Open access high-resolution 3D morphology models of cat, gerbil, rabbit, rat and human ossicular chains

Sound-seeking before and after hearing loss in mice

How we move our bodies affects how we perceive sound. For instance, head movements help us to better localize the source of a sound and to compensate for asymmetric hearing loss. However, many auditory experiments are designed to restrict head and body movements. To study the role of movement in hearing, we developed a behavioral task called sound-seeking that rewarded freely moving mice for tracking down an ongoing sound source. Over the course of learning, mice more efficiently navigated to the sound. Next, we asked how sound-seeking was affected by hearing loss induced by surgical removal of the malleus from the middle ear. After bilateral hearing loss sound-seeking performance drastically declined and did not recover. In striking contrast, after unilateral hearing loss mice were only transiently impaired and then recovered their sound-seek ability over about a week. Throughout recovery, unilateral mice increasingly relied on a movement strategy of sequentially checking potential locations for the sound source. In contrast, the startle reflex (an innate auditory behavior) was preserved after unilateral hearing loss and abolished by bilateral hearing loss without recovery over time. In sum, mice compensate with body movement for permanent unilateral damage to the peripheral auditory system. Looking forward, this paradigm provides an opportunity to examine how movement enhances perception and enables resilient adaptation to sensory disorders.

Experimental Study of Needle Insertion into Gerbil Tympanic Membrane

The perforation characteristics and fracture-related mechanical properties of the tympanic membrane (TM) greatly affect surgical procedures like myringotomy and tympanostomy performed on the middle ear. We analyzed the most important features of the gerbil TM perforation using an experimental approach that was based on force measurement during a 2-cycle needle insertion/extraction process. Fracture energy, friction energy, strain energy, and hysteresis loss were taken into consideration for the analysis of the different stages of needle insertion and extraction. The results demonstrated that (1) although the TM shows viscoelastic behavior, the contribution of hysteresis loss was negligible compared to other irreversible dissipated energy components (i.e., fracture energy and friction energy). (2) The TM puncture force did not substantially change during the first hours after animal death, but interestingly, it increased after 1 week due to the drying effects of soft tissue. (3) The needle geometry affected the crack length and the most important features of the force-displacement plot for the needle insertion process (puncture force, puncture displacement, and jump-in force) increased with increasing needle diameter, whereas the insertion velocity only changed the puncture and jump-in forces (both increased with increasing insertion velocity) and did not have a noticeable effect on the puncture displacement. (4) The fracture toughness of the gerbil TM was almost independent of the needle geometry and was found to be around 0.33 ± 0.10 kJ/m2.

Sound-seeking before and after hearing loss in mice

How we move our bodies affects how we perceive sound. For instance, we can explore an environment to seek out the source of a sound and we can use head movements to compensate for hearing loss. How we do this is not well understood because many auditory experiments are designed to limit head and body movements. To study the role of movement in hearing, we developed a behavioral task called sound-seeking that rewarded mice for tracking down an ongoing sound source. Over the course of learning, mice more efficiently navigated to the sound. We then asked how auditory behavior was affected by hearing loss induced by surgical removal of the malleus from the middle ear. An innate behavior, the auditory startle response, was abolished by bilateral hearing loss and unaffected by unilateral hearing loss. Similarly, performance on the sound-seeking task drastically declined after bilateral hearing loss and did not recover. In striking contrast, mice with unilateral hearing loss were only transiently impaired on sound-seeking; over a recovery period of about a week, they regained high levels of performance, increasingly reliant on a different spatial sampling strategy. Thus, even in the face of permanent unilateral damage to the peripheral auditory system, mice recover their ability to perform a naturalistic sound-seeking task. This paradigm provides an opportunity to examine how body movement enables better hearing and resilient adaptation to sensory deprivation.

