The organ of Corti in the bat Hipposideros bicolor

Correlating Cochlear Morphometrics from Parnell's Mustached Bat (Pteronotus parnellii) with Hearing

Morphometric analysis of the inner ear of mammals can provide information for cochlear frequency mapping, a species-specific designation of locations in the cochlea at which different sound frequencies are encoded. Morphometric variation occurs in the hair cells of the organ of Corti along the cochlea, with the base encoding the highest frequency sounds and the apex encoding the lowest frequencies. Changes in cell shape and spacing can yield additional information about the biophysical basis of cochlear tuning mechanisms. Here, we investigate how morphometric analysis of hair cells in mammals can be used to predict the relationship between frequency and cochlear location. We used linear and geometric morphometrics to analyze scanning electron micrographs of the hair cells of the cochleae in Parnell's mustached bat (Pteronotus parnellii) and Wistar rat (Rattus norvegicus) and determined a relationship between cochlear morphometrics and their frequency map. Sixteen of twenty-two of the morphometric parameters analyzed showed a significant change along the cochlea, including the distance between the rows of hair cells, outer hair cell width, and gap width between hair cells. A multiple linear regression model revealed that nine of these parameters are responsible for 86.9 % of the variation in these morphometric data. Determining the most biologically relevant measurements related to frequency detection can give us a greater understanding of the essential biomechanical characteristics for frequency selectivity during sound transduction in a diversity of animals.

Echolocating Whales and Bats Express the Motor Protein Prestin in the Inner Ear: A Potential Marker for Hearing Loss

Prestin is an integral membrane motor protein located in outer hair cells of the mammalian cochlea. It is responsible for electromotility and required for cochlear amplification. Although prestin works in a cycle-by-cycle mode up to frequencies of at least 79 kHz, it is not known whether or not prestin is required for the extreme high frequencies used by echolocating species. Cetaceans are known to possess a prestin coding gene. However, the expression and distribution pattern of the protein in the cetacean cochlea has not been determined, and the contribution of prestin to echolocation has not yet been resolved. Here we report the expression of the protein prestin in five species of echolocating whales and two species of echolocating bats. Positive labeling in the basolateral membrane of outer hair cells, using three anti-prestin antibodies, was found all along the cochlear spiral in echolocating species. These findings provide morphological evidence that prestin can have a role in cochlear amplification in the basolateral membrane up to 120–180 kHz. In addition, labeling of the cochlea with a combination of anti-prestin, anti-neurofilament, anti-myosin VI and/or phalloidin and DAPI will be useful for detecting potential recent cases of noise-induced hearing loss in stranded cetaceans. This study improves our understanding of the mechanisms involved in sound transduction in echolocating mammals, as well as describing an optimized methodology for detecting cases of hearing loss in stranded marine mammals.

Precise Doppler shift compensation in the hipposiderid bat, Hipposideros armiger

Bats of the Rhinolophidae and Hipposideridae families, and Pteronotus parnellii, compensate for Doppler shifts generated by their own flight movement. They adjust their call frequency such that the frequency of echoes coming from ahead fall in a specialized frequency range of the hearing system, the auditory fovea, to evaluate amplitude and frequency modulations in echoes from fluttering prey. Some studies in hipposiderids have suggested a less sophisticated or incomplete Doppler shift compensation. To investigate the precision of Doppler shift compensation in Hipposideros armiger, we recorded the echolocation and flight behaviour of bats flying to a grid, reconstructed the flight path, measured the flight speed, calculated the echo frequency, and compared it with the resting frequency prior to each flight. Within each flight, the average echo frequency was kept constant with a standard deviation of 110 Hz, independent of the flight speed. The resting and reference frequency were coupled with an offset of 80 Hz; however, they varied slightly from flight to flight. The precision of Doppler shift compensation and the offset were similar to that seen in Rhinolophidae and P. parnellii. The described frequency variations may explain why it has been assumed that Doppler shift compensation in hipposiderids is incomplete.

Ultrastructure of the Odontocete organ of Corti: scanning and transmission electron microscopy

The morphological study of the Odontocete organ of Corti, together with possible alterations associated with damage from sound exposure, represents a key conservation approach to assess the effects of acoustic pollution on marine ecosystems. By collaborating with stranding networks from several European countries, 150 ears from 13 species of Odontocetes were collected and analyzed by scanning (SEM) and transmission (TEM) electron microscopy. Based on our analyses, we first describe and compare Odontocete cochlear structures and then propose a diagnostic method to identify inner ear alterations in stranded individuals. The two species analyzed by TEM (Phocoena phocoena and Stenella coeruleoalba) showed morphological characteristics in the lower basal turn of high-frequency hearing species. Among other striking features, outer hair cell bodies were extremely small and were strongly attached to Deiters cells. Such morphological characteristics, shared with horseshoe bats, suggest that there has been convergent evolution of sound reception mechanisms among echolocating species. Despite possible autolytic artifacts due to technical and experimental constraints, the SEM analysis allowed us to detect the presence of scarring processes resulting from the disappearance of outer hair cells from the epithelium. In addition, in contrast to the rapid decomposition process of the sensory epithelium after death (especially of the inner hair cells), the tectorial membrane appeared to be more resistant to postmortem autolysis effects. Analysis of the stereocilia imprint pattern at the undersurface of the tectorial membrane may provide a way to detect possible ultrastructural alterations of the hair cell stereocilia by mirroring them on the tectorial membrane.

