Further observations on the fine structure of tip links between stereocilia of the guinea pig cochlea

Spontaneous calcium transients in hair cell stereocilia

The hair bundle of auditory and vestibular hair cells converts mechanical stimuli into electrical signals through mechanoelectrical transduction (MET). The MET apparatus is built around a tip link that connects neighboring stereocilia that are aligned in the direction of mechanosensitivity of the hair bundle. Upon stimulation, the MET channel complex responds to changes in tip-link tension and allows a cation influx into the cell. Ca 2+ influx in stereocilia has been used as a signature of MET activity. Using genetically encoded Ca 2+ sensors (GCaMP3, GCaMP6s) and high-performance fluorescence confocal microscopy, we detect spontaneous Ca 2+ transients in individual stereocilia in developing and fully formed hair bundles. We demonstrate that this activity is abolished by MET channel blockers and thus likely originates from putative MET channels. We observe Ca 2+ transients in the stereocilia of mice in tissue explants as well as in vivo in zebrafish hair cells, indicating this activity is functionally conserved. Within stereocilia, the origin of Ca 2+ transients is not limited to the canonical MET site at the stereocilia tip but is also present along the stereocilia length. Remarkably, we also observe these Ca 2+ transients in the microvilli-like structures on the hair cell surface in the early stages of bundle development, prior to the onset of MET. Ca 2+ transients are also present in the tallest rows of stereocilia in auditory hair cells, structures not traditionally thought to contain MET channels. We hypothesize that this newly described activity may reflect stochastic and spontaneous MET channel opening. Localization of these transients to other regions of the stereocilia indicates the presence of a pool of channels or channel precursors. Our work provides insights into MET channel assembly, maturation, function, and turnover.

Resistance to neomycin ototoxicity in the extreme basal (hook) region of the mouse cochlea

Aminoglycoside ototoxicity results in permanent loss of the sensory hair cells in the mammalian cochlea. It usually begins at the basal turn causing high-frequency hearing loss. Here we describe previously unreported resistance of hair cells to neomycin ototoxicity in the extreme basal (hook) region of the developing cochlea of the C57BL/6 mouse. Organ of Corti explants from mice at postnatal day 3 were incubated (37 °C, 5% CO₂) in normal culture medium for 19.5 h prior to and after exposure to neomycin (1 mM, 3 h). To study neomycin uptake in the hair cells, cochlear explants were incubated with Neomycin Texas-red (NTR) conjugate. As expected, exposure to neomycin significantly reduced the survival of inner (IHC) and outer hair cells (OHC). IHC survival rate was high in the apical segment and low in the basal segment. OHC were well preserved in the apical and hook regions, with substantial OHC loss in the basal segment. The NTR uptake study demonstrated that the high survival rate in the extreme basal turn OHC was associated with low NTR uptake. Treatment with a calcium chelator (BAPTA), which disrupts the opening of mechanoelectrical (MET) transduction channels, abolished or reduced NTR uptake in the hair cells throughout the cochlea. This confirmed the essential role of MET channels in neomycin uptake and implied that the transduction channels could be impaired in the hook region of the developing mouse cochlea, possibly as a result of the cadherin 23 mutation responsible for the progressive deafness in C57BL/6 mice.

The composition and role of cross links in mechanoelectrical transduction in vertebrate sensory hair cells

The key components of acousticolateralis systems (lateral line, hearing and balance) are sensory hair cells. At their apex, these cells have a bundle of specialized cellular protrusions, which are modified actin-containing microvilli, connected together by extracellular filaments called cross links. Stereociliary deflections open nonselective cation channels allowing ions from the extracellular environment into the cell, a process called mechanoelectrical transduction. This produces a receptor potential that causes the release of the excitatory neurotransmitter glutamate onto the terminals of the sensory nerve fibres, which connect to the cell base, causing nerve signals to be sent to the brain. Identification of the cellular mechanisms underlying mechanoelectrical transduction and of some of the proteins involved has been assisted by research into the genetics of deafness, molecular biology and mechanical measurements of function. It is thought that one type of cross link, the tip link, is composed of cadherin 23 and protocadherin 15, and gates the transduction channel when the bundle is deflected. Another type of link, called lateral (or horizontal) links, maintains optimal bundle cohesion and stiffness for transduction. This Commentary summarizes the information currently available about the structure, function and composition of the links and how they might be relevant to human hearing impairment.