Ecomorphological correlates of inner and middle ear anatomy within phyllostomid bats

Echolocation is the primary sense used by most bats to navigate their environment. However, the influence of echolocating behaviors upon the morphology of the auditory apparatus remains largely uninvestigated. While it is known that middle ear ossicle size scales positively with body mass across mammals, and that peak call frequency scales negatively with body mass among bats, there are still large gaps in our understanding of the degree to which allometry or ecology influences the morphology of the chiropteran auditory apparatus. To investigate this, we used μCT datasets to quantify three morphological components of the inner and middle ear: ossicle size, ossicle shape, and cochlear spirality. These data were collected across 27 phyllostomid species, spanning a broad range of body sizes, habitats, and dietary categories, and the relationships between these variables and ear morphology were assessed using a comparative phylogenetic approach. Ossicle size consistently scaled with strong negative allometry relative to body mass. Cochlear spirality was significantly (p = .025) associated with wing aspect ratio (a proxy for habitat use) but was not associated with body mass. From a morphological perspective, the malleus and incus exhibited some variation in kind with diet and call frequency, while stapes morphology is more closely tied to body size. Future work will assess these relationships within other chiropteran lineages, and investigate potential morphological differences in the middle and inner ear of echolocating-vs-non-echolocating taxa.

Micro-CT of the human ossicular chain: Statistical shape modeling and implications for otologic surgery

The ossicular chain is a middle ear structure consisting of the small incus, malleus and stapes bones, which transmit tympanic membrane vibrations caused by sound to the inner ear. Despite being shown to be highly variable in shape, there are very few morphological studies of the ossicles. The objective of this study was to use a large sample of cadaveric ossicles to create a set of three-dimensional models and study their statistical variance. Thirty-three cadaveric temporal bone samples were scanned using micro-computed tomography (μCT) and segmented. Statistical shape models (SSMs) were then made for each ossicle to demonstrate the divergence of morphological features. Results revealed that ossicles were most likely to vary in overall size, but that more specific feature variability was found at the manubrium of the malleus, the long process and lenticular process of the incus, and the crura and footplate of the stapes. By analyzing samples as whole ossicular chains, it was revealed that when fixed at the malleus, changes along the chain resulted in a wide variety of final stapes positions. This is the first known study to create high-quality, three-dimensional SSMs of the human ossicles. This information can be used to guide otological surgical training and planning, inform ossicular prosthesis development, and assist with other ossicular studies and applications by improving automated segmentation algorithms. All models have been made publicly available.

Functional anatomy of the middle and inner ears of the red fox, in comparison to domestic dogs and cats

Anatomical middle and inner ear parameters are often used to predict hearing sensitivities of mammalian species. Given that ear morphology is substantially affected both by phylogeny and body size, it is interesting to consider whether the relatively small anatomical differences expected in related species of similar size have a noticeable impact on hearing. We present a detailed anatomical description of the middle and inner ears of the red fox Vulpes vulpes, a widespread, wild carnivore for which a behavioural audiogram is available. We compare fox ears to those of the well‐studied and similarly sized domestic dog and cat, taking data for dogs and cats from the literature as well as providing new measurements of basilar membrane (BM) length and hair cell numbers and densities in these animals. Our results show that the middle ear of the red fox is very similar to that of dogs. The most obvious difference from that of the cat is the lack of a fully formed bony septum in the bulla tympanica of the fox. The cochlear structures of the fox, however, are very like those of the cat, whereas dogs have a broader BM in the basal cochlea. We further report that the mass of the middle ear ossicles and the bulla volume increase with age in foxes. Overall, the ear structures of foxes, dogs and cats are anatomically very similar, and their behavioural audiograms overlap. However, the results of several published models and correlations that use middle and inner ear measurements to predict aspects of hearing were not always found to match well with audiogram data, especially when it came to the sharper tuning in the fox audiogram. This highlights that, although there is evidently a broad correspondence between structure and function, it is not always possible to draw direct links when considering more subtle differences between related species.

