Mammalian Ear Research Articles

Altered expression of genes regulating inflammation and synaptogenesis during regrowth of afferent neurons to cochlear hair cells.

The spiral ganglion neurons constitute the primary connection between auditory hair cells and the brain. The spiral ganglion afferent fibers and their synapse with hair cells do not regenerate to any significant degree in adult mammalian ears after damage. We have investigated gene expression changes after kainate-induced disruption of the synapses in a neonatal cochlear explant model in which peripheral fibers and the afferent synapse do regenerate. We compared gene expression early after damage, during regeneration of the fibers and synapses, and after completion of in vitro regeneration. These analyses revealed a total of 2.5% differentially regulated transcripts (588 out of 24,000) based on a threshold of p<0.005. Inflammatory response genes as well as genes involved in regeneration of neural circuits were upregulated in the spiral ganglion neurons and organ of Corti, where the hair cells reside. Prominent genes upregulated at several time points included genes with roles in neurogenesis (Elavl4 and Sox21), neural outgrowth (Ntrk3 and Ppp1r1c), axonal guidance (Rgmb and Sema7a), synaptogenesis (Nlgn2 and Psd2), and synaptic vesicular function (Syt8 and Syn1). Immunohistochemical and in situ hybridization analysis of genes that had not previously been described in the cochlea confirmed their cochlear expression. The time course of expression of these genes suggests that kainate treatment resulted in a two-phase response in spiral ganglion neurons: an acute response consistent with inflammation, followed by an upregulation of neural regeneration genes. Identification of the genes activated during regeneration of these fibers suggests candidates that could be targeted to enhance regeneration in adult ears.

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.

Comparative micromechanics of bushcricket ears with and without a specialized auditory fovea region in the crista acustica.

In some insects and vertebrate species, the specific enlargement of sensory cell epithelium facilitates the perception of particular behaviourally relevant signals. The insect auditory fovea in the ear of the bushcricket Ancylecha fenestrata (Tettigoniidae: Phaneropterinae) is an example of such an expansion of sensory epithelium. Bushcricket ears developed in convergent evolution anatomical and functional similarities to mammal ears, such as travelling waves and auditory foveae, to process information by sound. As in vertebrate ears, sound induces a motion of this insect hearing organ (crista acustica), which can be characterized by its amplitude and phase response. However, detailed micromechanics in this bushcricket ear with an auditory fovea are yet unknown. Here, we fill this gap in knowledge for bushcricket, by analysing and comparing the ear micromechanics in Ancylecha fenestrata and a bushcricket species without auditory fovea (Mecopoda elongata, Tettigoniidae: Mecopodinae) using laser-Doppler vibrometry. We found that the increased size of the crista acustica, expanded by a foveal region in A. fenestrata, leads to higher mechanical amplitudes and longer phase delays in A. fenestrata male ears. Furthermore, area under curve analyses of the organ oscillations reveal that more sensory units are activated by the same stimuli in the males of the auditory fovea-possessing species A. fenestrata. The measured increase of phase delay in the region of the auditory fovea supports the conclusion that tilting of the transduction site is important for the effective opening of the involved transduction channels. Our detailed analysis of sound-induced micromechanics in this bushcricket ear demonstrates that an increase of sensory epithelium with foveal characteristics can enhance signal detection and may also improve the neuronal encoding.

Evolution of the Mammalian Ear: An Evolvability Hypothesis

Encapsulated within the temporal bone and comprising the smallest elements of the vertebrate skeleton, the ear is key to multiple senses: balance, posture control, gaze stabilization, and hearing. The transformation of the primary jaw joint into the mammalian ear ossicles is one of the most iconic transitions in vertebrate evolution, but the drivers of this complex evolutionary trajectory are not fully understood. We propose a novel hypothesis: The incorporation of the bones of the primary jaw joint into the middle ear has considerably increased the genetic, regulatory, and developmental complexity of the mammalian ear. This increase in the number of genetic and developmental factors may, in turn, have increased the evolutionary degrees of freedom for independent adaptations of the different functional ear units. The simpler ear anatomy in birds and reptiles may be less susceptible to developmental instabilities and disorders than in mammals but also more constrained in its evolution. Despite the tight spatial entanglement of functional ear components, the increased “evolvability” of the mammalian ear may have contributed to the evolutionary success and adaptive diversification of mammals in the vast diversity of ecological and behavioral niches observable today. A brief literature review revealed supporting evidence for this hypothesis.