Stereocilin connects outer hair cell stereocilia to one another and to the tectorial membrane

Stereocilin is defective in a recessive form of deafness, DFNB16. We studied the distribution of stereocilin in the developing and mature mouse inner ear and analyzed the consequences of its absence in stereocilin-null (Strc(-/-)) mice that suffer hearing loss starting at postnatal day 15 (P15) and progressing until P60. Using immunofluorescence and immunogold electron microscopy, stereocilin was detected in association with two cell surface specializations specific to outer hair cells (OHCs) in the mature cochlea: the horizontal top connectors that join the apical regions of adjacent stereocilia within the hair bundle, and the attachment links that attach the tallest stereocilia to the overlying tectorial membrane. Stereocilin was also detected around the kinocilium of vestibular hair cells and immature OHCs. In Strc(-/-) mice the OHC hair bundle was structurally and functionally normal until P9. Top connectors, however, did not form and the cohesiveness of the OHC hair bundle progressively deteriorated from P10. The stereocilia were still interconnected by tip links at P14, but these progressively disappeared from P15. By P60 the stereocilia, still arranged in a V-shaped bundle, were fully disconnected from each other. Stereocilia imprints on the lower surface of the tectorial membrane were also not observed in Strc(-/-) mice, thus indicating that the tips of the tallest stereocilia failed to be embedded in this gel. We conclude that stereocilin is essential to the formation of horizontal top connectors. We propose that these links, which maintain the cohesiveness of the mature OHC hair bundle, are required for tip-link turnover.

Cadherin 23-like polypeptide in hair bundle mechanoreceptors of sea anemones

We investigated hair bundle mechanoreceptors in sea anemones for a homolog of cadherin 23. A candidate sequence was identified from the database for Nematostella vectensis that has a shared lineage with vertebrate cadherin 23s. This cadherin 23-like protein comprises 6,074 residues. It is an integral protein that features three transmembrane alpha-helices and a large extracellular loop with 44 contiguous, cadherin (CAD) domains. In the second half of the polypeptide, the CAD domains occur in a quadruple repeat pattern. Members of the same repeat group (i.e., CAD 18, 22, 26, and so on) share nearly identical amino acid sequences. An affinity-purified antibody was generated to a peptide from the C-terminus of the cadherin 23-like polypeptide. The peptide is expected to lie on the exoplasmic side of the plasma membrane. In LM, the immunolabel produced punctate fluorescence in hair bundles. In TEM, immunogold particles were observed medially and distally on stereocilia of hair bundles. Dilute solutions of the antibody disrupted vibration sensitivity in anemones. We conclude that the cadherin 23-like polypeptide likely contributes to the mechanotransduction apparatus of hair bundle mechanoreceptors of anemones.

The dimensions and structural attachments of tip links in mammalian cochlear hair cells and the effects of exposure to different levels of extracellular calcium

The tip links between stereocilia of acousticolateral hair cells have been suggested to contain cadherin 23 (CDH23) comprising an upper branched portion that is bound to a lower portion composed of protocadherin 15 (PCDH15). The molecular conformation of CDH23, its binding to PCDH15, the tip links, and mechanoelectrical transduction have all been shown previously to be sensitive to exposure to low levels of calcium. The aim of this study was to compare the characteristics of tip links in guinea-pig cochlear hair cells with reported features of the CDH23-PCDH15 complex. Tip links were examined using field emission scanning electron microscopy and transmission electron microscopy in conventional preparations and after treatment with the detergent Triton-X-100 or varying calcium concentrations in the extracellular solution. The results showed that tip links have a twisted double-stranded appearance with a branched upper region. They survived demembranation of the stereocilia by detergent suggesting that they have transmembrane domains at both ends. Their lengths, when fixed in the presence of 2 mM extracellular calcium, were approximately 150 nm. With prior exposure to 1 mM calcium their lengths were approximately 164 nm. The lengths in 50 microM calcium are similar ( approximately 185 nm) to those reported for CDH23-PCDH15 complexes in 100 microM calcium ( approximately 180 nm). Exposure to 1 microM calcium caused loss of tip links and an increased distance between the residual attachment sites. The data indicate that extracellular calcium concentration affects tip-link length. One model compatible with the recently proposed tip-link structure is that the CDH23 double strand undergoes calcium-dependent unfolding, changing the length of the links. The bundle may also tilt in the direction of the tallest row of stereocilia as the tip link lengthens and then is lost. Overall, our data are consistent with a tip link composed of complexes of CDH23 and PCDH15 but do not rule out other possibilities.