Computed tomographic grading of middle ear disease in domestic rabbits (Oryctolagus cuniculi)

Ear disease, involving the middle and external ear, is commonly reported in pet rabbits, with lop-eared breeds predisposed. CT is the most reliable imaging modality for identifying otitis media (OM) in rabbits. This study aimed to develop a CT-based grading scale to classify the severity of CT changes in rabbits with presumed OM. Three observers independently reviewed CT images of 36 rabbits with OM. For both OM and material within the external ear canal, a grade of I to IV was used, (I=mild, IV=severe disease) and the level of agreement between reviewers was statistically evaluated. The study demonstrated that grades from the three reviewers showed a strong level of interobserver agreement. Comparisons between clinical signs and CT grade were also evaluated. Rabbits with grade IV OM had a high grade of material in the external ear canal. A high proportion of rabbits (83 per cent) underwent CT scanning conscious, demonstrating that diagnostic quality CT images are achievable without the need for anaesthetic intervention, minimising patient risk. This novel CT-based grading scale provides a methodology to correlate grade of disease with clinical signs, ear canal cytology, predisposing factors, treatment options and outcomes for rabbits with ear disease.

Tympanic membrane pressure buffering function at quasi-static and low-frequency pressure variations

Deformation of the tympanic membrane is known to contribute to the pressure regulation processes in the middle ear cleft. In this paper we investigated pressure variations in the rabbit middle ear in response to sinusoidal varying pressures applied to the ear canal, with frequencies ranging from 0.5 Hz to 50 Hz and pressure amplitudes ranging between 0.25 kPa and 1 kPa. The transtympanic pressure difference was found to be smallest in the quasi-static range, and quickly increased as a function of frequency. The response curves showed asymmetry, with larger transtympanic pressures when positive pressures were applied in the ear canal. Normalized transtympanic pressure amplitudes remained fairly constant as a function of input pressure, with values in the range of 60%-70% relative to the applied pressure. The total harmonic distortion of the middle ear pressure signal was calculated and was found to be very small (≤2%) for low-pressure amplitudes and low frequencies. For pressure amplitudes in the order of 0.25 kPa-0.5 kPa, it increased to about 10% at 50 Hz. When a 1 kPa pressure amplitude was applied, variation between animals became large and distortion values up to 30% at 50 Hz were observed. The results showed that pressure buffering due to tympanic membrane displacement was most effective for compensating small transtympanic pressure loads at low frequencies.

Areca nut extract protects against ovariectomy-induced osteoporosis in mice

Estrogen deficiency increases the generation of reactive oxygen species (ROS), which is a crucial pathogenic factor for osteoporosis. Areca nuts are rich in phenolics, which have high antioxidant activity. In the present study, an ovariectomy (OVX)-induced osteoporosis mouse model was used to investigate the protective effects of areca nut extract (ANE) on bone loss and related processes. A total of 24 8-week-old female mice were randomly divided into three groups (n=8 per group): I Sham-operated control; II, bilateral OVX; and III, bilateral OVX + ANE. Group III were treated orally with ANE at a single dose of 300 mg/kg body weight daily for 6 months. ANE supplementation for 6 months improved trabecular bone microarchitecture and significantly increased bone mineral density in the distal femur (P<0.05) compared with Group II. Furthermore, serum levels of the osteoclast differentiation-inducing factors, receptor activator of nuclear factor-κB ligand and osteoprotegerin were significantly increased and decreased, respectively (both P<0.05), in OVX mice and these effects were significantly inhibited by ANE treatment (both P<0.05). ANE supplementation also resulted in significantly decreased serum hydrogen peroxide and malondialdehyde levels compared with Group II, while the levels of glutathione and catalase activity were significantly increased (P<0.05 and P<0.01, respectively). The current study indicated that the protective effects of ANE against bone loss were mediated, at least in part, via inhibition of the release of ROS and bone resorption. These results suggested that ANE could have therapeutic value in the treatment of osteoporosis.

