First appearance and development of motile properties in outer hair cells isolated from guinea-pig cochlea

Generation of somatic electromechanical force by outer hair cells may be influenced by prestin-CASK interaction at the basal junction with the Deiter's cell

The motor protein, prestin, situated in the basolateral plasma membrane of cochlear outer hair cells (OHCs), underlies the generation of somatic, voltage-driven mechanical force, the basis for the exquisite sensitivity, frequency selectivity and dynamic range of mammalian hearing. The molecular and structural basis of the ontogenetic development of this electromechanical force has remained elusive. The present study demonstrates that this force is significantly reduced when the immature subcellular distribution of prestin found along the entire plasma membrane persists into maturity, as has been described in previous studies under hypothyroidism. This observation suggests that cochlear amplification is critically dependent on the surface expression and distribution of prestin. Searching for proteins involved in organizing the subcellular localization of prestin to the basolateral plasma membrane, we identified cochlear expression of a novel truncated prestin splice isoform named prestin 9b (Slc26A5d) that contains a putative PDZ domain-binding motif. Using prestin 9b as the bait in a yeast two-hybrid assay, we identified a calcium/calmodulin-dependent serine protein kinase (CASK) as an interaction partner of prestin. Co-immunoprecipitation assays showed that CASK and prestin 9b can interact with full-length prestin. CASK was co-localized with prestin in a membrane domain where prestin-expressing OHC membrane abuts prestin-free OHC membrane, but was absent from this area for thyroid hormone deficiency. These findings suggest that CASK and the truncated prestin splice isoform contribute to confinement of prestin to the basolateral region of the plasma membrane. By means of such an interaction, the basal junction region between the OHC and its Deiter's cell may contribute to efficient generation of somatic electromechanical force.

The LINC complex is essential for hearing

Hereditary hearing loss is the most common sensory deficit. We determined that progressive high-frequency hearing loss in 2 families of Iraqi Jewish ancestry was due to homozygosity for the protein truncating mutation SYNE4 c.228delAT. SYNE4, a gene not previously associated with hearing loss, encodes nesprin-4 (NESP4), an outer nuclear membrane (ONM) protein expressed in the hair cells of the inner ear. The truncated NESP4 encoded by the families' mutation did not localize to the ONM. NESP4 and SUN domain-containing protein 1 (SUN1), which localizes to the inner nuclear membrane (INM), are part of the linker of nucleoskeleton and cytoskeleton (LINC) complex in the nuclear envelope. Mice lacking either Nesp4 or Sun1 were evaluated for hair cell defects and hearing loss. In both Nesp4-/- and Sun1-/- mice, OHCs formed normally, but degenerated as hearing matured, leading to progressive hearing loss. The nuclei of OHCs from mutant mice failed to maintain their basal localization, potentially affecting cell motility and hence the response to sound. These results demonstrate that the LINC complex is essential for viability and normal morphology of OHCs and suggest that the position of the nucleus in sensory epithelial cells is critical for maintenance of normal hearing.

Human cochlea: anatomical characteristics and their relevance for cochlear implantation

This is a review of the anatomical characteristics of human cochlea and the importance of variations in this anatomy to the process of cochlear implantation (CI). Studies of the human cochlea are essential to better comprehend the physiology and pathology of man's hearing. The human cochlea is difficult to explore due to its vulnerability and bordering capsule. Inner ear tissue undergoes quick autolytic changes making investigations of autopsy material difficult, even though excellent results have been presented over time. Important issues today are novel inner ear therapies including CI and new approaches for inner ear pharmacological treatments. Inner ear surgery is now a reality, and technical advancements in the design of electrode arrays and surgical approaches allow preservation of remaining structure/function in most cases. Surgeons should aim to conserve cochlear structures for future potential stem cell and gene therapies. Renewal interest of round window approaches necessitates further acquaintance of this complex anatomy and its variations. Rough cochleostomy drilling at the intricate "hook" region can generate intracochlear bone-dust-inducing fibrosis and new bone formation, which could negatively influence auditory nerve responses at a later time point. Here, we present macro- and microanatomic investigations of the human cochlea viewing the extensive anatomic variations that influence electrode insertion. In addition, electron microscopic (TEM and SEM) and immunohistochemical results, based on specimens removed at surgeries for life-threatening petroclival meningioma and some well-preserved postmortal tissues, are displayed. These give us new information about structure as well as protein and molecular expression in man. Our aim was not to formulate a complete description of the complex human anatomy but to focus on aspects clinically relevant for electric stimulation, predominantly, the sensory targets, and how surgical atraumaticity best could be reached.

Development of Cochlear Amplification, Frequency Tuning, and Two-Tone Suppression in the Mouse

It is generally believed that the micromechanics of active cochlear transduction mature later than passive elements among altricial mammals. One consequence of this developmental order is the loss of transduction linearity, because an active, physiologically vulnerable process is superimposed on the passive elements of transduction. A triad of sensory advantage is gained as a consequence of acquiring active mechanics; sensitivity and frequency selectivity (frequency tuning) are enhanced and dynamic operating range increases. Evidence supporting this view is provided in this study by tracking the development of tuning curves in BALB/c mice. Active transduction, commonly known as cochlear amplification, enhances sensitivity in a narrow frequency band associated with the “tip” of the tuning curve. Passive aspects of transduction were assessed by considering the thresholds of responses elicited from the tuning curve “tail,” a frequency region that lies below the active transduction zone. The magnitude of cochlear amplification was considered by computing tuning curve tip-to-tail ratios, a commonly used index of active transduction gain. Tuning curve tip thresholds, frequency selectivity and tip-to-tail ratios, all indices of the functional status of active biomechanics, matured between 2 and 7 days after tail thresholds achieved adultlike values. Additionally, two-tone suppression, another product of active cochlear transduction, was first observed in association with the earliest appearance of tuning curve tips and matured along an equivalent time course. These findings support a traditional view of development in which the maturation of passive transduction precedes the maturation of active mechanics in the most sensitive region of the mouse cochlea.