Development and properties of stereociliary link types in hair cells of the mouse cochlea

The hair bundles of outer hair cells in the mature mouse cochlea possess three distinct cell-surface specializations: tip links, horizontal top connectors, and tectorial membrane attachment crowns. Electron microscopy was used to study the appearance and maturation of these link types and examine additional structures transiently associated with the developing hair bundle. At embryonic day 17.5 (E17.5), the stereocilia are interconnected by fine lateral links and have punctate elements distributed over their surface. Oblique tip links are also seen at this stage. By postnatal day 2 (P2), outer hair cell bundles have a dense cell coat, but have lost many of the lateral links seen at E17.5. At P2, ankle links appear around the base of the bundle and tectorial membrane attachment crowns are seen at the stereociliary tips. Ankle links become less apparent by P9 and are completely lost by P12. The appearance of horizontal top connectors, which persist into adulthood, occurs concomitant with this loss of ankle links. Treatment with the calcium chelator BAPTA or the protease subtilisin enabled these links to be further distinguished. Ankle links are susceptible to both treatments, tip links are only sensitive to BAPTA, and tectorial membrane attachment crowns are removed by subtilisin but not BAPTA. The cell-coat material is partially sensitive to subtilisin alone, while horizontal top connectors resist both treatments. These results indicate there is a rich, rapidly changing array of different links covering the developing hair bundle that becomes progressively refined to generate the mature complement by P19.

The structure of tip links and kinocilial links in avian sensory hair bundles

Recent studies have indicated that the tip links and kinocilial links of sensory hair bundles in the inner ear have similar properties and share a common epitope, and that cadherin 23 may also be a component of each link type. Transmission electron microscopy was therefore used to study and compare the fine structure of the tip links and kinocilial links in avian sensory hair bundles. Tannic acid treatment revealed a thin strand, 150-200 nm long and 8-11 nm thick, present in both link types. Fourier analysis of link images showed that the strand of both link types is formed from two filaments coiled in a helix-like arrangement with an axial period of 20-25 nm, with each filament composed of globular structures that are approximately 4 nm in diameter. Differences in the radius and period of the helix-like structure may underlie the observed variation in the length of tip and kinocilial links. The similar helix-like structure of the tip links and kinocilial links is in accord with the presence of a common cell-surface antigen (TLA antigen) and similarities in the physical and chemical properties of the two link types. The spacing of the globular structures comprising each filament of the two link types is similar to the 4.3 nm center-to-center spacing reported for the globular cadherin repeat, and is consistent with the suggestion that cadherin 23 is the tip link.

Ultrastructure of the abdominal sense organ of the scallop Mizuchopecten yessoensis (Jay)

The sensory epithelium of the abdominal sense organ (ASO) of the scallop Mizuchopecten yessoensis is composed of three cell types, sensory cells, mucous cells, and multiciliated cells. Sensory cells bear a single long (up to 250 microm) cilium surrounded by an inner ring of nine modified microvilli and an outer ring of ordinary microvilli paired with modified microvilli. Sensory cells make up about 90% of the total number of cells in the sensory epithelium. Mucous cells, which are much wider than sensory cells, bear only ordinary microvilli on their apical surface. Rare multiciliated cells with short (4-6 microm) cilia are scattered in the periphery of the sensory epithelium sheet. All hairs, cilium, and microvilli of each sensory cell are interconnected by a fibrous network. Nine modified microvilli of a single cell are interconnected by prominent laterally running fibrous links. Membrane-associated electron-dense material of modified microvilli is connected to the ciliary membrane-associated electron-dense material by fine string-like links. These links mechanically bridge the space between the cilium and modified microvilli, as do mechanical links, described for the stereocilia and kinocilium of vertebrate vestibular and cochlear hair cells. The proximal portion of a sensory cilium is about 100 microm long and has a typical 9 x 2+2 axoneme arrangement. The distal portion of a cilium is approximately 2 times thinner than the proximal one and is filled with homogeneous electron-dense material. Along the distal portion, diffuse material associated with the external surface of the membrane is found. The rigidity of distal portion of a cilium is much less than that of the proximal one.

Changes of Hair Cell Stereocilia and Threshold Shift after Acoustic Trauma in Guinea Pigs: Comparison between Inner and Outer Hair Cells

The vulnerability of inner hair cells (IHCs) and outer hair cells (OHCs) to acoustic overstimulation is still controversially discussed. The present study was undertaken to investigate the vulnerability of IHCs and OHCs and the relation between chronological changes of auditory threshold shifts and stereocilia damages on IHCs and OHCs in guinea pigs after moderate acoustic trauma, caused by a single continuous exposure to pink noise (20 Hz to 20 kHz) of around 106 +/- 2 dB SPL for 44 h. Stereocilia changes and threshold shifts of auditory brainstem responses (ABR) were assessed at regular intervals after noise exposure for 9 weeks. Scanning electron microscopy demonstrated the morphological changes of stereocilia as early as 1 day after noise exposure. The morphological changes included fused, bent, collapsed, and even missing stereocilia. These damages were more prominent on IHCs than on OHCs. The shift of ABR threshold was not parallel to the chronological change of the stereocilia on IHCs as well as OHCs. The elevation of the ABR threshold (40-60 dB SPL) was greatest on the 1st day after noise exposure, whereas the stereocilia showed the most damage 7 days after noise exposure. Combined with the results from previous studies, we conclude that moderate-level (around 105-110 dB) noise tends to induce more damage to the stereocilia of IHCs than of OHCs. Other damage (e.g., metabolic disturbance) than morphological damage of hair cell stereocilia may contribute partially to the hearing threshold shift induced by moderate acoustic overstimulation.