Finite-Element Modelling of the Response of the Gerbil Middle Ear to Sound

We present a finite-element model of the gerbil middle ear that, using a set of baseline parameters based primarily on a priori estimates from the literature, generates responses that are comparable with responses we measured in vivo using multi-point vibrometry and with those measured by other groups. We investigated the similarity of numerous features (umbo, pars-flaccida and pars-tensa displacement magnitudes, the resonance frequency and break-up frequency, etc.) in the experimental responses with corresponding ones in the model responses, as opposed to simply computing frequency-by-frequency differences between experimental and model responses. The umbo response of the model is within the range of variability seen in the experimental data in terms of the low-frequency (i.e., well below the middle-ear resonance) magnitude and phase, the main resonance frequency and magnitude, and the roll-off slope and irregularities in the response above the resonance frequency, but is somewhat high for frequencies above the resonance frequency. At low frequencies, the ossicular axis of rotation of the model appears to correspond to the anatomical axis but the behaviour is more complex at high frequencies (i.e., above the pars-tensa break-up). The behaviour of the pars tensa in the model is similar to what is observed experimentally in terms of magnitudes, phases, the break-up frequency of the spatial vibration pattern, and the bandwidths of the high-frequency response features. A sensitivity analysis showed that the parameters that have the strongest effects on the model results are the Young's modulus, thickness and density of the pars tensa; the Young's modulus of the stapedial annular ligament; and the Young's modulus and density of the malleus. Displacements of the tympanic membrane and manubrium and the low-frequency displacement of the stapes did not show large changes when the material properties of the incus, stapes, incudomallear joint, incudostapedial joint, and posterior incudal ligament were changed by ±10 % from their values in the baseline parameter set.

Structure and function of the mammalian middle ear. I: Large middle ears in small desert mammals

Many species of small desert mammals are known to have expanded auditory bullae. The ears of gerbils and heteromyids have been well described, but much less is known about the middle ear anatomy of other desert mammals. In this study, the middle ears of three gerbils (Meriones, Desmodillus and Gerbillurus), two jerboas (Jaculus) and two sengis (elephant-shrews: Macroscelides and Elephantulus) were examined and compared, using micro-computed tomography and light microscopy. Middle ear cavity expansion has occurred in members of all three groups, apparently in association with an essentially 'freely mobile' ossicular morphology and the development of bony tubes for the middle ear arteries. Cavity expansion can occur in different ways, resulting in different subcavity patterns even between different species of gerbils. Having enlarged middle ear cavities aids low-frequency audition, and several adaptive advantages of low-frequency hearing to small desert mammals have been proposed. However, while Macroscelides was found here to have middle ear cavities so large that together they exceed brain volume, the bullae of Elephantulus are considerably smaller. Why middle ear cavities are enlarged in some desert species but not others remains unclear, but it may relate to microhabitat.

Volume shrinkage of bone, brain and muscle tissue in sample preparation for micro-CT and light sheet fluorescence microscopy (LSFM)

Two methods are especially suited for tomographic imaging with histological detail of macroscopic samples that consist of multiple tissue types (bone, muscle, nerve or fat): Light sheet (based) fluorescence microscopy (LSFM) and micro-computed tomography (micro-CT). Micro-CT requires staining with heavy chemical elements (and thus fixation and sometimes dehydration) in order to make soft tissue imageable when measured alongside denser structures. LSMF requires fixation, decalcification, dehydration, clearing and staining with a fluorescent dye. The specimen preparation of both imaging methods is prone to shrinkage, which is often not mentioned, let alone quantified. In this paper the presence and degree of shrinkage are quantitatively identified for the selected preparation methods/stains. LSFM delivers a volume shrinkage of 17% for bone, 56% for muscle and 62% for brain tissue. The three most popular micro-CT stains (phosphotungstic acid, iodine with potassium iodide, and iodine in absolute ethanol) deliver a volume shrinkage ranging from 10 to 56% for muscle and 27-66% for brain, while bone does not shrink in micro-CT preparation.