Lateral wall protein content mediates alterations in cochlear outer hair cell mechanics before and after hearing onset

Specialized outer hair cells (OHCs) housed within the mammalian cochlea exhibit active, nonlinear, mechanical responses to auditory stimulation termed electromotility. The extraordinary frequency resolution capacity of the cochlea requires an exquisitely equilibrated mechanical system of sensory and supporting cells. OHC electromotile length change, stiffness, and force generation are responsible for a 100-fold increase in hearing sensitivity by augmenting vibrational input to non-motile sensory inner hair cells. Characterization of OHC mechanics is crucial for understanding and ultimately preventing permanent functional deficits due to overstimulation or as a consequence of various cochlear pathologies. The OHCs' major structural assembly is a highly-specialized lateral wall. The lateral wall consists of three structures; a plasma membrane highly-enriched with the motor-protein prestin, an actin-spectrin cortical lattice, and one or more layers of subsurface cisternae. Technical difficulties in independently manipulating each lateral wall constituent have constrained previous attempts to analyze the determinants of OHCs' mechanical properties. Temporal separations in the accumulation of each lateral wall constituent during postnatal development permit associations between lateral wall structure and OHC mechanics. We compared developing and adult gerbil OHC axial stiffness using calibrated glass fibers. Alterations in each lateral wall component and OHC stiffness were correlated as a function of age. Reduced F-actin labeling was correlated with reduced OHC stiffness before hearing onset. Prestin incorporation into the PM was correlated with increased OHC stiffness at hearing onset. Our data indicate lateral wall F-actin and prestin are the primary determinants of OHC mechanical properties before and after hearing onset, respectively.

Developmental expression of the outer hair cell motor prestin in the mouse

The development of motor protein activity in the lateral membrane of the mouse outer hair cell (OHC) from postnatal day 5 (P5) to P18 was investigated under whole-cell voltage clamp. Voltage-dependent, nonlinear capacitance (C (v)), which represents the conformational fluctuations of the motor molecule, progressively increased during development. At P12, the onset of hearing in the mouse, C (v) was about 70% of the mature level. C (v) saturated at P18 when hearing shows full maturation. On the other hand, C (lin), which represents the membrane area of the OHC, showed a relatively small increase with development, reaching steady state at P10. This early maturation of linear capacitance is further supported by morphological estimates of surface area during development. These results, in light of recent prestin knockout experiments and our results with quantitative polymerase chain reaction, suggest that, rather than the incorporation of new motors into the lateral membrane after P10, molecular motors mature to augment nonlinear capacitance. Thus, current estimates of motor protein density based on charge movement may be exaggerated. A corresponding indicator of motor maturation, the motor's operating voltage midpoint, V (pkcm), tended to shift to depolarized potentials during postnatal development, although it was unstable prior to P10. However, after P14, V (pkcm) reached a steady-state level near -67 mV, suggesting that intrinsic membrane tension or intracellular chloride, each of which can modulate V (pkcm), may mature at P14. These developmental data significantly alter our understanding of the cellular mechanisms that control cochlear amplification and provide a foundation for future analysis of genetic modifications of mouse auditory development.

The concentrations of calcium buffering proteins in mammalian cochlear hair cells

Calcium buffers are important for shaping and localizing cytoplasmic Ca2+ transients in neurons. We measured the concentrations of the four main calcium-buffering proteins (calbindin-D28k, calretinin, parvalbumin-alpha, and parvalbumin-beta) in rat cochlear hair cells in which Ca2+ signaling is a central element of fast transduction and synaptic transmission. The proteins were quantified by calibrating immunogold tissue counts against gels containing known amounts of each protein, and the method was verified by application to Purkinje cells in which independent estimates exist for some of the protein concentrations. The results showed that, in animals with fully developed hearing, inner hair cells had 110 of the proteinaceous calcium buffer of outer hair cells in which the cell body contained parvalbumin-beta (oncomodulin) and calbindin-D28k at levels equivalent to 5 mm calcium-binding sites. Both proteins were partially excluded from the hair bundles, which may permit fast unbuffered Ca2+ regulation of the mechanotransducer channels. The sum of the calcium buffer concentrations decreased in inner hair cells and increased in outer hair cells as the cells developed their adult properties during cochlear maturation. The results suggest that Ca2+ has distinct roles in the two types of hair cell, reflecting their different functions in auditory transduction. Ca2+ is used in inner hair cells primarily for fast phase-locked synaptic transmission, whereas Ca2+ may be involved in regulating the motor capability underlying cochlear amplification of the outer hair cell. The high concentration of calcium buffer in outer hair cells, similar only to skeletal muscle, may protect against deleterious consequences of Ca2+ loading after acoustic overstimulation.

Early maturation of evoked otoacoustic emissions and medial olivocochlear reflex in preterm neonates

The present study was designed to investigate the early maturation of the brainstem regulation of the cochlear function in preterm neonates. Evoked otoacoustic emissions (EOAE) and their regulation via the medial olivocochlear efferent (MOC) reflex were investigated in a large population of preterm neonates and compared with full-term neonates and young babies from birth to 4 y and school-aged children. In 28-wk preterm neonates, EOAE were seen in the mid-frequency range. These responses extended both to the low (down to 1025 Hz) and high (up to 6152 Hz) frequency ranges at 38 wk of gestational age and remained stable up to 4 mo. At this stage, the amplitude of EOAE overlapped adult values. EOAE amplitudes then decreased to reach adult values at 3 y of age. Maturation of MOC efferents innervating the outer hair cells was investigated by studying the suppressive effect of contralateral sound on the EOAE amplitudes (MOC reflex). The first MOC responses were recorded in preterm neonates of 32-33 wk of gestational age, reaching adult-like values at 37 wk of gestational age. The maximum effect of MOC efferent activation occurred between 2000 and 4000 Hz. These results suggest that, in humans, MOC efferents mature in utero. Thus, testing the MOC reflex may have a clinical relevance to detect an abnormal development of the auditory pathways, particularly of a brainstem circuitry not explored through conventional testing.