A novel antigen sensitive to calcium chelation that is associated with the tip links and kinocilial links of sensory hair bundles

Tip links are extracellular, cell-surface-associated filaments of unknown molecular composition that are thought to gate the mechanotransducer channel of the sensory hair cell. They disappear from the hair bundle in response to calcium chelation and lanthanum treatment and resist degradation by the protease subtilisin. A monoclonal antibody derived from a hybridoma screen identified a novel antigen associated with tip links, the tip-link antigen. The tip-link antigen is also associated with kinocilial links, subtilisin-resistant filaments that are sensitive to calcium chelation and connect the kinocilium to the tallest stereocilia of the hair bundle. Furthermore, the tip-link antigen is expressed in the retina, where it is associated with the ciliary calyx, a ring of microvilli that surrounds the outer segment of the photoreceptor. The tip-link antigen rapidly disappears from the surface of the hair bundle in response to calcium chelation. It is also subtilisin resistant, relative to the ankle-link antigen, an antigen associated with another type of hair bundle link. The tip-link antigen is lanthanum sensitive and, like tip links, reappears on the surface of the hair bundle after calcium chelation. The monoclonal antibody to the tip-link antigen immunoprecipitates two concanavalin A-reactive polypeptides with apparent molecular masses of 200 and 250 kDa from detergent extracts of the retina. These results provide the first identification of a cell surface antigen associated with tip links, indicate that tip links share properties in common with kinocilial links, and reveal a second epitope that, along with the ankle-link antigen, is common to both sensory hair bundles and the ciliary calyx of photoreceptors.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

The glycocalyx and stereociliary interconnections of the vestibular sensory epithelia of the guinea pig. A freeze-fracture, low-voltage cryo-SEM, SEM and TEM study

In this study freeze fracture, low-voltage cryo-SEM, SEM and TEM were used to characterise the glycocalyx and stereociliary interconnections in the hair cell bundle of the vestibular sensory epithelia of the guinea pig. The glycocalyx resembles a shell-like structure separately surrounding each stereocilium and kinocilium over its entire length. The lateral interciliary connections emerge from the glycocalyx layer, forming an extensive extracellular network maintaining the stereocilia as a bundle. These connections are morphologically similar to the glycocalyx, and grossly oriented in the same direction, indicative of a role in the mechanical transduction system. The matrix material around the glycocalyx also appears to be morphologically similar to the glycocalyx, suggesting the glycocalyx to be even more important in the mechanical transduction system. The tip-links are covered with a layer, which is a continuation of the glycocalyx covering the stereocilia.

Structure of outer hair cell stereocilia side and attachment links in the chinchilla cochlea

The structure and symmetry of chinchilla outer hair cell (OHC) stereocilia side and attachment links were investigated by transmission electron microscopy using tannic acid and Cuprolinic blue histochemical procedures. The side links run laterally between and across the rows of the stereocilia and connect the stereocilia together within the bundle. Attachment links form a crown-like array around the tips of only the tallest OHC stereocilia and attach these stereocilia to the Type B fibrils of the tectorial membrane. Computer averaging of the side links from tannic acid-treated tissues showed a central dense region of the link between adjacent stereocilia and a smaller dense portion at the plasma membrane end of the link. Computer averaging of Cuprolinic blue-treated tissues showed low electron density of the central region of the link, and the plasma membrane ends of the link were electron dense. After tannic acid treatment, the attachment links showed a diffused radial distribution around the tips of the tallest OHC stereocilia. After Cuprolinic blue treatment, the attachment links appeared as electron-dense, membrane-bound granular structures arranged with radial symmetry. The central regions of the side links are reactive to tannic acid. These regions appear to contain neutral and basic residues of proteins and participate in side-by-side association of the side links in regular aggregates. Cuprolinic blue-reactive regions of the side and attachment links appear to contain acidic sulfated residues of glycoproteins or proteoglycans, which may be involved in the attachment of these links to the stereocilium membrane.