Three-dimensional vibration of the malleus and incus in the living gerbil

In previous studies, 3D motion of the middle-ear ossicles in cat and human was explored, but models for hearing research have shifted in the last few decades to smaller mammals, and gerbil, in particular, has become a popular hearing model. In the present study, we have measured with an optical interferometer the 3D motion of the malleus and incus in anesthetized gerbil for sound of moderate intensity (90-dB sound pressure level) over a broad frequency range. To access the ossicles, the pars flaccida was removed exposing the neck and head of the malleus and the incus from the malleus-incus joint to the plate of the lenticular process. Vibration measurements were done at six to eight points per ossicle while the angle of observation was varied over approximately 30 ° to enable calculation of the 3D rigid-body velocity components. These components were expressed in an intrinsic reference frame, with one axis along the anatomical suspension axis of the malleus-incus block and a second axis along the stapes piston direction. Another way of describing the motion that does not assume an a priori rotation axis is to calculate the instantaneous rotation axis (screw axis) of the malleus/incus motion. Only at frequencies below a few kilohertz did the screw axis have a maximum rotation in a direction close to that of the ligament axis. A slight slippage in the malleus-incus joint developed with increasing frequency. Our findings are useful in determining the sound transfer characteristics through the middle ear and serve as a reference for validation of mathematical middle-ear models. Last but not least, comparing our present results in gerbil with those of previously measured species (human and cat) exposes similarities and dissimilarities among them.

Viscoelastic properties of the human tympanic membrane studied with stroboscopic holography and finite element modeling

A new anatomically-accurate Finite Element (FE) model of the tympanic membrane (TM) and malleus was combined with measurements of the sound-induced motion of the TM surface and the bony manubrium, in an isolated TM-malleus preparation. Using the results, we were able to address two issues related to how sound is coupled to the ossicular chain: (i) Estimate the viscous damping within the tympanic membrane itself, the presence of which may help smooth the broadband response of a potentially highly resonant TM, and (ii) Investigate the function of a peculiar feature of human middle-ear anatomy, the thin mucosal epithelial fold that couples the mid part of the human manubrium to the TM. Sound induced motions of the surface of ex vivo human eardrums and mallei were measured with stroboscopic holography, which yields maps of the amplitude and phase of the displacement of the entire membrane surface at selected frequencies. The results of these measurements were similar, but not identical to measurements made in intact ears. The holography measurements were complemented by laser-Doppler vibrometer measurements of sound-induced umbo velocity, which were made with fine-frequency resolution. Comparisons of these measurements to predictions from a new anatomically accurate FE model with varied membrane characteristics suggest the TM contains viscous elements, which provide relatively low damping, and that the epithelial fold that connects the central section of the human manubrium to the TM only loosely couples the TM to the manubrium. The laser-Doppler measurements in two preparations also suggested the presence of significant variation in the complex modulus of the TM between specimens. Some animations illustrating the model results are available at our website (www.uantwerp.be/en/rg/bimef/downloads/tympanic-membrane-motion).

Three-dimensional histological specimen preparation for accurate imaging and spatial reconstruction of the middle and inner ear

PURPOSE

This paper presents a highly accurate cross-sectional preparation technique. The research aim was to develop an adequate imaging modality for both soft and bony tissue structures featuring high contrast and high resolution. Therefore, the advancement of an already existing micro-grinding procedure was pursued. The central objectives were to preserve spatial relations and to ensure the accurate three-dimensional reconstruction of histological sections.

METHODS

Twelve human temporal bone specimens including middle and inner ear structures were utilized. They were embedded in epoxy resin, then dissected by serial grinding and finally digitalized. The actual abrasion of each grinding slice was measured using a tactile length gauge with an accuracy of one micrometre. The cross-sectional images were aligned with the aid of artificial markers and by applying a feature-based, custom-made auto-registration algorithm. To determine the accuracy of the overall reconstruction procedure, a well-known reference object was used for comparison. To ensure the compatibility of the histological data with conventional clinical image data, the image stacks were finally converted into the DICOM standard.

RESULTS

The image fusion of data from temporal bone specimens' and from non-destructive flat-panel-based volume computed tomography confirmed the spatial accuracy achieved by the procedure, as did the evaluation using the reference object.