Making a mammalian ear. Modular decoupling of the mammalian middle ear and jaw discovered in a new species of Cretaceous stem therian mammals

Evolution of the definitive mammalian middle ear (DMME) as a textbook example in vertebrate evolution has been extensively studied during the last 200 years. Fossils provide the direct evidence on evolutionary stages of the DMME, but because of delicacy of the miniscule ossicles, unequivocal evidence about them has always been rare. Recent work on a stem therian mammal (124 million years old) shows presence of the surangular bone in the basal mammals as a primitive feature and potentially retained in the embryonic stage of some extant mammals. The work also proposed that the DMME and mammalian jaw evolved in a modular fashion. It started as a highly integrated complex in structures and functions, the two modules were regulated by similar developmental genetic mechanisms and eventually decoupled under natural selection so that the physical constraint the two modules imposed on each other was removed, allowing future improvement of each module for better function.

YAP Mediates Hair Cell Regeneration in Balance Organs of Chickens, But LATS Kinases Suppress Its Activity in Mice.

Loss of sensory hair cells causes permanent hearing and balance deficits in humans and other mammals, but for nonmammals such deficits are temporary. Nonmammals recover hearing and balance sensitivity after supporting cells proliferate and differentiate into replacement hair cells. Evidence of mechanical differences between those sensory epithelia and their supporting cells prompted us to investigate whether the capacity to activate YAP, an effector in the mechanosensitive Hippo pathway, correlates with regenerative capacity in acceleration-sensing utricles of chickens and mice of both sexes. After hair cell ablation, YAP accumulated in supporting cell nuclei in chicken utricles and promoted regenerative proliferation, but YAP remained cytoplasmic and little proliferation occurred in mouse utricles. YAP localization in supporting cells was also more sensitive to shape change and inhibition of MST1/2 in chicken utricles than in mouse utricles. Genetic manipulations showed that in vivo expression of the YAP-S127A variant caused robust proliferation of neonatal mouse supporting cells, which produced progeny that expressed hair cell markers, but proliferative responses declined postnatally. Expression of YAP-5SA, which more effectively evades inhibitory phosphorylation, resulted in TEAD-dependent proliferation of striolar supporting cells, even in adult utricles. Conditional deletion of LATS1/2 kinases abolished the inhibitory phosphorylation of endogenous YAP and led to striolar proliferation in adult mouse utricles. The findings suggest that damage overcomes inhibitory Hippo signaling and facilitates regenerative proliferation in nonmammalian utricles, whereas constitutive LATS1/2 kinase activity suppresses YAP-TEAD signaling in mammalian utricles and contributes to maintaining the proliferative quiescence that appears to underlie the permanence of sensory deficits.SIGNIFICANCE STATEMENT Loud sounds, ototoxic drugs, infections, and aging kill sensory hair cells in the ear, causing irreversible hearing loss and balance deficits for millions. In nonmammals, damage evokes shape changes in supporting cells, which can divide and regenerate hair cells. Such shape changes are limited in mammalian ears, where supporting cells develop E-cadherin-rich apical junctions reinforced by robust F-actin bands, and the cells fail to divide. Here, we find that damage readily activates YAP in supporting cells within balance epithelia of chickens, but not mice. Deleting LATS kinases or expressing YAP variants that evade LATS-mediated inhibitory phosphorylation induces proliferation in supporting cells of adult mice. YAP signaling eventually may be harnessed to overcome proliferative quiescence that limits regeneration in mammalian ears.

