde and 0.1% glutaraldehyde and processed for paraffin wax embedding. The dewaxed sections were incubated with 7 biotinylated lectins, followed by avidin-biotin-peroxidase complex. A different staining pattern was observed in the stria vascularis, spiral ligament and spiral limbus in the age groups examined. The staining intensity varied between lectins and the reaction product exhibited limited disparity. The staining intensity for WGA increased with age in all the 3 nonsensory regions. The staining patterns for the other lectins differed in the various nonsensory regions examined indicating tissue specificity. The limited variations in the lectin binding patterns after 2nd wk of postnatal life also indicate that the changes in the carbohydrate moieties are established during the fetal period of cochlear development and limited changes take place during postnatal maturation of the nonsensory regions.

Cloning and expression of Shaker alpha- and beta-subunits during inner ear development

Sensory cells of the chicken cochlea exhibit different ion channels relative to their position along the epithelium. One of these channels conducts an A-type potassium current which is found primarily in 'short' hair cells. Here, we report the first full length cloning and developmental expression of Shaker genes from this endorgan. Clones were obtained by screening a chicken (Gallus gallus) cochlea cDNA library, using probes made from RHK1 (i.e., Kvalpha1.4) cDNA, a Shaker homologue isolated from rat heart, and hKvbeta1.2 cDNA, a beta homologue isolated from human heart. Sequence analysis revealed a chick homologue of Kvalpha1.4, with a deduced amino acid similarity of 76-79% to mammalian Kvalpha1.4, and a chick homologue of Kvbeta1.1, with a similarity of 95% to mammalian Kvbeta1.1. In addition, we isolated a variant of cKvalpha1. 4 (cKvalpha1.4(m)) that differs in its untranslated regions and shows complete similarity in its coding region, except for the deletion of a single nucleotide. During development of the inner ear, reverse transcription-polymerase chain reaction (RT-PCR) studies show that the beta-subunit is expressed as early as embryonic day 3, whereas alpha- and beta-subunits are coexpressed on embryonic days 7 to 10, 14, and in adult.

Age- and strain-related differences in dehydrogenase activity and glycogen levels in CBA and C57 mouse cochleas

In the C57 mouse strain, loss of sensory hair cells (HCs) begins during early adulthood, starting in the base of the cochlea and progressing toward the apex as aging continues. In contrast, the CBA mouse strain exhibits no significant cochlear histopathology until relatively late in life. These strain and age differences may be related to differences in cochlear energy metabolism. To examine this possibility, we used dehydrogenase and glycogen histochemistry to evaluate the metabolic capacities of HCs and stria vascularis (SV) in cochleas of C57 and CBA mice. Reaction product density was quantified and compared as a function of strain (1.5-month-old C57 mice vs. CBA mice) and age (CBA mice, 1.5, 18 and 36 months). Young C57 mice had significantly less HC dehydrogenase activity than CBA mice of any age, lower HC glycogen levels than 18-month-old CBA mice and lower SV glycogen levels than 18- or 36-month-old CBA animals. Within the CBA strain, HC dehydrogenase activity decreased significantly between 1.5 and 18 months of age, while glycogen levels in both HCs and SV increased over the same time period. Between 18 and 36 months, HC dehydrogenase activity and SV glycogen levels remained stable. The results show that there are significant age-related changes in energy metabolism in the inner ear of CBA mice that are correlated with age-related hearing loss. Genetically determined deficits in cochlear metabolic capacity in C57 mice could be linked to the early onset of hearing loss in this strain.

Murine model of autoimmune hearing loss induced by myelin protein P0

Myelin protein P0 has been identified as an autoantigen in inner ear diseases. In order to study autoimmune hearing loss, we performed brain stem auditory-evoked potential (BAEP) studies on P0-sensitized mice. Two P0-sensitized mice showed hunched posture, poor coat, loss of body weight, and abnormal walking with a waddling gait. About 25% of the P0-sensitized mice developed hearing loss. In the BAEP study, peak latencies of waves I, III, and V and the interpeak latency I-III were prolonged in the P0-sensitized hearing loss group of mice. Hearing thresholds were elevated in this group of mice in comparison with the control mice. Inflammatory cell infiltration was observed in the cochlear nerve region, and a reduced number of spiral ganglion cells was also detected. These results suggest that P0-sensitized mice are a useful model for studying autoimmune inflammation of the peripheral portion of the auditory system.