Development of the human fetal cochlear nerve: a morphometric study

Ontogenesis of the human peripheral auditory pathway is relatively less explored. While the distal part of the auditory perception apparatus (i.e. the cochlea) received attention, studies on the neural element carrying information to the brainstem (i.e. the cochlear nerve) are scarce. In the present study, axonal differentiation, maturation and myelination of the distal end of the human cochlear nerve (CN) were assessed using light and electron microscopy. Seven human fetuses of 12, 15, 18, 20, 22, 28 and 38 weeks' gestation (WG) were analyzed. Light microscopy revealed nerve fascicles as early as 12 WG, initially arranged loosely but later compacted by 18 WG. Myelinated fibers were clearly detected at 28 WG. Ultrastructurally, at 12 WG developing Schwann cells were present between the thin unmyelinated axons. At 15 WG, the fascicular arrangement was distinct with blood vessels in the perineurium. The maximum number of axons was found at 20 WG, which subsequently reduced to reach the adult level at 22 WG. The myelinated axons in the CN were first observed on the left side at 20 WG, following which the number and proportion of myelinated axons increased until term, incorporating both small and large axons. The right CN lagged behind in maturation. Axon size also increased with age. Thus, the maturation of the human CN commences during the mid-gestation period and produces exuberant axons that are eventually pruned at a time when axons start to myelinate. During this developmental period the human CN maintains maturational asymmetry, the functional consequences of which remain to be elucidated.

The voltage-sensitive motor protein and the Ca2+-sensitive cytoskeleton in developing rat cochlear outer hair cells

Cochlear outer hair cells (OHCs) possess a unique fast voltage-driven motility associated with a voltage-sensitive motor protein embedded in the basolateral membrane. This mechanism is believed to underlie the cochlear amplification in mammals. OHCs also have a Ca2+/calmodulin-dependent mechanical pathway which involves a submembranous circumferential cytoskeleton. The purpose of this study was to compare the functional appearance of the voltage-sensitive motor proteins with that involving the Ca2+-sensitive cytoskeleton during postnatal development of rat OHCs. We demonstrate that whole-cell electromotility and Ca2+-voked mechanical responses, by ionomycin, develop concomitantly after postnatal day 5 (P5). These two mechanical properties also develop simultaneously in OHCs isolated from two-week-old cultures of P0-P1 organs of Corti. This excludes the participation of neural innervation in the postnatal maturation of the OHCs' motile properties. In addition, we show that the expression of the membranous voltage-sensitive motor protein precedes, by several days, the appearance of whole-cell electromotility. The concomitant development of whole-cell electromotility and Ca2+-sensitive motility, both in vivo and in vitro, underlines the cytoskeleton as an important factor in the functional organization of the voltage-sensitive motor proteins within the plasma membrane.

Expression density and functional characteristics of the outer hair cell motor protein are regulated during postnatal development in rat

1. The non-linear capacitance (Cnon-lin) of postnatal outer hair cells (OHCs) of the rat was measured by a patch-clamp lock-in technique. Cnon-lin is thought to result from a membrane protein that provides the molecular basis for the unique electromotility of OHCs by undergoing conformational changes in response to changes in membrane potential (Vm). Protein conformation is coupled to Vm by a charged voltage sensor, which imposes Cnon-lin on the OHC. Cnon-lin was investigated in order to characterize the surface expression and voltage dependence of this motor protein during postnatal development. 2. On the day of birth (P0), Cnon-lin was not detected in OHCs of the basal turn of the cochlea, whilst it was 89 fF in apical OHCs. Cnon-lin increased gradually during postnatal development and reached 2.3 pF (basal turn, P9) and 7.5 pF (apical turn, P14) at the oldest developmental stages covered by our measurements. The density of the protein in the plasma membrane, deduced from non-linear charge movement per membrane area, increased steeply between P6 and P11 and reached steady state (4200 e- microm-2) at about P12. 3. Voltage at peak capacitance (V) shifted with development from hyperpolarized potentials shortly after birth (-88.3 mV, P2) to the depolarized potential characteristic of mature OHCs (-40.8 mV, P14). This developmental difference in V was also observed in outside-out patches immediately after patch excision. During subsequent wash-out V shifted towards the depolarized value found in the adult state, suggesting a direct modulation of the molecular motor. 4. Thus, the density of the motor protein in the plasma membrane and also its voltage dependence change concomitantly in the postnatal period and reach adult characteristics right at the onset of hearing.

Influence of spontaneous otoacoustic emissions on distortion product otoacoustic emission amplitudes

Although the influence of the levels and ratios of the primary stimulus on the amplitude of distortion product otoacoustic emissions (DPOAEs) has been studied intensely, the influence of the presence of spontaneous otoacoustic emissions (SOAEs) has been investigated less thoroughly. The present investigation analysed whether the unilateral presence of 58 SOAEs in 43 normal-hearing adults was related to larger DPOAEs in the ear with SOAEs compared to the contralateral ear having no SOAEs. The study was designed such that the only factor that could influence the amplitude of DPOAEs was the presence of SOAEs. Input/output (I/O) functions were collected in response to primary tones that were presented in 5-dB steps from 70 to 40 dB SPL at the frequency of the unilaterally recorded SOAE of each subject. The primary outcome was the demonstration of statistically significant (P < 0.05) larger DPOAEs in ears exhibiting SOAEs than in ears without measurable SOAEs, except at the highest stimulus level of 70 dB SPL. These results suggest that SOAEs play an additive role in the measurement of DPOAEs. The enhancing effect of the unilateral presence of SOAEs on DPOAEs was statistically significant for 65 dB SPL and lower levels of primary tones. The authors speculate that passive cochlear properties begin to participate in the generation of DPOAEs at primary-stimulus levels greater than 65 dB SPL.

Relationship between the development of outer hair cell electromotility and efferent innervation: a study in cultured organ of corti of neonatal gerbils

Outer hair cell (OHC) electromotility, which powers the cochlear amplifier, develops at a later stage of hearing ontogeny. There has been speculation whether efferents play a necessary role in directing or achieving OHC maturation in mammals. In this study, we examine whether the development of OHC motility depends on the establishment of efferent innervation of the cells' synaptic pole by measuring electromotility of OHCs grown in cultures, deprived of efferent innervation. Tissue cultures of the organ of Corti were prepared from the cochleas of newborn gerbils. Solitary OHCs were obtained from 4- to 15-d-old cultures by enzymatic digestion and mechanical trituration. Length changes evoked by transcellular electrical stimulation were detected and measured with a photodiode sensor. Results show that OHCs develop electromotility between 6 and 13 d in culture without the presence of efferent innervation. The timetable for the onset of OHC electromotility is comparable with that in vivo. This demonstrates that the ontogeny of OHC electromotility is an intrinsic process that does not require the influence of efferent innervation.