Comparative aspects of cochlear functional organization in mammals

This review addresses the functional organization of the mammalian cochlea under a comparative and evolutionary perspective. A comparison of the monotreme cochlea with that of marsupial and placental mammals highlights important evolutionary steps towards a hearing organ dedicated to process higher frequencies and a larger frequency range than found in non-mammalian vertebrates. Among placental mammals, there are numerous cochlear specializations which relate to hearing range in adaptation to specific habitats that are superimposed on a common basic design. These are illustrated by examples of specialist ears which evolved excellent high frequency hearing and echolocation (bats and dolphins) and by the example of subterranean rodents with ears devoted to processing low frequencies. Furthermore, structural functional correlations important for tonotopic cochlear organization and predictions of hearing capabilities are discussed.

Efferent innervation to the auditory basilar papilla of scincid lizards

Hair cells of the inner ear of vertebrates are innervated by afferent neurons that transmit sensory information to the brain as well as efferent neurons that receive feedback from the brainstem. The function of the efferent feedback system is poorly understood and may have changed during evolution when different tetrapod groups acquired sensitivity to airborne sound and extended their hearing ranges to higher frequencies. Lizards show a unique subdivision of their basilar papilla (homologous to the mammalian organ of Corti) into a low-frequency (<1 kHz) and a high-frequency (approximately 1-5 kHz) region. The high-frequency region was reported to have lost its efferent innervation, suggesting it was insignificant or even functionally detrimental at higher frequencies. We re-examined the innervation to the basilar papilla of five species of Australian scincid lizards, by using immunohistochemistry. Anti-choline acetyltransferase (ChAT) was used as an efferent marker. Co-localization with anti-synaptic vesicle protein 2 confirmed the synaptic identity of label. Cholinergic terminals were observed along the whole length of the basilar papilla, including the regions that had previously been described as devoid of efferent innervation. However, there was a clear decrease in terminal density from apical, low-frequency to basal, high-frequency locations. Our findings suggest that efferent innervation is a general feature of the hair cells in the basilar papilla of lizards, irrespective of tonotopic location. This re-enforces the notion that efferent feedback control of hair cells is a fundamental and important property of all vertebrate hearing organs.

Characteristics of echolocating bats’ auditory stereocilia length, compared with other mammals

The stereocilia of the Organ of Corti in 4 different echolocating bats, Myotis adversus, Murina leucogaster, Nyctalus plancyi (Nyctalus velutinus), and Rhinolophus ferrumequinum were observed by using scanning electron microscopy (SEM). Stereocilia lengths were estimated for comparison with those of non-echolocating mammals. The specialized lengths of outer hair cells (OHC) stereocilia in echolocating bats were shorter than those of non-echolocating mammals. The specialized lengths of inner hair cells (IHC) stereocilia were longer than those of outer hair cells stereocilia in the Organ of Corti of echolocating bats. These characteristics of the auditory stereocilia length of echolocating bats represent the fine architecture of the electromotility process, helping to adapt to high frequency sound and echolocation.

Protection from acoustic trauma is not a primary function of the medial olivocochlear efferent system