A comparison of hair bundle mechanoreceptors in sea anemones and vertebrate systems

Hair bundle mechanoreceptors of the acousticolateralis system of vertebrates are similar to hair bundles found on tentacles of sea anemones, primitive marine invertebrates. In each case, hair bundles consist of actin-based stereocilia interconnected by extracellular linkages. Recently, considerable attention has been directed to one class of linkages called "tip links." Tip links interconnect the tip of one stereocilium to the adjacent, taller stereocilium. According to the currently favored hypothesis for signal transduction, tip links may be "gating springs" that gate cation channels opened during deflection of the hair bundle. To depolarize the membrane, deflections must be directed so as to induce strain on the tip links. Deflections in the opposite direction lead to hyperpolarization of the membrane. Hair cells adapt to prolonged deflection of hair bundles. Whereas in some vertebrates, adaptation is incomplete (i.e., the current fails to return to baseline), adaptation in anemones appears to be complete. Signal transduction is reversibly blocked by agents thought to interact with the transduction channel including streptomycin. In addition, signal transduction is abolished following exposure to agents thought to attack tip links including calcium-depleted buffers or elastase. Hair cells of lower vertebrates can be replaced by division and differentiation of supporting cells. In chickens, a repair system exists wherein tip links are replaced via a mechanism that does not involve protein synthesis. The repair mechanism of anemones involves synthesis of new proteins that may constitute replacement linkages and accessory proteins that attach the linkages to appropriate integral proteins.

Repair of hair bundles in sea anemones by secreted proteins

Sea anemones are sessile invertebrates that detect movements of prey using numerous hair bundles located on tentacles surrounding their mouth. Previously we found that hair bundles of anemones are structurally and functionally similar to those of vertebrates. After 10-15 min exposure to calcium depleted buffers, hair bundles in chickens suffer moderate damage from which they recover in 12 h without requiring new protein synthesis [Zhao, Yamoah and Gillespie, Proc. Natl. Acad. Sci. USA 94 (1996) 15469-15474]. We find that after 1 h exposure to calcium free seawater, hair bundles of anemones suffer extensive damage from which they recover in 4 h, apparently because of newly synthesized, secretory proteins called 'repair proteins'. Recovery is delayed in a dose dependent fashion by cycloheximide. In the presence of exogenously added repair proteins, recovery occurs within 8 min and is cycloheximide insensitive. Recovery is ascertained by a bioassay performed on intact specimens, by electrophysiology, and by timelapse video microscopy. Fraction beta, a chromatographic fraction with bioactivity comparable to the complete mixture of repair proteins, consists of complexes having an estimated mass of 2000 kDa. Avidin based cytochemistry suggests that biotinylated fraction beta binds to damaged hair bundles. SDS-PAGE gel electrophoresis demonstrates that fraction beta contains 8-10 polypeptides of 90 kDa or smaller. At least four of these polypeptides apparently are consumed during the repair process. Negatively stained samples of fraction beta are shown by transmission electron microscopy to include filamentous structures similar in length (150 nm) and width (6 nm) to linkages between stereocilia. The filamentous structures can be associated with globular structures (20 nm in diameter). A model is presented wherein repair proteins comprise replacement linkages and enzymes that attach linkages to appropriate membrane proteins.

Hair bundles of sea anemones as a model system for vertebrate hair bundles

Sea anemones are marine invertebrates that use hair bundles to detect swimming movements of prey. Prey are captured by nematocysts (stinging capsules) that discharge into the prey. To further characterize anemone hair bundles and to compare hair bundles in anemones with hair bundles in vertebrates, we investigated fine structure and cytochemistry of anemone hair bundles. In addition, using a biological assay based on counting nematocysts discharged into vibrating test probes, we examined sensitivity of vibration detection to aminoglycoside antibiotics, Ca(2+)-free seawater, and amiloride. Like vertebrate hair bundles, anemone hair bundles are composed of stereocilia, possess lateral linkages between stereocilia whose preservation for transmission electron microscopy is enhanced by ruthenium red, and possess tip links morphologically similar to vertebrate tip links. Furthermore, vibration-dependent discharge of nematocysts is reversibly inhibited by 10(-4) M streptomycin and abolished by brief exposure to Ca(2+)-free seawater. However, unlike vertebrate hair bundles, anemone hair bundles appear to be insensitive to amiloride since vibration-dependent discharge of nematocysts is unaffected by up to mM amiloride. Thus, anemone hair bundles may serve as a useful model system for vertebrate hair bundles with the interesting feature of being insensitive to amiloride.

Calcium imaging of single stereocilia in hair cells: localization of transduction channels at both ends of tip links

Mechanically gated "transduction" channels in inner ear hair cells are thought to be connected to tip links stretched between adjacent stereocilia. To locate active channels, calcium-green fluorescence in single stereocilia was measured with two-photon laser scanning microscopy. Bundle deflection increased fluorescence in many but not all stereocilia; the increase was blocked by depolarization. The number of stereocilia responding was proportional to the transduction current, consistent with Ca2+ influx through transduction channels. Fluorescence rose first in the tips of stereocilia and then in the bases, in agreement with channel localization at the tips. Some of the shortest stereocilia in a bundle showed a fluorescence increase, as did some of the tallest, indicating that transduction channels can be at either or both ends of tip links.