The postnatal development of F-actin in tension fibroblasts of the spiral ligament of the gerbil cochlea

The tension fibroblasts of the spiral ligament of the mammalian cochlea are thought to create radial tension on the basilar membrane. Their postnatal development was investigated in the gerbil (Meriones unguiculatus) with confocal fluorescence microscopy using phallotoxin as a specific marker for F-actin. In the adult cochlea, tension fibroblasts were restricted to the basal cochlear turn and were arranged in 2-4 rows in the marginal region of the spiral ligament. They contained intensely stained parallel bundles of F-actin. In upper cochlear turns, the marginal region of the spiral ligament was occupied by sparsely distributed, unobtrusively labeled fibrocytes, the bone lining cells. The spiral ligament of young postnatal stages (newborn--6 days after birth (DAB)) lacked F-actin labeling patterns that are characteristic for tension fibroblasts in the adult. Rather, the whole inner surface of the otic capsule throughout all cochlear turns was outlined by cell layers with distinct but diffuse cytoplasmic F-actin label. These cells may represent perichondrial fibrocytes. Around 9 DAB, the perichondrium revealed changes in morphology and F-actin patterns that indicate a further differentiation into tension fibroblasts (basal turn) or bone lining cells (more apical turns). At 12 DAB, around onset of hearing, adult-like bone lining cells were found in the marginal regions of the spiral ligament of upper cochlear turns. In the basal turn, tension fibroblasts were present, but their F-actin cytoskeleton was not fully developed. During the following days, F-actin label increased in tension fibroblasts and reached adult-like configuration at 17 DAB, coinciding with mature hearing characteristics. The role of tension fibroblasts in development of hearing characteristics is discussed.

Development of the organ of Corti in horseshoe bats: scanning and transmission electron microscopy

The late prenatal and early postnatal development of the organ of Corti were studied in the horseshoe bat (Rhinolophus rouxi) by using scanning and transmission electron microscopy. Arrangements and dimensions of stereocilia bundles, together with their contacts with the tectorial membrane, were found to be adult-like shortly before birth, and thus before the biological onset of hearing (3-5 days after birth). During the first postnatal week, there were baso-apical gradients in disappearing kinocilia on inner hair cells (IHC), microvillis of supporting cells, and marginal pillars. The lower basal cochlear turn was mature with respect to these regressing structures at 3 days after birth, the apical turn at 10 days after birth. At birth, cytodifferentiation was found to be completed, and the tunnel of Corti and innermost spaces of Nuel had opened. The ultrastructure of IHCs was not markedly different from that at later ages. In outer hair cells (OHC), the adult-like regular arrangement of a single layer of subsurface cisternae and pillars was seen as soon as protrusions of supporting cells had withdrawn from the lateral wall of OHCs (basal turn at birth and throughout the cochlea 2 days after birth). Numerous efferent endings contacted the somata of IHCs up to the second postnatal week. Since the medial olivocochlear system is absent in horseshoe bats, the adult-like innervation pattern of OHCs was established at the biological onset of hearing. During the first 2 postnatal weeks, the cytoskeleton of pillar and Deiters cells, and the specialized Deiters cups developed. The organ of Corti appeared adult-like at 14 days, apart from the persistence of a reduced tympanic cover layer attached to the basilar membrane. Morphological data support physiological findings that the first broadly tuned auditory responses arise from the basal turn. The distinct low to high frequency gradient in development of sensitivity during the first 2 postnatal weeks of the horseshoe bat was not, however, matched by morphological gradients, and it would appear that the development of the cytoskeleton of supporting cells contributed to the establishment of tuning in the auditory fovea. Adult-like morphology of the organ of Corti coincided with the emergence of sharply tuned responses from the auditory fovea, but there was no clear-cut correlate for the shift in tuned foveal frequency representation that occurred during the following 3 weeks.

Development of cochlear active mechanisms in humans differs between gender

Despite onset of function early during the third term of gestation, the human auditory system demonstrates continued maturation, thought previously to occur primarily at the neural level. The electromotile properties of outer hair cells appear to contribute substantially to hearing sensitivity and frequency selectivity and lead to the generation of otoacoustic emissions (OAEs). This report demonstrates continued development of cochlear active mechanisms (i.e. end-organ level) after onset of cochlear function, as reflected by OAEs. Significant gender differences also are reported, corresponding to recently observed intersex differences in cochlear length and precursory to gender differences observed in the adult.

The early postnatal development of F-actin patterns in the organ of Corti of the gerbil (Meriones unguiculatus) and the horseshoe bat (Rhinolophus rouxi)

The arrangements of F-actin in hair cells and non-sensory cells were studied in paraformaldehyde-fixed cochleae of horseshoe bats and gerbils in several postnatal stages and in the adult. Phallotoxin-labeled midmodiolar cryostat sections of the organ of Corti were analyzed with confocal fluorescence microscopy. In both species, the arrangement of F-actin in the adult organ of Corti was essentially similar to that described in other mammals; however, both species showed their own species-typical specializations in staining of the Deiters cells. In the gerbil, a distinct baso-apical gradient in morphology and staining properties was found in the upper compartment of the Deiters cells. In the bat, F-actin label within the Deiters cups was most pronounced in the basal cochlear turn and less abundant in the apical turns. During the first postnatal week, the sensory epithelium of the gerbil lacked the tunnel of Corti and the spaces of Nuel. Only the reticular lamina and the surface of the greater epithelial ridge were intensely labeled for F-actin. At 9 days after birth (DAB), when the tunnel of Corti and the inner spiral sulcus were formed, the footplates of Deiters and pillar cells and the apices of pillar cells began to show intense F-actin label. At 12 DAB, corresponding to onset of hearing, F-actin staining was found throughout the supporting cell bodies, but was less intense than in the adult. The specialized upper compartment of the Deiters cells differentiated around 15-20 DAB. In the neonate bat, gross-morphology of the organ of Corti was almost adult-like, but only the reticular lamina and the head- and footplates of pillar cells showed intense F-actin staining. The F-actin cytoskeleton of the Deiters cells bodies was poorly developed. At the onset of hearing (between 3rd and 5th DAB), supporting cells showed only a slight increase of F-actin mainly at mechanically important cell regions, namely the Deiters cups, the contact zone of pillar headplates and the footplates of supporting cells. The most intense increase of F-actin occurred between onset of hearing and 16 DAB. At 16 DAB, the F-actin distribution within the supporting cells was similar to the adult. In both species, there were no clear baso-apical gradients in development of F-actin patterns. It is proposed that F-actin insertion in supporting cells after the onset of hearing contributes to maturation of cochlear function.