EGF and a GSK3 Inhibitor Deplete Junctional E-cadherin and Stimulate Proliferation in the Mature Mammalian Ear.

Sensory hair cell losses underlie the vast majority of permanent hearing and balance deficits in humans, but many nonmammalian vertebrates can fully recover from hearing impairments and balance dysfunctions because supporting cells (SCs) in their ears retain lifelong regenerative capacities that depend on proliferation and differentiation as replacement hair cells. Most SCs in vertebrate ears stop dividing during embryogenesis; and soon after birth, vestibular SCs in mammals transition to lasting quiescence as they develop massively thickened circumferential F-actin bands at their E-cadherin-rich adherens junctions. Here, we report that treatment with EGF and a GSK3 inhibitor thinned the circumferential F-actin bands throughout the sensory epithelium of cultured utricles that were isolated from adult mice of either sex. That treatment also caused decreases in E-cadherin, β-catenin, and YAP in the striola, and stimulated robust proliferation of mature, normally quiescent striolar SCs. The findings suggest that E-cadherin-rich junctions, which are not present in the SCs of the fish, amphibians, and birds which readily regenerate hair cells, are responsible in part for the mammalian ear's vulnerability to permanent balance and hearing deficits.SIGNIFICANCE STATEMENT Millions of people are affected by hearing and balance deficits that arise when loud sounds, ototoxic drugs, infections, and aging cause hair cell losses. Such deficits are permanent for humans and other mammals, but nonmammals can recover hearing and balance after supporting cells regenerate replacement hair cells. Mammalian supporting cells lose the capacity to proliferate around the time they develop unique, exceptionally reinforced, E-cadherin-rich intercellular junctions. Here, we report the discovery of a pharmacological treatment that thins F-actin bands, depletes E-cadherin, and stimulates proliferation in long-quiescent supporting cells within a balance epithelium from adult mice. The findings suggest that high E-cadherin in those supporting cell junctions may be responsible, in part, for the permanence of hair cell loss in mammals.

Developmental aspects of the tympanic membrane: Shedding light on function and disease.

SummaryThe ear drum, or tympanic membrane (TM), is a key component in the intricate relay that transmits air‐borne sound to our fluid‐filled inner ear. Despite early belief that the mammalian ear drum evolved as a transformation of a reptilian drum, newer fossil data suggests a parallel and independent evolution of this structure in mammals. The term “drum” belies what is in fact a complex three‐dimensional structure formed from multiple embryonic cell lineages. Intriguingly, disease affects the ear drum differently in its different parts, with the superior and posterior parts being much more frequently affected. This suggests a key role for the developmental details of TM formation in its final form and function, both in homeostasis and regeneration. Here we review recent studies in rodent models and humans that are beginning to address large knowledge gaps in TM cell dynamics from a developmental biologist's point of view. We outline the biological and clinical uncertainties that remain, with a view to guiding the indispensable contribution that developmental biology will be able to make to better understanding the TM.

The Cation Channel TMEM63B is an Osmosensor Required for Hearing

Life has evolved appropriate adaptive responses to maintain cell shape and volume when facing osmotic stress. Hypotonic stress causes the activation of swelling-activated nonselective cation channels (NSCCs), which leads to a Ca2+-dependent regulatory volume decrease (RVD) and the adaptive maintenance of cell volume; however, the molecular identities of the osmosensitive NSCCs remain unclear. Here, we identified TMEM63B as an osmosensitive NSCC activated by hypotonic stress. TMEM63B is enriched in the inner ear sensory hair cells. Genetic deletion of TMEM63B results in necroptosis of outer hair cells (OHCs) and progressive hearing loss. Mechanistically, the TMEM63B channel mediates hypoosmolarity-induced Ca2+ influx, which activates Ca2+-dependent K+ channels required for the maintenance of OHC morphology. These findings demonstrate that TMEM63B is the osmosensor of mammalian ear and the long-sought cation channel mediating Ca2+-dependent RVD.