Low-voltage field-emission scanning electron microscopy of non-coated guinea-pig hair cell stereocilia

The stereociliar structures of the guinea-pig cochlear organ of Corti were studied at low-voltage (1-5 kV) with field-emission scanning electron microscope (SEM) using various pre- and post-fixation methods, such as OTOTO (OsO4/thiocrbohydrazide/OsO4/thiocarbohydrazide/OsO4) and TAO (tannic acid/arginine/OsO4), and different dissection procedures of the cochlea. A perfusion and immersion pre-fixation with glutaraldehyde, in combination with removal of the bony wall and stria vascularis from the cochlea, followed by the TAO non-coating treatment gave the best result at 2 kV acceleration voltage. Due to these new technique, several interesting delicate structures of the stereocilia, in particular fine surface structures, were detected for the first time using SEM. These findings include the different types of cross-links and tip links, i.e., the fine surface morphology of the stereocilia and their attachments and imprints in the tectorial membrane (TM). One of the most interesting findings in this study is a network of long filamentous structures, which has been identified mainly at the top of the longest stereocilia and the undersurface of the TM and which may represent the glycocalyx. These findings and their possible implications in the process of mechanoelectrical transduction will be discussed.

Abolition of the receptor potential response of isolated mammalian outer hair cells by hair-bundle treatment with elastase: a test of the tip-link hypothesis

To test the hypothesis that the tip-links of hair-cell stereocilia are essential for mechanoelectrical transduction, tip-links of isolated outer hair cells (OHCs) of the guinea-pig cochlea were eliminated with a proteolytic enzyme, elastase, and the influence on the receptor potential measured with the whole-cell patch-clamp technique. Within 45 s of immersion of the hair bundle in 20 IU/ml elastase, the receptor potential in response to direct deflection of the hair bundle was irreversibly abolished. The electrical input impedance of the cell remained unchanged, implying that the channels of the basolateral membrane were not affected by elastase. The effect of elastase on the receptor potential was comparable to changes seen after mechanically induced hair-bundle damage. As a further control, a putative transduction-channel blocker, dihydrostreptomycin (68 microM), which does not affect tip-links, was applied to the hair bundle. Although the receptor potential was also blocked by dihydrostreptomycin, the effect was reversible. The results suggest that tip-links are required for mechanoelectrical transduction of mammalian OHCs.

Molecular machinery of auditory and vestibular transduction

The hunt for molecules that conduct mechanoelectrical transduction in hair cells has recently intensified. A hair cell's transduction apparatus adapts to sustained stimuli, and myosin I beta and myosin VIIA have been advanced as candidates for the motor that mediates this process. The identity of the transduction channel remains unknown, although a viable suggestion proposes that it belongs to the amiloride-sensitive Na+ channel family.

A model for the mechanics of the stereociliar bundle on acousticolateral hair cells

The stereociliar bundle on acousticolateral hair cells was modelled as a series of stiff rods (stereocilia), and springs (stereociliary links and rootlets). Predictions were made for the coupling of stimulus-induced deflections between the stereocilia on the hair bundle, and for the stretches of the different classes of link. Comparison of the results with the measured mechanical properties of hair bundles suggests that in the bullfrog sacculus the stiffness of a side link and a tip link are related to the rootlet's contribution to the stiffness of a stereocilium to deflection in approximately the ratio > or = 400:100:1. The results show that stretch of the tip links is closely related to the deflection of the hair bundle over a wide range of model parameters, while the stretch of the side links is more variable, and in some types of bundle the mean stretch of the side links may be zero or negative. The results are in accordance with the view that the tip links are in an appropriate position to detect the deflections, while the main role for the side links may be to couple the deflections between the stereocilia. The mechanical consequences of bundles of different configurations, as seen in different hair cell types, are investigated.

A model of stereociliary tip-link stretches

A model of the tip-link stretches produced by angular deflections of the stereocilia of vertebrate acoustico-lateralis hair cells is presented. It is shown that tip-link stretch in the model is proportional to the angle of stereociliary deflection. By contrast, the stretch of a horizontal (e.g., row-to-row) link is proportional to the square of the angle of stereociliary deflection. Possible roles of these stretches in sensory transduction are discussed.