Cholinergic responses in developing outer hair cells of the rat cochlea

Acetylcholine-evoked currents were investigated using the conventional whole-cell patch-clamp recording technique in developing outer hair cells (OHCs). The cells were isolated from the rat cochlea at different stages of postnatal development ranging from day 4 (P4) to P30. Acetylcholine-evoked currents could be recorded at P6 and P8. At this developmental stage, the majority of OHCs displayed inward nicotinic-like currents near the resting membrane potential. These cholinergic currents zeroed near 0 mV, as expected for a non-selective cation current, and could be reversibly blocked by d-tubocurarine. At P12 and adult stage, the cholinergic response of OHCs switched to an outward current reversing near EK and displaying a bell shape peaking between -40 and -30 mV. This change in polarity of the acetylcholine response during postnatal development might be explained by progressive functional coupling between acetylcholine ionotropic receptors permeable to Ca2+ and nearby Ca(2+)-activated K+ channels at the synaptic pole of OHCs.

Distortion generated by the ear: its emergence and evolution during development

Physiologically vulnerable active mechanics associated with the transduction of sounds in adults distort cochlear output. Specifically, frequencies not present in the incoming acoustic signal are represented in its output (ie., the spike trains of auditory nerve fibers). The purpose of this investigation was to study the appearance and evolution of intermodulation distortion during development. Established surgical and electrophysiologic techniques were used to record the extracellular electrical activity of individual auditory nerve fibers from developing kittens. Discharge-rate or synchrony-based responses to two tones (f1 and f2) presented simultaneously at various ratios of f2/f1 were recorded. The cubic distortion product (CDP; 2f1-f2, where f1<f2) and the difference tone (f2-f1) were the intermodulation distortion products (DPs) emphasized in the investigation. CDPs were first observed nearly 10 days after the earliest auditory nerve fiber responses to simple tone-burst stimuli could be elicited in neonatal cats. Furthermore, the spectrum of DPs generated in the cochlea changed dramatically during the second postnatal week and achieved adultlike characteristics after the first postnatal month. Findings reported here support the notion that cochlear transduction is initially linear in neonatal cats, possibly representing a critical period of auditory development, and that the content of distortion-related spectral information carried in auditory nerve fiber spike trains increases during development, in concert with other peripheral auditory attributes.

Development of intracellular Ca-ATPase in the gerbil outer hair cell lateral wall

The development of Ca-ATPase immunoreactivity in gerbil outer hair cells (OHCs), assayed by immunofluorescence and postembedding immunocytochemistry, is reported here. In the adult, a linear array of label is seen inside the lateral plasma membrane. The ultrastructural distribution of Ca-ATPase near the OHC lateral plasma membrane was examined using immunogold cytochemistry and showed this calcium pumping enzyme to be present throughout the subsurface cisternal complex (SSC), especially near the innermost layers. During development, Ca-ATPase immunoreactivity appeared in patches near the lateral plasma membrane of some OHCs of the third row by 12 days after birth (DAB). By 15-16 DAB, punctate immunoreactivity was detected in the second and first rows. At 20 DAB, immunostaining near OHC lateral plasma membrane was increased, but was less continuous than OHC staining in the adult cochlea. The appearance of Ca-ATPase in OHCs coincides with the onset of auditory function and isolated OHC motility in the gerbil. The ultrastructural demonstration of abundant sites of calcium pumps in the SSC supports a role for this structure in the intracellular storage of calcium. These findings suggest a possible role of Ca-ATPase and the SSC in the regulation of slow motility of OHCs which has been reported to depend on intracellular calcium concentration.

Development of the auditory receptors of the rat: a SEM study

Fetal and postnatal ontogenesis of the rat cochlea, from the 16th gestational day (16DG) until 3 months post partum, were studied using scanning electron microscopy with emphasis on the stereocilia during the earliest stages of development. The epithelium of the cochlear duct in 16DG rat consisted of plygonal cells topped with numerous microvilli and one central kinocilium, which form the so-called Kölliker's organ. Inner hair cells (IHCs) appeared at 18DG in the basal cochlea. They were characterized by tufts of cilia of the same height and with a kinocilium. The first outer hair cells (OHCs) can be seen at 20DG. The earliest stages of ciliary differentiation, at 18DG for IHCs and 20DG for OHCs, were similar on both types of cells and were characterized by the presence of round bundles of cilia arising from the surrounding microvilli. A three-dimensional V-shaped organization for OHCs and the linear arrangement for IHCs appeared by the end of the first postnatal week, accompanied by the disappearance of transient cilia on the modiolar side of the hair cell and the kinocilium on the external side. The apical pole of OHCs reached adult-like morphology before that of IHCs. Various links between stereocilia were detected already at birth. Morphometric analysis showed that auditory cells from the base of the cochlea reached adult size by the end of the first postnatal week while those from the apex increased their size later. A review of the literature including comparative observations across species on the ontogenesis of the stereocilia shows that hair cells of the stato-acoustic system may present the same early ontogenesis.

Expression of plasma membrane Ca-ATPase in the adult and developing gerbil cochlea

The distribution of the plasma membrane Ca-ATPase (PMCA) was mapped in the adult and developing gerbil cochlea by immunostaining with a monoclonal antibody against the human erythrocyte PMCA. In the mature cochlea, intense immunoreactivity was present at the surface of stereocilia of both inner (IHC) and outer (OHC) hair cells. The basolateral plasma membrane of IHCs but not OHCs stained strongly whereas that of strial marginal cells and the epithelial cell layer of Reissner's membrane showed only weak reactivity. Nerve terminals underlying IHCs were also selectively stained. At birth, strong to moderate reactivity for PMCA was present in the basolateral plasma membrane of IHCs and OHCs, strial marginal cells, and epithelial cells lining the scala media surface of Reissner's membrane and in the neurolemma of spiral ganglion cells. Immunostaining in the basolateral plasmalemma of OHCs, strial marginal cells, and epithelial cells lining Reissner's membrane remained strong to moderate up to 14 days after birth when it diminished or disappeared entirely, suggesting a developmental role for PMCA activity in these sites. Expression of PMCA at the surface of IHC and OHC stereocilia was first observed at 10 days after birth and staining reached adult levels by 14 days after birth. The abundance of PMCA in the stereociliary plasma membrane of mature hair cells supports the suggested involvement of Ca2+ in regulating transduction and adaptation mechanisms.