CONCLUSION

This systematic and easy-to-follow preparation technique enables the three-dimensional (3D) histological reconstruction of complex soft and bony tissue structures. It facilitates the creation of detailed and spatially correct 3D anatomical models. Such models are of great benefit for image-based segmentation and planning in the field of computer-assisted surgery as well as in finite element analysis. In the context of human inner ear surgery, three-dimensional histology will improve the experimental evaluation and determination of intra-cochlear trauma after the insertion of an electrode array of a cochlear implant system.

Micro-channels in the mastoid anatomy. Indications of a separate blood supply of the air cell system mucosa by micro-CT scanning

The mastoid air cell system has traditionally been considered to have a passive role in gas exchange and pressure regulation of the middle ear possibly with some acoustic function. However, more evidence has focused on the mucosa of the mastoid, which may play a more active role in regulation of middle ear pressure. In this study we have applied micro-CT scanning on a series of three human temporal bones. This approach greatly enhances the resolution (40-60 μm), so that we have discovered anatomical details, which has not been reported earlier. Thus, qualitative analysis using volume rendering has demonstrated notable micro-channels connecting the surface of the compact bone directly to the mastoid air cells as well as forming a network of connections between the air cells. Quantitative analysis on 2D slices was employed to determine the average diameter of these micro-channels (158 μm; range = 40-440 μm) as well as their density at a localized area (average = 75 cm(-2); range = 64-97 cm(-2)). These channels are hypothesized to contain a separate vascular supply for the mastoid mucosa. However, future studies of the histological structure of the micro-channels are warranted to confirm the hypothesis. Studies on the mastoid mucosa and its blood supply may improve our knowledge of its physiological properties, which may have important implications for our understanding of the pressure regulation of the middle ear. This article is part of a special issue entitled "MEMRO 2012".

MicroCT versus sTSLIM 3D imaging of the mouse cochlea

We made a qualitative and quantitative comparison between a state-of-the-art implementation of micro-Computed Tomography (microCT) and the scanning Thin-Sheet Laser Imaging Microscopy (sTSLIM) method, applied to mouse cochleae. Both imaging methods are non-destructive and perform optical sectioning, respectively, with X-rays and laser light. MicroCT can be used on fresh or fixed tissue samples and is primarily designed to image bone rather than soft tissues. It requires complex back-projection algorithms to produce a two-dimensional image, and it is an expensive instrument. sTSLIM requires that a specimen be chemically fixed, decalcified, and cleared; but it produces high-resolution images of soft and bony tissues with minimum image postprocessing and is less expensive than microCT. In this article, we discuss the merits and disadvantages of each method individually and when combined.

A reexamination of the Carnivora malleus (Mammalia, Placentalia)

Authoritative anatomical references depict domestic dogs and cats as having a malleus with a short rostral (anterior) process that is connected via a ligament to the ectotympanic of the auditory bulla. Similar mallei have been reported for representatives of each of the 15 extant families of Carnivora, the placental order containing dogs and cats. This morphology is in contrast to a malleus with a long rostral process anchored to the ectotympanic that is considered to be primitive for mammals. Our reexamination of extant carnivorans found representatives from 12 families that possess an elongate rostral process anchored to the ectotympanic. Consequently, the malleus also is a component of the bulla. In a subset of our carnivoran sample, we confirmed that the elongate rostral process on the ectotympanic is continuous with the rest of the malleus through a thin osseous lamina. This morphology is reconstructed as primitive for Carnivora. Prior inaccurate descriptions of the taxa in our sample having mallei continuous with the bulla were based on damaged mallei. In addition to coupling to the ectotympanic, the rostral process of the malleus was found to have a hook-like process that fits in a facet on the skull base in representatives from seven families (felids, nandiniids, viverrids, canids, ursids, procyonids, and mustelids); its occurrence in the remaining families could not be ascertained. This feature is named herein the mallear hook and is likewise reconstructed to be primitive for Carnivora. We also investigated mallei in one additional placental order reported to have mallei not connected to the ectotympanic, Pholidota (pangolins), the extant sister group of Carnivora. We found pholidotans to also have anchored mallei with long rostral processes, but lacking mallear hooks. In light of our results, other mammals previously reported to have short rostral processes should be reexamined.