Glycocalyx and ciliary interconnections in bullfrog vestibular end organs: normal structure and changes after incubation in frog Ringer's solution

The ultrastructure of ciliary interconnecting systems as well as changes after incubation in frog Ringer's solution of the bullfrog vestibular end organs were investigated by using a scanning electron microscopy. The plasma membrane of the sensory hairs showed rather rough appearance. Side links emerged from the plasma membrane and tightly connected neighboring cilia. The tip links stretched from the tips of the stereocilia to their taller neighbors. These tip links were arranged in a same direction towards the kinocilium. These findings indicate that the side links may keep the cilia arranged in a bundle and the tip links may be involved in sensory cell transduction system. Changes in the surface texture and ciliary interconnecting systems were detected 2 hrs after incubation in frog Ringer's solution. Such changes included an increase in granularity of the surface membrane, fracturing and disappearance of all types of ciliary interconnections, especially side links. The tip links were more durable than side links. This indicates that the physiological studies of bullfrog hair bundle are justified across a 1-hr interval from the time the temporal bones are removed from the animal.

Mechanoelectrical transduction by hair cells

Hair cells of the inner ear are one of nature's great success stories, appearing early in vertebrate evolution and having a similar form in all vertebrate classes. They are specialized columnar epithelial cells, with an array of modified microvilli or stereocilia on their apical surface, interconnected by a series of linkages. The mechanical stimulus causes deflection of the stereocilia, stretching linkages between them, and opening the mechanotransducer channels. On a slower timescale, hair cells adapt in order to maintain optimum sensitivity, with an adaptation motor within the stereocilia acting to keep the resting tension on channels constant.

Ultrastructure of the horseshoe bat's organ of Corti. II. Transmission electron microscopy

The fine structure of the organ of Corti was investigated in the echolocating horseshoe bat (Rhinolophus rouxi) by transmission electron microscopy. Particular emphasis was placed on the receptor cells and their supporting cells. The receptor cells, inner hair cells (IHC) and outer hair cells (OHC), possess the typical mammalian shape, but OHCs are extremely short (length: 12-15 microns in the basal turn and up to 28-30 microns in the apical turn). The afferent innervation of both types of receptor cells and the efferent innervation of the IHC system conform to the general mammalian scheme; however, confirming earlier reports, an efferent innervation to the OHCs is absent. Throughout the cochlea, IHCs and OHCs possess a single layer of subsurface cisternae. Above the level of the nucleus of the OHCs, the arrangements of the subsurface cisternae and their connection to the lateral cell membrane via pillars are highly regular, whereas in IHCs, the cisternae are of irregular shape and the pillar system is much less distinct. In the basal turn of the cochlea, the attachment sites of the OHCs to the supporting cells possess specialized features: (a) in the reticular lamina, the contact sites of the cuticular plates of OHCs with the outer pillar cells and the Deiters cell phalanges are of exaggerated length, and (b) the cup formation of the Deiters cell body, which houses the bottom of the OHC, has a specialized shape and is packed with electron-dense material and microtubules. The results are discussed in relation to cochlear ultrastructure in other mammals and in the context of active processes in cochlear mechanics.

The development of links between stereocilia in hair cells of the chick basilar papilla

Auditory papillae of chicks (embryonic age 6-21 days) were examined by scanning and transmission electron microscopy, in order to trace the development of the tip links between the stereocilia, and in order to trace the development of the spatial organisation of the tip links. In the most immature bundles, stereocilia were not graded in height, while strands of tenuous material interconnected adjacent stereocilia, this material being concentrated in a band near the tips of the stereocilia. The material joined the stereocilia in all directions, with no preferential direction for the interconnecting material being visible. Similarly, no columnar organisation of the stereocilia was visible. As soon as a gradation in height of the stereocilia began to appear, material could be seen running upwards from the shorter stereocilia to the adjacent lengthening stereocilia. There was a continuum in appearance between (i) the material running laterally between short immature stereocilia, (ii) the material running upwards between stereocilia which were developing a gradation in height, and (iii) the tip links seen in more mature bundles. It is suggested that tip links are a specialisation of the links which join immature stereocilia laterally near their tips. It is also suggested that the orientation of tip links, parallel to the hair cell axis of bilateral symmetry, is produced by the gradient in growth of the stereocilia.

Influence of collagenase on tip links in hair cells of the chick basilar papilla

Chick basilar papillae were incubated in collagenase, using concentrations previously employed for the isolation of viable hair cells. When assessed by scanning electron microscopy, the hair bundles had a normal conformation, with no loss of tip links compared with control incubations. The results suggest that collagenase dose not destroy the integrity of structures on the apical surface of hair cells, and that tip links are not composed of collagen. The results are in agreement with the hypothesis that tip links are involved in mechanotransduction.