Presence of diverse Rickettsia spp. and absence of Borrelia burgdorferi sensu lato in ticks in an East Texas forest with reduced tick density associated with controlled burns

As tick-borne diseases continue to emerge across the United States, there is need for a better understanding of the tick and pathogen communities in the southern states and of habitat features that influence transmission risk. We surveyed questing and on-host ticks in pine-dominated forests with various fire management regimes in the Sam Houston National Forest, a popular recreation area near Houston, Texas. Four linear transects were established- two with a history of controlled burns, and two unburned. Systematic drag sampling yielded 112 ticks from two species, Ixodes scapularis (n=73) and Amblyomma americanum (n=39), with an additional 106 questing ticks collected opportunistically from drag cloth operators. There was a significant difference in systematically-collected questing tick density between unburned (15 and 18 ticks/1000 m2) and burned (2 and 4 ticks/1000 m2) transects. We captured 106 rodents and found 74 ticks on the rodents, predominantly Dermacentor variabilis. One unburned transect had significantly more ticks per mammal than any of the other three transects. DNA of Rickettsia species was detected in 146/292 on and off-host ticks, including the ‘Rickettsial endosymbiont of I. scapularis’ and Rickettsia amblyommatis, which are of uncertain pathogenicity to humans. Borrelia lonestari was detected in one A. americanum, while Borrelia burgdorferi sensu stricto, the agent of Lyme disease, was not detected in any tick samples. Neither Borrelia nor Rickettsia spp. were detected in any of the mammal ear biopsies (n=64) or blood samples (n=100) tested via PCR. This study documents a high prevalence in ticks of Rickettsia spp. thought to be endosymbionts, a low prevalence of relapsing fever group Borrelia in ticks, and a lack of detection of Lyme disease-group Borrelia in both ticks and mammals in an east Texas forested recreation area. Additionally, we observed low questing tick density in areas with a history of controlled burns. These results expand knowledge of tick-borne disease ecology in east Texas which can aid in directing future investigative, modeling, and management efforts.

The frequency limit of outer hair cell motility measured in vivo.

Outer hair cells (OHCs) in the mammalian ear exhibit electromotility, electrically driven somatic length changes that are thought to mechanically amplify sound-evoked vibrations. For this amplification to work, OHCs must respond to sounds on a cycle-by-cycle basis even at frequencies that exceed the low-pass corner frequency of their cell membranes. Using in vivo optical vibrometry we tested this theory by measuring sound-evoked motility in the 13-25 kHz region of the gerbil cochlea. OHC vibrations were strongly rectified, and motility exhibited first-order low-pass characteristics with corner frequencies around 3 kHz- more than 2.5 octaves below the frequencies the OHCs are expected to amplify. These observations lead us to suggest that the OHCs operate more like the envelope detectors in a classical gain-control scheme than like high-frequency sound amplifiers. These findings call for a fundamental reconsideration of the role of the OHCs in cochlear function and the causes of cochlear hearing loss.

Analytical Modelling of Natural Frequency of Tympanic Membrane as the Low-Frequency Limit of Hearing.

We examine the low-frequency limit of hearing of the mammalian ear through the analytical modelling of the natural frequency of the tympanic membrane. The resulting equation of the natural frequency of the modelled tympanic membrane is numerically verified against previous theoretical studies, and is statistically validated against the experimental data on the low-frequency limit of hearing. By utilizing the Wilcoxon signed-rank test; W-values of 29 (p value = 0.25014) and 23 (p value = 0.11642) are respectively obtained for the 0.2% and 0.3% prestrain (at 5% significance level for sample size of 13). We fail to reject the null hypothesis as the W-values are within the critical values of the test statistics, and therefore conclude that the tympanic membrane acts as a low-frequency limiter of acoustic stimulus. Based on our study, wecan predict the low-frequency limit of hearing inmammals (e.g., for the whale as 3.6Hz and for the zebra as 44.0Hz).