Postnatal development of membrane specialisations of gerbil outer hair cells

Using a combination of freeze-fracture and thin sections, this study examines the maturation of the membrane specialisations of the gerbil outer hair cells (OHC) between 2 and 16 days after birth (DAB). The apical membrane, the junctional region around the neck of the cell, and the lateral and basal membranes are described. The results suggest a sequential development of the different components of the lateral wall. Intramembrane protein particles (IMP), the putative OHC motor elements, were found to be present at low density at 2 DAB and increased in density from 2200 IMP/microns 2 at 2 DAB to 4131/ microns 2 at 8 DAB. OHCs have been reported as showing electromotility from 8 DAB onward. IMPs continue to increase in density until mature values are attained at 16 DAB. Sub-surface cisternae did not appear until 8 DAB, with a single layer being complete by 10 DAB. Pillar structures, proposed to be related to the cytoskeletal lattice, first appear at 10 DAB. The apical membrane of the immature hair cell is characterised by the presence of pits related to the endocytosis of vesicles, and tip-links between stereocilia, thought to be associated with sites of ion channel opening, are present at 2 DAB. The junctional region comprises two areas which mature at differing rates: an apical-most region which attains an adult-like appearance by 8 DAB and a basal-ward region which continues to increase in complexity until mature at 16 DAB. The functional significance of the results are discussed in relation to the possible roles of the junctional regions and the proposed sites of the OHC motor elements.

Structural and ultrastructural aspects of isolated immature cochlear outer hair cells maintained in short-term culture

Immature outer hair cells (OHCs), isolated from developing rat cochlea without using proteolytic enzymes, were maintained in short-term culture in a clot of coagulated plasma. Cell viability was assessed by a laser scanning image cytometer, using double-fluorescent labeling. Light and transmission electron microscopy was used to study the morphology of isolated cells. Ten to 60 healthy OHCs were obtained from one cochlea, either as single isolated cells or clusters containing 2-10 cells from the same row. Although dead cells were observed only 1 h after dissociation, there were still viable cells after 6 h. Isolated OHCs were not perfectly cylindrical, due to the immaturity of their cortical structures. One hour after dissociation the ultrastructural organization of the isolated cells was generally well preserved, but this was followed by dilatation of the Golgi apparatus and endoplasmic reticulum. Specific changes in isolated OHCs were also observed at the subsurface cisternae and cuticular plate. Although degenerating OHCs generally showed a classic pattern of necrosis, certain morphological features reminiscent of apoptosis were also observed. This study emphasises the difficulty involved in investigating isolated immature OHCs in vitro and provides a basis for future research into the physiological requirements of isolated immature OHCs.

Cytological changes related to maturation of the organ of Corti and opening of Corti's tunnel

Maturation of the organ of Corti in the gerbil was analyzed between 2 and 16 days after birth (DAB) by electron microscopy and immunostaining for beta-tubulin. At 2 DAB, the organ of Corti consisted of stratified epithelium bearing immature sensory hair cells (HCs) and supporting cells. Maturation of OHCs and Deiters cells progressed in a medial-to-lateral direction and cytoskeletal development in inner pillar cells preceded that in outer pillar cells at the single location studied along the frequency-place map. Pillar cell differentiation progressed through a unique stage characterized by the appearance and stratification of structural features apparently concerned with opening of Corti's tunnel and subsequently showed other structural changes related to maturity toward the adult form. Development of the microtubule cytoskeleton occurred first in the cell's apex and proceeded basally. Ruffling of a middle region of the cell surface by microvilli appeared to promote separation between inner and outer pillar cells and initiate tunnel opening at 4 DAB. Proliferation of distended cisternae of granular reticulum evidenced proteinaceous secretion by these cells between 4 and 8 DAB. Subsequent tunnel expansion at about 14 DAB coincided with appearance in outer pillar cells of tubulocisternal endoplasmic reticulum and associated Golgi complexes that are thought to mediate fluid and ion secretion. Sixteen days postnatally after disappearance of granular and tubulocisternal reticula and Golgi complexes and at the time of clearing of tunnel fluid, lysosomes interpreted as mediating catabolism of endocytosed protein congregated beneath the apical and apicolateral plasmalemmae of inner pillar cells. As with pillar cells, development of the microtubule system in Deiters cells proceeded from the cell's apex to base. Following differentiation of their microtubule system by 8 DAB, Deiters cells showed expansion of Golgi cisternae between 10 and 15 DAB and development of tubulocisternal endoplasmic reticulum at 15 DAB. Hair cells possessed abundant, distinctively large mitochondria from 4 to 10 DAB. The subsurface cisternae matured earlier in medial as opposed to lateral outer hair cells. Vesicles budding from underlying cisternae appeared associated with development of subsurface cisternae and at 16 DAB were still observed in third row but not in more mature first row HCs.

Postnatal expression of the facilitated glucose transporter, GLUT 5, in gerbil outer hair cells

The fifth isoform from the family of facilitative glucose transporters (GLUT 5) was identified in the gerbil cochlea by light and electron microscopic immunohistochemistry. The immunostaining procedure employed a polyclonal antibody raised against a synthetic peptide representing the carboxy terminus of human GLUT 5. Immunoreactive GLUT 5 was abundant in the basolateral plasma membrane of outer hair cells (OHCs) in the mature gerbil cochlea. No immunostaining was observed in any other site in the cochlea or vestibular system. During postnatal development of the gerbil inner ear, OHCs were first observed to express GLUT 5 in a punctate pattern at 10 days after birth. Immunostaining intensity increased gradually and developed a linear pattern along the entire OHC basolateral plasmalemma between 10 and 16 days after birth when it approached adult levels. The developmental expression of GLUT 5 corresponded with the appearance of glycogen in OHCs and structural maturation of the organ of Corti. This time period also coincided with the onset and rapid maturation of auditory function in the gerbil. GLUT 5 apparently supports OHC function by facilitating uptake of glucose which enables these cells to sustain a high level of metabolic activity in a relatively anaerobic environment.