The medial olivocochlear (MOC) efferent system is an important component of an active mechanical outer hair cell system in mammals. An extensive neurophysiological literature demonstrates that the MOC system attenuates the response of the cochlea to sound by reducing the gain of the outer hair cell mechanical response to stimulation. Despite a growing understanding of MOC physiology, the biological role of the MOC system in mammalian audition remains uncertain. Some evidence suggests that the MOC system functions in a protective role by acting to reduce receptor damage during intense acoustic exposure. For the MOC system to have evolved as a protective mechanism, however, the inner ears of mammals must be exposed to potentially damaging sources of noise that can elicit MOC-mediated protective effects under natural conditions. In this review, we evaluate the possibility that the MOC system evolved to protect the inner ear from naturally occurring environmental noise. Our survey of nonanthropogenic noise levels shows that while sustained sources of broadband noise are found in nearly all natural acoustic environments, frequency-averaged ambient noise levels in these environments rarely exceed 70 dB SPL. Similarly, sources reporting ambient noise spectra in natural acoustic environments suggest that noise levels within narrow frequency bands are typically low in intensity (<40 dB SPL). Only in rare instances (e.g., during frog choruses) are ambient noise levels sustained at moderately high intensities (~70-90 dB SPL). By contrast, all experiments in which an MOC-mediated protective effect was demonstrated used much higher sound intensities to traumatize the cochlea (100-150 dB SPL). This substantial difference between natural ambient noise levels and the experimental conditions necessary to evoke MOC-mediated protection suggests that even the noisiest natural acoustic environments are not sufficiently intense to have selected for the evolution of the MOC system as a protective mechanism. Furthermore, although relatively intense noise environments do exist in nature, they are insufficiently distributed to account for the widespread distribution of the MOC system in mammals. The paucity of high-intensity noise and the near ubiquity of low-level noise in natural environments supports the hypothesis that the MOC system evolved as a mechanism for "unmasking" biologically significant acoustic stimuli by reducing the response of the cochlea to simultaneous low-level noise. This suggested role enjoys widespread experimental support.

Further studies on the mechanics of the cochlear partition in the mustached bat. I. Ultrastructural observations on the tectorial membrane and its attachments

From semithin and ultrathin sections of the mustached bat cochlea, baso-apical gradients in ultrastructural composition, shape and attachment site of the tectorial membrane (TM) were determined in relation to gradients in hair cell size and stereocilia size. These provide a data base for estimates of the mechanical properties of the organ of Corti as they relate to specialized aspects of the cochlear frequency map (Kössl and Vater, 1996). As in other mammals, the TM is composed to type A and type B protofibrils. Measurements of the packing density of type A protofibrils reveal gradients in both the radial and longitudinal direction. Distinct variations in packing density of type A protofibrils across the radial extent of the TM allow the definition of more subregions than in other mammals. Throughout the cochlea, packing density is highest in the 'stripe' region located close to the spiral limbus. The centrally located 'core' region of the middle zone contains distinctly fewer type A protofibrils than the laterally located 'mantle' region of the middle zone. The TM in the specialized basal turn (first and second half-turns) features a higher packing density of type A protofibrils in the 'mantle' than the TM in the apical cochlea (upper third to fifth half-turns), and in incorporation of longitudinally directed type A protofibrils in the marginal zone. Among cochlear turns, there are pronounced changes in cross-sectional area of the TM and the extent of its limbal attachment site. Within the densely innervated second half-turn that contains an expanded representation of the 60 kHz constant frequency (CF) component of the echolocation signal, both the cross-sectional area (see also Henson and Henson, 1991) and the attachment site of the TM are enlarged. An extended limbal attachment site is also observed in the densely innervated region of the lower first half-turn that represents the upper harmonics of the call. Within the sparsely innervated region of the upper first half-turn, the limbal attachment site of the TM is significantly diminished. Size of outer hair cells (OHC) ranges between 12 and 13 microns throughout the basal 80% of cochlear length and reaches maximal values of 20 microns in the apex. Size of OHC stereocilia ranges between 0.7 and 0.8 microns throughout the basal 60% of cochlear length and reaches a maximal size of 2.2 microns in the apex. These data corroborate and extend previous notions that morphological specializations of the TM in concert with specializations of the basilar membrane and perilymphatic spaces play an integral role in creating specialized cochlear tuning in the mustached bat.

The cochlea of Tadarida brasiliensis: specialized functional organization in a generalized bat

Tadarida brasiliensis mexicana employs a broad-band sonar system at frequencies between 80 and 20 kHz and is characterized by non-specialized hearing capabilities. The cochlear frequency map was determined with extracellular horseradish peroxidase tracing in relation to quantitative morphological data obtained with light, scanning and transmission electron microscopy. These data reveal distinct species characteristic specializations clearly separate from the patterns observed in other bats with either broad-band or narrow-band sonar systems. The basilar membrane (BM) is coiled to 2.5 turns and about 12 mm long. Its thickness and width only change within the extreme basal and apical ends. The frequency range from about 30 to 80 kHz is represented in the lower basal turn with a typically mammalian mapping coefficient of about 3 mm/octave. This region exhibits morphological features correlated with non-specialized processing of high frequencies. (1) The BM is radially segmented by thickenings of pars tecta and pars pectinata. (2) The 3 rows of outer hair cells (OHCs) have similar morphology. Between 35 and 86% distance from base, frequencies between 30 and 12 kHz are represented with a slightly expanded mapping coefficient of about 6 mm/octave. In analogy to previous work, this cochlea region is termed acoustic fovea. It includes the frequency range of maximum sensitivity and sharpest tuning (21-27 kHz) but also frequencies below the sonar signals. The fovea is characterized by several morphological specializations. (1) The BM features a continuous radial thickening mainly composed of hyaline substance. (2) There is an increased number of layers of tension fibroblasts in the spiral ligament. (3) There are morphological differences in the arrangements of stereocilia bundles among the 3 rows of OHCs. The transitions between non-specialized and specialized cochlear regions occur gradually within a distance of about 600 microns. The gradients in stereocilia length of both receptor cell types and the gradations in length of the OHC bodies match specialized aspects of the frequency map.