Involvement of insulin-like growth factor-I in inner ear organogenesis and regeneration

The verterbrate inner ear is an excellent model system to study signalling mechanisms in embryonic development. During the last years, insulin-like growth factor-I (IGF-I) has attracted attention in relation to the regulation of inner ear ontogenesis. IGF-I and its high-affinity tyrosine-kinase receptor are expressed during early stages of inner ear development. IGF-I is a powerful mitogen for the otic vesicle, where it stimulates cell-division and mitogenic signalling cascades. Later in development, IGF-I also promotes survival and neurogenesis of the otic neurones in the cochleovestibular ganglion (CVG). The actions of IGF-I are associated with the generation of lipidic messengers and the activation of Raf kinase, which results in the rapid induction of the expression of the proliferative cell nuclear antigen (PCNA) and the nuclear proto-oncogenes c-fos and c-jun. Regulation of organogenesis involves a dynamic balance of the mechanisms regulating cell division, differentiation and death. A model is proposed where this balance is the consequence of the action of IGF-I and NGF, which converge in Raf activation or suppression. The combinatorial expression of jun and Fos family members in particular domains of the otic vesicle would be the final result of such cascade. Some of these mechanisms may be also implicated in otic regeneration.

Local substances regulating cochlear blood flow

The regulation of cochlear lateral wall blood flow was investigated in rats using histochemistry, immunohistochemistry and transmission electron microscopy. The contractile protein, tropomyosin (TM) was localized in the pericyte around the vessels of spiral ligament but little was seen around strial capillaries. Prostaglandin I2 (PGI2) is a vasodilatory substance produced by PGI2 synthase (PGI2S), which is present in the endothelium of the vessels of spiral ligament but not much is present around strial capillaries. Endothelial nitric oxide synthase (eNOS) was present in the endothelium of both the vessels of spiral ligament and strial capillaries. These findings indicate that the blood flow of the cochlear lateral wall is regulated mainly in the spiral ligament by locally produced substances and that strial blood flow is not significantly regulated by pericytes responsible for enlargement or constriction of capillaries.

Select types of supporting cell in the inner ear express aquaporin-4 water channel protein

Aquaporins (AQPs) confer a high water permeability on cell membranes and play important parts in secretory and absorptive epithelia in kidney and other organs. Here we investigate whether AQPs are expressed in the sensory epithelia of the inner ear, where a precise volume regulation is crucial. By use of specific antibodies it was found that the inner ear contains AQP1 and 4 while being devoid of detectable levels of AQP2, 3 or 5. Immunofluorescence and postembedding immunogold labelling revealed a strictly non-epithelial distribution of AQP1, confirming previous data. In contrast, AQP4 protein and mRNA (visualized by in situ hybridization) were concentrated in select types of supporting cell, including Hensen's cells and inner sulcus cells. Immunogold particles signalling AQP4 were confined to the basolateral plasma membrane of Hensen's cells and to the basal plasma membrane of Claudius cells and inner sulcus cells. AQP4 was also found in supporting cells of the vestibular end organs, but was absent from transitional epithelial cells and dark cells. Strong labelling for AQP4 and AQP4-mRNA was associated with the central part of the cochlear and vestibular nerves. Hair cells were consistently unlabelled. Our findings indicate that AQP4 may facilitate osmotically driven water fluxes in the sensory epithelia of the inner ear and thus contribute to the volume and ion homeostasis at these sites.