Experimental and Theoretical Explorations of Traveling Waves and Tuning in the Bushcricket Ear

The ability to detect airborne sound is essential for many animals. Examples from the inner ear of mammals and bushcrickets demonstrate that similar detection strategies evolved in taxonomically distant species. Both mammalian and bushcricket ears possess a narrow strip of sensory tissue that exhibits an anatomical gradient and traveling wave motion responses used for frequency discrimination. We measured pressure and motion in the bushcricket ear to investigate physical properties, stiffness, and mass, which govern the mechanical responses to sound. As in the mammalian cochlea, sound-induced fluid pressure and motion responses were tonotopically organized along the longitudinal axis of the crista acustica, the bushcricket’s hearing organ. The fluid pressure at the crista and crista motion were used to calculate the acoustic impedance of the organ-bounded fluid mass (Zmass). We used a theoretical wave analysis of wavelength data from a previous study to predict the crista acustica stiffness. The wave analysis also predicts Zmass, and that result agreed reasonably well with the directly measured Zmass, lending support to the theoretical wave analysis. The magnitude of the crista stiffness was similar to basilar membrane stiffness in mammals, and as in mammals, the stiffness decreased from the high-frequency to the low-frequency region. At a given location, the stiffness increased with increasing frequency, corresponding to increasing curvature of the traveling wave (decreasing wavelength), indicating that longitudinal coupling plays a substantial role in determining crista stiffness. This is in contrast to the mammalian ear, in which stiffness is independent of frequency and longitudinal coupling is relatively small.

The effect of single-ossicle ear flexibility and eardrum cone orientation on quasi-static behavior of the chicken middle ear

In the single-ossicle ear of chickens, the quasi-static displacement of the umbo shows great asymmetry; umbo displacements are much larger for negative than for positive pressure in the middle ear, which is opposite to the typical asymmetry observed in mammal ears. To better understand this behavior, a finite-element model was created of the static response of the chicken middle ear. The role of flexibility of the extracolumella in the model was investigated, and the potential effect of the outward orientation of the tympanic-membrane cone was studied by building two adapted models with a flat membrane and an inverted conical membrane. It is found that the extracolumella must be made of flexible material to explain the large inward displacements of the umbo, and that displacements of the footplate are much smaller due to bending of the flexible extracolumella. However, increasing extracolumellar stiffness mostly reduces umbo displacement rather than increasing footplate displacement. The results suggest that the inverted orientation of the membrane cone is responsible for the change in asymmetry of the umbo displacement curve. The asymmetry of the footplate displacement curve in the normal model is smaller, but increases towards positive middle-ear pressure in the case of a flat or inverted membrane geometry.

Prenatal and postnatal development of the mammalian ear.

The ear can be subdivided into three distinct parts, each with significantly distinct structural and functional differences, the outer, middle, and inner ear, the latter housing the specialized sensory hair cells that act as transducers. There are numerous manuscripts documenting the anatomical development of the inner, middle, and outer ear in humans, rodents, chick, and zebrafish, dating back to the early 20th Century, and these developmental processes of these components are further compared in a number of review articles (Anthwal & Thompson, ; Basch, Brown, Jen, & Groves, ; Sai & Ladher, ). This article presents a review of both pre- and postnatal development of the inner ear, discusses recent molecular genetic advances toward our understanding of hair cells responsible for the sensory functions of the inner ear. Finally, a survey of comparative ear biology is used to pull together our understanding of the species differences, similarities, and key time points of definitive organ development of the ear.