The effect of contralateral stimulation on cochlear resonance and damping in the mustached bat: the role of the medial efferent system

In the unanesthetized mustached bat, stimulation of the ear with an acoustic transient produces damped oscillations which are evident in the cochlear microphonic potential. In this report we demonstrate how the decay time of these oscillations is affected by broadband noise presented to the contralateral ear (CLN). In the absence of CLN, the mean decay time was 1.94 +/- 0.23 ms, but during the presentation of CLN the decay time consistently decreased. The changes were finely graded, the higher the CLN, the greater the change. The effect could be maintained at a constant level for extended periods of time and this was evident when the CLN exceeded 40 dB SPL. The latency of the reflex for 64 dB noise was about 11 ms and near maximum changes occurred within 15 ms of CLN onset. Sectioning medial efferent nerve fibers in the floor of the fourth ventricle or the administration of a single dose of gentamicin eliminated changes produced by CLN. The prominence of CM responses to damped oscillations and the robust changes in response to CLN make the mustached bat an excellent model for studying the influence of the medial efferent system on cochlear mechanics.

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.

Postnatal development of the rat organ of Corti. I. General morphology, basilar membrane, tectorial membrane and border cells

The development of the rat organ of Corti was studied during the first postnatal weeks. The temporal and the spatial patterns of cochlear development were investigated between 4 and 24 days after birth by means of semi-thin sections at approx. ten equidistant positions along the entire cochlear duct. At all examined positions width, thickness and cross sectional area of basilar membrane, cross-sectional area of tectorial membrane, of cells of Hensen, Claudius and Boettcher and of the organ of Corti were quantitatively analyzed. The most conspicuous maturational changes occur between 8 and 12 days after birth. These are the detachment of the tectorial membrane, the first appearance of filaments within the basilar membrane, the formation of the tunnel of Corti and the opening of the inner spiral sulcus. Quantitative analysis revealed that structures of a given position along the cochlear duct do not develop synchronously. Width of the basilar membrane and cross-sectional area of the tectorial membrane are already mature at the onset of hearing (10-12 days after birth). Length, thickness and cross-sectional area of the basilar membrane as well as cross-sectional area of the organ of Corti and of the cells of Hensen, Claudius and Boettcher still develop after the onset of hearing (up to 20-24 days after birth). We suggest that basic cochlear function is established by structures which are mature before the onset of hearing. Cochlear structures which develop after the onset of hearing might be involved in this improvement during this period.

Ultrastructure of the horseshoe bat's organ of Corti. I. Scanning electron microscopy

The organ of Corti of the echolocating horseshoe bat (Rhinolophus rouxi) was investigated with scanning electron microscopy in order to provide a comparison with non-echolocating mammals. Throughout the cochlea of horseshoe bats, each outer hair cell (OHC) possesses three rows of stereocilia and there are no morphological distinctions among the different rows of OHCs. However, there are morphological differences between different regions along the cochlea. In the lower and upper basal turn, the receptor surfaces of OHCs are characterized by extremely wide W-shaped stereocilia bundles and wingshaped cuticular plates. The cuticular plates of OHCs of the middle and outermost rows are arranged parallel to each other. Stereocilia length is only 0.8 microns and there is an exaggerated angle of inclination of the shortest row of stereocilia towards the next taller one. Stereocilia arrangements in the apex of the horseshoe bat's cochlea closely resembles those observed in the midbasal region of the rat cochlea. Inner hair cells (IHC) in the lower basal turn appear specialized. They possess only two rows of stereocilia and only 7-8 stereocilia per row. Their cuticular plates are small and oval and widely separated from one another in the longitudinal direction. IHCs at all other locations possess three and up to four rows of stereocilia and 17-20 stereocilia per row. Their cuticular plates are elongated and closely spaced. The transition from specialized to typical mammalian morphology occurs abruptly (over a distance of about 100-150 microns) at the border between the lower and the upper basal turn. This transition is not accompanied by a change in OHC morphology. In the subsurface of the tectorial membrane, throughout the cochlea, there are distinct imprints of the tallest row of stereocilia of all three rows of OHCs and of the IHCs. Data are discussed in relation to specialized aspects of the cochlear frequency map in horseshoe bats and as possible micromechanical adaptations to ultra-high frequency hearing.

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.