Expression of the gap-junction connexins 26 and 30 in the rat cochlea

Gap junction channels which are responsible for direct intercellular communication are composed of connexin proteins. Different connexins are distributed in a tissue-specific manner. Up to now only connexin26 has been identified to be widely expressed in the inner ear. In order to investigate the role of additional gap junction proteins, the expression of connexin30 and 43 was investigated in the rat cochlea. Connexin26 and connexin30 were both expressed in the spiral limbus, the spiral ligament, the stria vascularis and between supporting cells of the organ of Corti. Double-labeling experiments suggest that both connexins are partly colocalized between cells. Weak staining of connexin43 could only be detected in the stria vascularis, the spiral ligament and between organ of Corti supporting cells. The corresponding transcripts for connexin26, 30 and 43 could be detected by Northern blot analysis. The expression of different gap junction channels in the cochlea suggests functional diversity. Gap junctions in the inner ear may control ion concentrations of cochlear fluids or act as conduits through which glucose and other metabolites diffuse.

HPLC detection of serotonin within the rat cochlea

This study was performed to analyse the cochlear concentrations of serotonin (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA), their sources and modifications induced by noise exposure. Superior cervical ganglionectomy did not modify these concentrations. However, removal of the blood by aortic perfusion reduced significantly (about 76%) the cochlear concentration of 5-HT without affecting the 5-HIAA concentration. These results indicate that blood constitutes an important source of 5-HT to the cochlea, opposite to the superior cervical ganglion. Exposure to noise at 90 dB SPL did not modify the total cochlear concentrations of 5-HT and 5-HIAA, or the concentrations remaining after removal of the blood, suggesting that 5-HT could have a modulatory role in the cochlea distinct from that of olivocochlear neurotransmitters.

[Regional variations of the cytokeratin expression along the guinea pig cochlear turn]

Expression of cytokeratin (Ck) proteins in the guinea pig cochlea was examined using a recently developed immunohistochemical method for celloidin section. This method has allowed us to observe a whole cochlea with excellent morphological preservation in a visual field under the light microscope. Anti-Ck antibodies FIL-7 (AE-1, Signet Lab.) and NCL-CK19 (Novocastra Lab.) stained all supporting cells of the organ of Corti, in which pillar cells, Deiters' cells, Hensen's cells, and Boettcher's cells showed a strong reaction. The histochemical labeling of pillar and Deiter's cells showed a gradual increment from the base to the apex. The immunohistochemically stained area in Hensen's cells increased toward the apex, which was attributed mainly to the increase in both the number and size of Hensen's cells from the base to the apex. In an electron-microscopic examination, changes in histochemical labeling of Hensen's cells did not appear from the base to the apex. Close examination revealed that reaction precipitates were distributed along a filament structure which seemed to correspond to the cytokeratin filaments. These results suggest that the Ck filaments give elasticity and flexibility to the supporting cells in the organ of Corti and provide mechanical strength to the organ of Corti at the upper turns, which vibrate with larger amplitudes, and that the Ck filaments in the supporting cell must be of considerable importance as a tuning mechanism. Immunohistochemical labeling for Ck was also observed in the spiral prominence cells, outer sulcus cells, interdental cells, and Reissner's membrane. The positively stained area in the outer sulcus increased from the apex to the base.

Use of electron spectroscopic imaging to determine element composition of the melanin granules in the stria vascularis of the guinea pig

Electron spectroscopic imaging (ESI) was used to analyze the element content of melanin granules in the stria vascularis seen in ultrathin sections of Spurr-embedded cochleae of the guinea pig. To determine element composition, ESI images were taken at different ionization edges, and non-specific background signals were subtracted digitally by an image processing system. The presence of calcium and nitrogen in the melanin granules could be demonstrated clearly. The calcium identified in the melanin granules was then compared with the spatial distributions of calcium binding sites after the application of an antimonate precipitation method, which was used to localize loosely bound calcium. Despite a high calcium concentration within the granules, only very small single scattered calcium precipitates could be detected between these structures as compared with the amount of calcium precipitates attached to the plasma membrane or located within the cell nuclei. The nearly complete absence of precipitates within the melanin granules after the application of antimonate suggests differences in calcium binding and mobility involved in various physiological processes of ion balance regulation within the stria vascularis.

[Levels of the pineal hormone melatonin and its circadian variations in the cochlea of Wistar rats]

The normal inner ear contains different elements belonging to the immune system. Some pathology of the inner ear has been attributed to autoimmune mechanisms, both humoral and cellular. Melatonin, the principal hormone of the pineal gland, modulates the immune system, is involved in autoimmune processes, and mediates free-radical and interleukine production. In a study of the presence of melatonin in rat cochlea, a day-night rhythm of melatonin in cochlea was observed. Melatonin levels are high during dark periods and low during light periods. This rhythm is synchronic with peripheral melatonin circulation. More work, both experimental and clinical, is needed to determine the role of melatonin in the inner ear and its therapeutic usefulness.