ADAM10 and γ-secretase regulate sensory regeneration in the avian vestibular organs

The loss of sensory hair cells from the inner ear is a leading cause of hearing and balance disorders. The mammalian ear has a very limited ability to replace lost hair cells, but the inner ears of non-mammalian vertebrates can spontaneously regenerate hair cells after injury. Prior studies have shown that replacement hair cells are derived from epithelial supporting cells and that the differentiation of new hair cells is regulated by the Notch signaling pathway. The present study examined molecular influences on regeneration in the avian utricle, which has a particularly robust regenerative ability. Chicken utricles were placed in organotypic culture and hair cells were lesioned by application of the ototoxic antibiotic streptomycin. Cultures were then allowed to regenerate in vitro for seven days. Some specimens were treated with small molecule inhibitors of γ-secretase or ADAM10, proteases which are essential for transmission of Notch signaling. As expected, treatment with both inhibitors led to increased numbers of replacement hair cells. However, we also found that inhibition of both proteases resulted in increased regenerative proliferation. Subsequent experiments showed that inhibition of γ-secretase or ADAM10 could also trigger proliferation in undamaged utricles. To better understand these phenomena, we used RNA-Seq profiling to characterize changes in gene expression following γ-secretase inhibition. We observed expression patterns that were consistent with Notch pathway inhibition, but we also found that the utricular sensory epithelium contains numerous γ-secretase substrates that might regulate cell cycle entry and possibly supporting cell-to-hair cell conversion. Together, our data suggest multiple roles for γ-secretase and ADAM10 in vestibular hair cell regeneration.

Reduced Membrane Insertion of CLC-K by V33L Barttin Results in Loss of Hearing, but Leaves Kidney Function Intact.

In the mammalian ear, transduction of sound stimuli is initiated by K+ entry through mechano-sensitive channels into inner hair cells. K+ entry is driven by a positive endocochlear potential that is maintained by the marginal cell layer of the stria vascularis. This process requires basolateral K+ import by NKCC1 Na+−2Cl−−K+ co-transporters as well as Cl− efflux through ClC-Ka/barttin or ClC-Kb/barttin channels. Multiple mutations in the gene encoding the obligatory CLC-K subunit barttin, BSND, have been identified in patients with Bartter syndrome type IV. These mutations reduce the endocochlear potential and cause deafness. As CLC-K/barttin channels are also expressed in the kidney, patients with Bartter syndrome IV typically also suffer from salt-wasting hyperuria and electrolyte imbalances. However, there was a single report on a BSND mutation that resulted only in deafness, but not kidney disease. We herein studied the functional consequences of another recently discovered BSND mutation that predicts exchange of valine at position 33 by leucine. We combined whole-cell patch clamp, confocal microscopy and protein biochemistry to analyze how V33L affects distinct functions of barttin. We found that V33L reduced membrane insertion of CLC-K/barttin complexes without altering unitary CLC-K channel function. Our findings support the hypothesis of a common pathophysiology for the selective loss of hearing due to an attenuation of the total chloride conductance in the stria vascularis while providing enough residual function to maintain normal kidney function.

Meckel’s cartilage breakdown offers clues to mammalian middle ear evolution

A key transformation in mammalian ear evolution was incorporation of the primary jaw joint of premammalian synapsids into the definitive mammalian middle ear of living mammals. This evolutionary transition occurred in two-steps, starting with a partial or "transitional" mammalian middle ear in which the ectotympanic and malleus were still connected to the mandible by an ossified Meckel's Cartilage (MC), as observed in many Mesozoic mammals. This was followed by MC breakdown, freeing the ectotympanic and the malleus from the mandible and creating the definitive mammalian middle ear. Here we report novel findings on the role of chondroclasts in MC breakdown, shedding light on how therian mammals lost MC connecting the ear to the jaw. Genetic or pharmacological loss of clast cells in mice and opossums leads to persistence of embryonic MC beyond juvenile stages, with MC ossification in mutant mice. The persistent MC causes a distinctive postnatal groove on the mouse dentary. This morphology phenocopies the ossified MC and Meckelian groove observed in Mesozoic mammals. Clast cell recruitment to MC is not observed in reptiles, where MC persists as a cartilaginous structure. We hypothesize that ossification of MC is an ancestral feature of mammaliaforms, and that a shift in the timing of clast cell recruitment to MC prior to its ossification is a key developmental mechanism for the evolution of the definitive mammalian middle ear in extant therians.