Early hair-cell degeneration in the extreme apex of the guinea pig cochlea

Guinea pigs, aged from 3 weeks before term to 31 weeks after birth, were prepared for scanning electron microscopy. Examination of the extreme apex of the cochlea showed apparently pathological hair cells, even 3 weeks before term. The pathologies included loss and fusion of stereocilia, and the formation of giant stereocilia. The pathologies were most prevalent on row 3 of outer hair cells, declining to outer hair cell rows 2 and 1, with the inner hair cells being least affected. The abnormalities increased with time, increasing rapidly over the first few weeks of life, and more slowly thereafter. It is suggested that early degeneration in the extreme cochlear apex forms a novel model for spontaneous hair cell degeneration, with applicability to other types of spontaneous hair cell degeneration.

Development of calretinin immunoreactivity in the mouse inner ear

Calretinin is a calcium-binding protein of the EF-hand family. It has been previously identified in particular cell types of adult guinea pig, rat, and chinchilla inner ear. Development of calretinin immunoreactivity in the mouse inner ear was investigated from embryonic day 13 (E13) to the adult stage. In the adult mouse vestibule, calretinin immunoreactivity was present in the same structures as described for the rat and guinea pig: the population of afferent fibers forming calyx units and a small number of ganglion neurons. The earliest immunoreactivity was found at E17 in vestibular hair cells (VHCs), then, at E19, in afferent fibers entering the sensory epithelia and in rare ganglion neurons. At postnatal day 4 (P4), a few vestibular nerve fibers and ganglion neurons were reactive. From this stage until P14, immunoreactivity developed in the calyx units and disappeared from VHCs. At P14, immunostaining was adult-like. In the adult mouse cochlea, immunoreactivity was present in the same cell populations as described in the rat: the inner hair cells (IHCs) and most of Corti's ganglion neurons. Calretinin immunoreactivity appeared at E19-P0 in IHCs and ganglion neurons of the basal turn. At P1, outer hair cells (OHCs) of the basal turn were positive. Calretinin immunoreactivity then appeared in IHCs, OHCs, and ganglion neurons of the medial turn, then of the apical turn. At P4, all IHCs and OHCs and most of the ganglion neurons were immunostained. Immunoreactivity gradually disappeared from the OHCs starting at P10 and, at P22, only IHCs and ganglion neurons were positive. The sequences of appearance of calretinin were specific to each cell type of the inner ear and paralleled their respective maturation. Calretinin was transiently expressed in VHCs and OHCs.

First appearance and development of electromotility in neonatal gerbil outer hair cells

With the purpose of pinpointing the time of onset of electromotility, outer hair cells (OHCs) from apical and basal turns of the cochleae of postnatal gerbils, ranging in age from 6 to 19 days, were isolated and drawn into a glass microchamber. Length changes evoked by transcellular electrical stimulation were detected and measured with a photodiode detector. Motile responses first appeared in 3 out of 14 basal turn OHCs at 7 days after birth (DAB). At 8 DAB, 3 out of 13 apical turn cells also responded to the electrical stimulation. By 12 DAB, all the OHCs from both turns showed motile responses. Input-output functions relating applied stimulus and change in cell length revealed that the motile response threshold improved from 7 DAB to 12 DAB and the response amplitude kept increasing from 7 DAB until 13-14 DAB, when mature amplitudes were reached. Measurements of OHC length revealed only minor changes in basal turn hair cell length while apical hair cells continued to elongate until approximately 16 DAB. Since the onset of auditory function in gerbils occurs around 12 DAB and fine tuning develops between 14 and 17 DAB, our results suggest that the onset of OHC motility occurs earlier than that of auditory function and the maturation of the motility amplitude occurred earlier than the development of fine tuning. The maturation of OHC motility and the development of otoacoustic emissions are also compared and discussed.

Responses of peripheral auditory neurons to two-tone stimuli during development: II. Factors related to neural responsiveness

In an accompanying paper (Fitzakerley et al., 1994), it was demonstrated that there is a significant developmental correlation between the appearance of tuning and two-tone suppression. However, it was also found that some sharply tuned neurons meeting minimal adult standards did not exhibit suppression. Therefore, in order to investigate other factors that may be related to the demonstration of suppression in peripheral auditory neurons, the relationship of two-tone suppression with various parameters related to neural responsiveness was studied in perinatal kittens. A positive correlation was made between the observance of suppression and driven discharge rate, characteristic frequency (CF), and threshold, all properties which change significantly during the final stages of cochlear differentiation. In older animals, suppression was also observed in higher percentages of neurons having low spontaneous rates (< 1 spk/s). Suppression was evoked less often by test tones placed below CF than above CF in neurons recorded from younger animals, and was generally produced by a narrower range of test tone intensities than those recorded from adults. As a result, the conventional description of suppression as observed in peripheral auditory neurons in mature animals must be extended to include these factors when responses of immature neurons are considered.

Effects of endolymphatic and perilymphatic application of salicylate in the pigeon. II: Fine structure of auditory hair cells

Large doses of salicylate are known to cause reversible ototoxic effects including fine structural alterations of the auditory hair cells in mammals. To investigate possible fine structural correlates of salicylate effects on pigeon auditory hair cells, the basilar papillae following perilymphatic or endolymphatic application of salicylate were fixed and processed for transmission electron microscopy. The pigeon auditory hair cells possessed organelles typically described in avians. A single or multi-layered array of cisternae along the cytoplasmic side of the lateral plasma membrane, i.e. subsurface cisternae that are characteristic for mammalian outer hair cells, was not seen. The most prominent fine structural alterations of hair cells after salicylate application were an increase in the luminal width of smooth and rough endoplasmic reticulum as well as the frequent occurrence of prominent single-membrane-bound vesicles filled with electron-dense bodies. Based on the assumption that subsurface cisternae represent a specialized form of endoplasmic reticulum, the present findings indicate that the structural correlates of salicylate toxicity are similar in mammalian and avian auditory hair cells.