[The study of ultrastructure of atrial natriuretic peptides immunoreactive in cochlear of guinea pigs]

To further explore ultrastructure of atrial natriuretic peptides immunoreactive (ANP-IR) products in cochlear of guinea pig, and secretory type, ultrastructure of ANP-IR products of stria vascularis were studied with immunoelectronic microscopy. The result indicated: ANP-IR granules were seen in the cytoplasm, more obviously at the ends of the nucleus. There were two types of granule: storage and secretory granule. The study suggests that ANPs were secreted by rupture of restriction membrane, not by exocytosis secretion.

Sound-evoked efflux of excitatory amino acids in the guinea-pig cochlea in vitro

We have used the perfused guinea-pig temporal-bone preparation to study the sound-evoked efflux of aspartate and glutamate, which are putative afferent transmitters in the cochlea. The cochlea was stimulated with white noise at 89, 95, and 101 dB SPL. Cochlear function was monitored by recording the endocochlear potential, the cochlear microphonic, and the summating potential. In silence, there was a low basal efflux of both amino acids. A significant and intensity dependent sound-evoked efflux of aspartate was observed at all levels, whereas a significant efflux of glutamate was found only at the 101 dB SPL level. Immunohistochemistry of sections from the organ of corti showed an ubiquitous distribution of glutamate-like immunoreactivity in the sensory organ and ganglion, whereas aspartate-like immunoreactivity was found in the region of the inner hair cells and in the spiral ganglion. In view of these findings, we suggest that not only glutamate, but also aspartate may have a neurotransmitter role in the afferent pathway of the cochlea.

Expression of growth factors and their receptors in the postnatal rat cochlea

RT-PCR was used to assay for growth factors and receptors from seven different protein families in cochlea tissues of the juvenile rat. There was a broad representation of the growth factor families in all the cochlea tissues examined, though the organ of Corti and stria vascularis expressed a greater variety than the spiral ganglion. This broad expression suggests that a variety of known growth factors play significant roles in the development, maintenance, and repair of the inner ear. The results of this survey serve as a basis for the design of future in vitro experiments that will address the ability of growth factors to protect hair cells from damage and to evoke a repair-regeneration response by injured hair cells.

Mutation of a gene encoding a protein with extracellular matrix motifs in Usher syndrome type IIa

Usher syndrome type IIa (OMIM 276901), an autosomal recessive disorder characterized by moderate to severe sensorineural hearing loss and progressive retinitis pigmentosa, maps to the long arm of human chromosome 1q41 between markers AFM268ZD1 and AFM144XF2. Three biologically important mutations in Usher syndrome type IIa patients were identified in a gene (USH2A) isolated from this critical region. The USH2A gene encodes a protein with a predicted size of 171.5 kilodaltons that has laminin epidermal growth factor and fibronectin type III motifs; these motifs are most commonly observed in proteins comprising components of the basal lamina and extracellular matrixes and in cell adhesion molecules.

Expression of mRNA encoding vasopressin V1a, vasopressin V2, and ANP-B receptors in the rat cochlea

The expression of mRNAs encoding vasopressin V1a, V2, and ANP-B receptors in the rat cochlea was examined by PCR and in situ hybridization. After reverse-transcription of rat cochlear RNA, cDNA was amplified by PCR using pairs of primers specific to these receptors. After subcloning of the PCR products, clones with sequences identical to those cloned previously from the rat liver (V1a receptor), kidney (V2 receptor) and brain (ANP-B receptor) were obtained. The localization of expression of those receptors in the developing and adult rat cochlea was examined by in situ hybridization using 35S-labeled cRNA probes. The V1a and V2 receptors were expressed throughout the whole of the neonatal rat cochlea, while no expression was detected in the adult cochlea. The ANP-B receptor was expressed throughout the whole of the neonatal cochlea. In the adult cochlea, expression was observed in the spiral ganglion and the spiral ligament. These results suggest that vasopressin may play a role in the development of the cochlea, and that natriuretic peptide may play a role in the function of the spiral ganglion and the spiral ligament.