Energy Output from a Single Outer Hair Cell

Electromotility of outer hair cells (OHCs) has been extensively studied with in vitro experiments because of its physiological significance to the cochlear amplifier, which provides the exquisite sensitivity and frequency selectivity of the mammalian ear. However, these studies have been performed largely under load-free conditions or with static load, while these cells function in vivo in a dynamic environment, receiving electrical energy to enhance mechanical oscillation in the inner ear. This gap leaves uncertainties in addressing a key issue, how much mechanical energy an OHC provides. This study is an attempt of bridging the gap by introducing a simple one-dimensional model for electromotility of OHC in a dynamic environment. This model incorporates a feedback loop involving the receptor potential and the mechanical load on OHC, and leads to an analytical expression for the membrane capacitance, which explicitly describes the dependence on the elastic load, viscous drag, and the mass. The derived equation of motion was examined in a mass-less model system with realistic parameter values for OHC. It was found that viscous drag is more effective than elastic load in enhancing the receptor potential that drives the cell. For this reason, it is expected that OHCs are more effective in counteracting viscous drag than providing elastic energy to the system.

Inner ear labyrinth anatomy of monotremes and implications for mammalian inner ear evolution.

The monophyletic clade Monotremata branches early from the rest of the mammalian crown group in the Jurassic and members of this clade retain many ancestral mammalian traits. Thus, accurate and detailed anatomical descriptions of this group can offer unique insight into the early evolutionary history of Mammalia. In this study, we examine the inner ear anatomy of two extant monotremes, Ornithorhynchus anatinus and Tachyglossus aculeatus, with the primary goals of elucidating the ancestral mammalian ear morphology and resolving inconsistencies found within previous descriptive literature. We use histological serial sections and high-resolution microcomputed tomography (µCT) for correlating soft tissue features of the vestibule and cochlea to the osseous labyrinth endocast. We found that in both monotremes the scala tympani coils to a lesser degree than scala vestibuli and scala media, although all three scalae show an apical coil inside the osseous cochlear tube. The helicotrema (conduit between scala tympani and scala vestibuli) is in subapical position, and the cochlear and lagenar ganglia and their associated nerve fibers are not enclosed by bone. In comparison, in extant therian mammals (i.e., marsupials and placentals) the helicotrema is located at the apex of the osseous cochlear canal, the three scalae coil to the same degree and the cochlear ganglion is enclosed by the primary bony lamina. Whether the lagenar ganglion is lost in therian mammals or integrated into the cochlear ganglion is still debated. The presence of a sensory lagenar macula at the apex of the membranous cochlear duct, innervated by a separate lagenar nerve and ganglion is a plesiomorphic condition of amniotes that monotremes share. A separate osseous lagenar canaliculus for the lagenar nerve, and the coiling of the distended lagenar sac at the end of the cochlear duct are autapomorphies of monotremes. Based on our findings we hypothesize that the ancestral inner ear of stem mammaliaforms is characterized by a straight or slightly curved osseous cochlear canal, a lagenar macula, lagenar nerve fibers separated from a larger bundle of cochlear nerve fibers, the presence of an organ of Corti and an intra-otic cochlear ganglion suspended by membranous connective tissue. Among the major Mesozoic clades of crown mammals, cladotherians and gondwanatherians most likely acquired a fully functioning organ of Corti but lost the sensory lagenar macula, like extant therians. However, Mesozoic spalacotherioids, multituberculates and eutriconodonts likely retained the mammaliaform condition. J. Morphol. 278:236-263, 2017. © 2016 Wiley Periodicals,Inc.