Monoclonal antibodies specific for endoplasmic membranes of mammalian cochlear outer hair cells

Monoclonal antibodies were raised in vitro against an antigen associated with the lateral cisternal membranes of outer hair cells. Two of the antibodies were class IgM and one of these retained its specific reactivity in tissue fixed with aldehydes and embedded in the resin LR White. Immunogold labelling for electron microscopy showed that the antigen was closely associated with the membranes rather than the cytoplasmic or lumenal regions of the cisternae. The third antibody was an IgG. All three weakly labelled a protein band with an apparent molecular weight of about 60 kD on a Western blot. The antibodies did not cross-react with any other cell in the organ of Corti, including the inner hair cells. Furthermore, they showed no cross-reactivity with skeletal muscle, kidney, gut, brain, skin, blood or retina from the guinea pig. The results suggest that the lateral cisternae in outer hair cells may be functionally different from those of inner hair cells. The antibodies may provide useful markers for outer hair cells in studies of hair cell regeneration.

Distribution of actin in developing outer hair cells in the gerbil

During the two weeks following the onset of cochlear function in the gerbil, active cochlear processes appear to mature. The active cochlear processes likely involve outer hair cells with their specialized lateral wall structures, including the subsurface cisternae and associated cytoskeletal elements. We have previously demonstrated that the subsurface cisternae mature gradually during the time that active cochlear processes mature in the gerbil. In the study reported here, we used postembedding immunocytochemical electron microscopy to investigate whether actin labelling associated with the cortical cytoskeleton of the gerbil outer hair cell increased concomitantly. In contrast to the gradual development of the subsurface cisternae, actin labelling in the region of the cortical cytoskeleton significantly increased during the onset of cochlear function and maintained this level during the time that active cochlear processes mature. Thus, it appears that increased actin adjacent to the lateral plasma membrane of the outer hair cell is related to the onset of cochlear function rather than to the maturation of active cochlear processes.

Development of gerbil outer hair cells after the onset of cochlear function: an ultrastructural study

It has recently been proposed that elements which contribute to active cochlear processes develop at the same time (between postnatal day (PND) 12 and 21) as the shift of the place code in the developing gerbil cochlea. Since outer hair cells (OHCs) have been implicated in these processes, we have hypothesized that developing OHCs will exhibit changes in anatomical features that contribute to cochlear maturation. Our results demonstrate that the ultrastructural characteristics of OHCs change after the onset of hearing (PND 12), during the time that cochlear nonlinearities are being established (PND 12-21). Differences are primarily associated with the distribution of cytoplasmic organelles. The subsurface cisternae (SSC), which are thought to be related to the mechanical support of the outer hair cell, to cell motility, and therefore to cochlear mechanics, are present at PND 10 but remain immature, with cisternal layers added during the preweanling period. In immature OHCs, more mitochondria are centrally-located than in mature OHCs. During development mitochondria come to form a continuous row near the innermost leaflet of the SSC. These ultrastructural features undergo rapid change during the maturation of peripheral auditory function.

Dissociation between the calcium-induced and voltage-driven motility in cochlear outer hair cells from the waltzing guinea pig

The waltzing guinea pig, possessing an hereditary progressive deafness, shows pathology to the actin-bearing structures within the hair cells of the organ of Corti. In particular, the affected structures include the stereocilia, the cuticular plate and, as shown in the present study, swollen and disorganized subsurface cisternae. To test whether this pathology affected outer hair cell motility, cells were isolated from waltzing guinea pigs and their age-matched controls and were subjected to either electrical or chemical stimulation. Visual detection thresholds and the magnitude of the electrically-induced length changes were equivalent for both groups. However, when intracellular calcium was increased with either the calcium ionophore, ionomycin or Ca2+/ATP (under permeabilized conditions with DMSO), length changes were significantly reduced for the outer hair cells from waltzing guinea pigs compared to the controls. The average percent length increase induced by 10 microM ionomycin for the outer hair cells from control animals was 2.3 +/- 1.7 whereas for postnatal day 4 waltzing guinea pigs it was 1.3 +/- 1.7. Postnatal day 7 and 10 waltzing guinea pigs responded with significantly smaller percent length changes. The intracellular concentration of ionic calcium increased similarly for both groups after the application of ionomycin as revealed with the indicator fluo-3. In the permeabilized cells in the presence of Ca2+/ATP, control cells responded with a percent length change of 3.5, whereas, age-matched waltzing outer hair cells responded with barely detectable length changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenesis of F-actin in hair cells

This report describes the ontogenesis of cochlear stereocilia using scanning electron microscopy for analysis of cilia appearance, and fluorescence microscopy of phalloidin, a label for F-actin, to determine the maturation of the cilia framework. Surface and frozen-sectioned preparations of the otic capsule were obtained from several stages of rat pup development beginning at the 16th gestational day and at various stages until adulthood. In the earliest stage investigated, strong fluorescence labeling was visible on the apical part of Kölliker's organ, revealing a reticular outline of cell junctions. Hair cells started to differentiate at the 18th day of gestation from cells within the primordial receptor area. Phalloidin labeling revealed a sequential appearance of F-actin as the hair cells differentiated from the cells with the Kölliker's organ. The differentiation of receptor cells occurred first with the appearance of a junctional complex between the hair cell and the surrounding cells. Then a cuticular plate appeared followed by the progressive emergence of stereocilia. The F-actin labeling also revealed a progressive differentiation of receptor cells from the cochlear base to its apex. There was also an inner to outer hair cell developmental gradient of label. Inner hair cells developed stereocilia before outer hair cells. The third row of outer hair cells was the last to acquire stereocilia. The adult pattern of stereocilia was reached around the 6th postnatal day. We conclude that the appearance of actin filaments in developing receptor cells and the emergence of stereocilia can be regraded as markers for correlating function and other structural differentiation.

Development of frequency tuning in the auditory periphery

The peripheral auditory organ, the cochlea, acts as a spectral analyzer resolving the frequency components of sound. During development the cochlea first responds to loud low-frequency sounds, and only gradually acquires the adult pattern of increased sensitivity and an expanded high-frequency range. This evolution of function may result in part from the gradual maturation of hair cell properties.