Induction of immune-mediated hearing loss in SCID mice by injection of MRL/lpr mouse spleen cells

Induction of immune-mediated hearing loss in SCID mice by injection of MRL/lpr mouse spleen cells The MRL/lpr mouse, which has a mutation in the Fas gene encoding a cell-surface receptor for apoptosis, shows an accumulation of abnormal immunocompetent cells and SLE-like disease. It has recently been reported that this mouse also manifests sensorineural hearing loss (SHL) with cochlear pathology at 20 weeks of age. We examined the effects of injecting MRL/lpr spleen cells on the development of SHL in severe combined immunodeficient (SCID) mice, which originally develop neither SHL nor cochlear pathology. Immune-mediated SHL and cochlear pathology were, indeed, transferred to the SCID mice by the injection of spleen cells from the MRL/lpr mice. These findings suggest that cell-mediated immunity is involved in the development of SHL and cochlear pathology.

Distribution of Eph-related molecules in the developing and mature cochlea

Receptors and ligands of the Eph family have recently been shown to influence the development of a variety of tissues. In the present study, the temporal and spatial distribution of Eph receptors and ligands were investigated in the embryonic and postnatal cochlea using Northern blot and immunohistochemical analysis. The results of Northern blot experiments revealed that a large number of Eph family members were present in embryonic cochlear and vestibular ganglia. Immunohistochemical studies revealed that ligands and receptors of the GPI subclass were distributed in complementary patterns within the differentiating spiral limbus, inner sulcus and outer sulcus. The distribution of these molecules became more restricted beginning in the first postnatal week. In contrast, members of the transmembrane subclass of Eph ligands were largely associated with cochlear neurons and their target hair cells. Expression of these ligands appeared to increase during the second postnatal week, corresponding to the period of peripheral nerve fiber reorganization in the cochlea. Together, these studies suggest that multiple Eph family members play unique roles in formation of the cochlea.

Apoptosis in the human inner ear. Detection by in situ end-labeling of fragmented DNA and correlation with other markers

The aim of this study was to obtain baseline data on the recently described special form of single cell death, apoptosis, in normal human inner ears. For this purpose, in situ end-labeling of the fragmented DNA was applied, in conjunction with apoptosis-related markers, to detect cellular elements showing programmed cell death in decalcified and paraffin-embedded tissues. Over 20 specimens were analyzed which were obtained from autopsy cases with no history of acoustic lesions confirmed by histopathology. Based on staining results, we saw no apoptotic signs in the majority of normal adult inner ears. An apoptotic cell captured in the Reissner's membrane of the cochlea from an old patient may, however, indicate an age-related subtle cell loss with the process of apoptosis. Nevertheless, the fact that more apoptosis was not found in our cases suggests that this phenomenon does not contribute significantly to the tissue homeostasis in the adult inner ear under normal conditions. These data are in accordance with our immunohistochemical findings on the p53 nucleoprotein, and proliferating cell nuclear antigen expression since there was no staining in any of the cellular elements, including the mesenchymal cells. This reflects a stationary and stable condition of cells of the vestibular and the cochlear structures, probably to maintain their integrity and the fine sensory functions. As opposed to the above findings, during inner ear development, the epithelial cells lining the cochlear lumen, the ossifying cartilage of the temporal bone, and the mesenchymal cells show different degrees of proliferation in combination with single cell death as signs of maturation of the vestibular and the cochlear apparatus. In addition, apoptosis has been demonstrated in cells of the cochlear stria vascularis from an adult patient treated with high doses of cisplatin, vinblastine and bleomycin prior to death. Furthermore, a wide range of apoptosis could be induced experimentally in a normal ear by an external perfusion of actinomycin D (ActD), which is known to produce programmed cell death in many cell types of different origins. The potential role of cytostatic agents in the apoptotic process of the inner ear needs, however, to be confirmed in large-scale specimens from patients treated with genotoxins. The fact, however, that apoptotic cells are also seen in association with ActD indicates that the fine sensory structure of the cochlea may also be a target for certain chemotherapeutic agents when administered in high doses.