Action of elastase, collagenase and other enzymes upon linkages between stereocilia in the guinea-pig cochlea

Enzymes, which degrade elements of the extracellular environment, were studied for their actions upon stereocilia and their cross-linkages by scanning electron microscopy. Chondroitinase, hyaluronidase and keratanase, which attack carbohydrate moieties of the extracellular matrix, had little effect upon hair bundles. Collagenase and plasmin (fibrinolysin) also had only marginal effects. Elastase produced dramatic effects upon hair bundles. Both lateral and tip links were degraded resulting in separation and splaying of stereocilia. Many stereocilia showed no marked loss of rigidity, although some were bent or kinked. In general, inner hair cells were the most susceptible to elastase followed by row 3, row 2, row 1 of the outer hair cells. The proteolytic enzyme trypsin did not noticeably disrupt the hair bundles. Protease caused loss of rigidity and fracture of stereocilia resulting in considerable collapse of hair bundles. Crosslinkages between stereocilia were less noticeably degraded. These results indicate that both lateral and tip links of stereocilia comprise a proteinaceous moiety which could be elastin or some chemically related structure.

Effect of furosemide upon morphology of hair bundles in guinea pig cochlear hair cells

Furosemide was administered by intraperitoneal injection and intracochlear perfusion. The peak-to-peak amplitude of the cochlear microphonic at high sound intensities was reduced however the drug was administered. After a period of time ranging from 1-4 h following drug application, cochleae were fixed and prepared for scanning electron microscopy to examine the hair cells. Changes in the hair bundles from basal turns tended to be more extensive than those of more apical turns. Initially there was an increase in the granularity of the surface of the stereociliary membrane and a tendency for cross-links to swell, stretch and break. Later, the surface texture of stereocilia became smoother than normal, and hair cells in the basal turns showed extensive erosion and fracture of cross-links. Tip links could survive even when extensively stretched. It is possible that the morphological changes in stereocilia reported here do not arise from direct actions of furosemide, but indirectly, from perturbations of the ionic composition of cochlear fluids induced by effects of the drug upon the stria vascularis.

Effect of acoustic overstimulation on the glycocalyx and the ciliary interconnections in the organ of Corti: high resolution scanning electron microscopic investigation

Changes in ciliary interconnections in the organ of Corti are described after acoustic overstimulation using a special high resolution scanning electron microscope and tannic acid-osmium staining technique, giving an almost three dimensional view. Guinea pigs were exposed to a 3.85 kHz pure tone at an intensity of 120 dB for 22.5 minutes. The first detectable change was a disarrangement of the cilia with a loosening of the interconnections. The ciliary plasma membrane presented with an abnormally smooth appearance. The tip links connecting the tips of the stereocilia to their taller neighbours were also affected showing elongation or even disappearance. The fine granules which cover the tips of the tallest stereocilia of the outer hair cells were decreased. These findings suggest that acoustic overstimulation may affect the carbohydrate metabolism exceeding to degeneration of ciliary interconnections resulting in a disarrangement and detachment of cilia. The tip links, which may participate in sensory cell transduction, seem also to be affected by acoustic overstimulation.

How the ear's works work

The senses of hearing and equilibrium depend on sensory receptors called hair cells which can detect motions of atomic dimensions and respond more than 100,000 times a second. Biophysical studies suggest that mechanical forces control the opening and closing of transduction channels by acting through elastic components in each hair cell's mechanoreceptive hair bundle. Other ion channels, as well as the mechanical and hydrodynamic properties of hair bundles, tune individual hair cells to particular frequencies of stimulation.

Post-mortem changes in hair bundles of the guinea pig and human cochlea studied by high-resolution scanning microscopy

High-resolution scanning electron microscopic studies have been made on the guinea pig cochlea on material fixed from 15 min to 4 h post-mortem. Changes in the surface texture and cross-links of stereocilia were detected after only 15 min. Such changes included an increase in granularity of the surface membrane, thickening, stretching and fracturing of all types of cross-links accompanied by splaying apart and loss of rigidity of stereocilia. These changes were more pronounced in the basal turns of the cochlea and in general increased in severity and spread more apically with increasing times post-mortem. By 4 h, many hair bundles consisted of a fused amorphous mass in which individual stereocilia were not discernible. Remarkably at this time, some hair bundles appeared to have suffered little damage. These results will facilitate better discrimination between effects solely due to post-mortem necrotic changes and those due to specific actions of ototoxic drugs and other insults.

Tip-link organization in anomalously-oriented hair cells of the guinea pig cochlea

Stereocilia are described in a group of guinea pigs with abnormal conformation of the outer hair cell stereociliary bundles. Rows of stereocilia were found which were circular instead of V- or W-shaped, and rotated or even reversed in orientation with respect to normal. The stereocilia have however a normal gradation in heights of the rows of stereocilia, and have tip links running in the direction of gradation in height.