Presence of the pineal hormone melatonin in rat cochlea: its variations with lighting conditions

Within the normal inner ear, there are elements that belong to the immune system. Different inner ear disorders can be explained by autoimmune mechanisms, affecting both humoral and cellular immunity. Melatonin, the principal hormone of the pineal gland, modulates the immune system and extensively participates in the autoimmune processes related to type II collagen. Therefore, we have studied the presence of melatonin in rat cochlea, proving that its concentrations change depending on lighting conditions. Rats show high levels when confined to darkness and low levels when subject to continuous light exposure. The results correlate with the concentration of melatonin in peripheral circulation. Further experimental and clinical studies are necessary to clarify the role and the possible therapeutic applications of melatonin.

The optical properties of the cochlear bone

The transmittance, reflectance and the angular dependence of coherent light at wavelengths of 632.8 and 750 nm in the cochlear bone in guinea pigs were measured by a single integrating sphere and a goniometer. The strict one-dimensional transport theory was applied to calculate the absorption and scattering coefficients. The Henyey-Greenstein phase function was applied to determine the anisotropy factor g, from the goniometer measurements. A linear inverse correlation was found between the calculated g and sample thickness. We found that the cochlear bone in guinea pigs strongly scatters light in the forward direction. Its absorption and scattering coefficients are much larger than those of dermis. Delrin has a similar optical absorption property to human dermis tissue.

Localization of soluble guanylate cyclase activity in the guinea pig cochlea suggests involvement in regulation of blood flow and supporting cell physiology

Although the nitric oxide/cGMP pathway has many important roles in biology, studies of this system in the mammalian cochlea have focused on the first enzyme in the pathway, nitric oxide synthase (NOS). However, characterization of the NO receptor, soluble guanylate cyclase (sGC), is crucial to determine the cells targeted by NO and to develop rational hypotheses of the function of this pathway in auditory processing. In this study we characterized guinea pig cochlear sGC by determining its enzymatic activity and cellular localization. In cytosolic fractions of auditory nerve, lateral wall tissues, and cochlear neuroepithelium, addition of NO donors resulted in three- to 15-fold increases in cGMP formation. NO-stimulated sGC activity was not detected in particulate fractions. We also localized cochlear sGC activity through immunocytochemical detection of NO-stimulated cGMP. sGC activity was detected in Hensen's and Deiters' cells of the organ of Corti, as well as in vascular pericytes surrounding small capillaries in the lateral wall tissues and sensory neuroepithelium. sGC activity was not observed in sensory cells. Using NADPH-diaphorase histochemistry, NOS was localized to pillar cells and nerve fibers underlying hair cells. These results indicate that the NO/cGMP pathway may influence diverse elements of the auditory system, including cochlear blood flow and supporting cell physiology.

Pathology of the cochlea following a spontaneous mutation in DBA/2 mice

The DBA/2 strain of mice usually presents with noise-induced epileptic seizures and hearing disorders. After a spontaneous mutation a strain with early hearing loss and circling behaviour was produced. This strain presents with clinical symptoms found in diseases connected to inner ear disorders. These animals do not suffer from periodical disorders, however, but have functional disturbances continuously and can therefore serve as an animal model for diseases originating from both parts of the inner ear. The genetic inheritance appears to be autosomal recessive. Offspring showed circling behaviour and severe pathology in the vestibular part of the inner ear. In the present study pathology of the cochlear part of the inner ear was visualized using conventional microscopical techniques. The content of actin and fodrin was labelled immunohistochemically, and hearing was assessed with auditory brainstem recordings. After 1 month the animals showed deterioration of the cochlear part of the inner ear. At 6 months no organ of Corti remained and the animals were deaf. Transmission and scanning electron microscopy revealed severe apical hair cell changes. The content of alpha-actinin and fodrin in the DBA/2 mouse was already fainter than that in age-matched CBA control mice at the age of 1 month. Labelling of antibodies against fodrin increased in the supporting cells of the older animals, probably owing to the replacement of hair cells.