The development of the mammalian outer and middle ear

An Essential Requirement for Fgf10 in Pinna Extension Sheds Light on Auricle Defects in LADD Syndrome

The pinna (or auricle) is part of the external ear, acting to capture and funnel sound toward the middle ear. The pinna is defective in a number of craniofacial syndromes, including Lacrimo-auriculo-dento-digital (LADD) syndrome, which is caused by mutations in FGF10 or its receptor FGFR2b. Here we study pinna defects in the Fgf10 knockout mouse. We show that Fgf10 is expressed in both the muscles and forming cartilage of the developing external ear, with loss of signaling leading to a failure in the normal extension of the pinna over the ear canal. Conditional knockout of Fgf10 in the neural crest fails to recapitulate this phenotype, suggesting that the defect is due to loss of Fgf10 from the muscles, or that this source of Fgf10 can compensate for loss in the forming cartilage. The defect in the Fgf10 null mouse is driven by a reduction in proliferation, rather than an increase in cell death, which can be partially phenocopied by inhibiting cell proliferation in explant culture. Overall, we highlight the mechanisms that could lead to the phenotype observed in LADD syndrome patients and potentially explain the formation of similar low-set and cup shaped ears observed in other syndromes.

Development of hearing in the big brown bat

We studied the development of hearing in newborn pups of the big brown bat, Eptesicus fuscus. In the majority of pups, the opening of both outer auditory canals occurred on or before postnatal day (PND) 7, but in some, it extended to PND 11. Using repeated auditory brainstem response (ABR) recordings, we tracked the progressive development and maturation of auditory sensitivity in 22 E. fuscus pups every 3 days, from PND 10 to PND 31, with additional recordings in a subset of bats at 2 months, 3 months and 1 year of life. There was a profound increase in auditory sensitivity across development for frequencies between 4 and 100 kHz, with the largest threshold shifts occurring early in development between PND 10 and 19. Prior to PND 13-16 and when pups were still non-volant, most bats were unable to hear frequencies above 48 kHz; however, sensitivity to these higher ultrasonic frequencies increased with age. Notably, this change occurred near the age when young bats started learning how to fly and echolocate.

On the Role of Fibrocytes and the Extracellular Matrix in the Physiology and Pathophysiology of the Spiral Ligament

The spiral ligament in the cochlea has been suggested to play a significant role in the pathophysiology of different etiologies of strial hearing loss. Spiral ligament fibrocytes (SLFs), the main cell type in the lateral wall, are crucial in maintaining the endocochlear potential and regulating blood flow. SLF dysfunction can therefore cause cochlear dysfunction and thus hearing impairment. Recent studies have highlighted the role of SLFs in the immune response of the cochlea. In contrast to sensory cells in the inner ear, SLFs (more specifically type III fibrocytes) have also demonstrated the ability to regenerate after different types of trauma such as drug toxicity and noise. SLFs are responsible for producing proteins, such as collagen and cochlin, that create an adequate extracellular matrix to thrive in. Any dysfunction of SLFs or structural changes to the extracellular matrix can significantly impact hearing function. However, SLFs may prove useful in restoring hearing by their potential to regenerate cells in the spiral ligament.

Uncovering the secreted signals and transcription factors regulating the development of mammalian middle ear ossicles

The mammalian middle ear comprises a chain of ossicles, the malleus, incus, and stapes that act as an impedance matching device during the transmission of sound from the tympanic membrane to the inner ear. These ossicles are derived from cranial neural crest cells that undergo endochondral ossification and subsequently differentiate into their final functional forms. Defects that occur during middle ear development can result in conductive hearing loss. In this review, we summarize studies describing the crucial roles played by signaling molecules such as sonic hedgehog, bone morphogenetic proteins, fibroblast growth factors, notch ligands, and chemokines during the differentiation of neural crest into the middle ear ossicles. In addition to these cell-extrinsic signals, we also discuss studies on the function of transcription factor genes such as Foxi3, Tbx1, Bapx1, Pou3f4, and Gsc in regulating the development and morphology of the middle ear ossicles.

Pneumatized middle ear ossicle - A normal variant

It is important to be familiar with normal anatomic variants in order to not to mistake them for pathology. It is well known that the middle ear ossicles have varying normal morphology on gross anatomy, but can also be malformed in certain congenital syndromes or eroded in the setting of chronic ear infection or cholesteatoma. This case series describes the pneumatized ossicle on imaging, a rare normal anatomic variant.

Modeling Normal and Pathological Ear Cartilage in vitro Using Somatic Stem Cells in Three-Dimensional Culture

Microtia (underdeveloped ear) is a rare congenital dysmorphology affecting the development of the outer ear. Although human microtic cartilage has not been fully characterized, chondrogenic cells derived from this tissue have been proposed as a suitable source for autologous auricular reconstruction. The aim of this study was to further characterize native microtic cartilage and investigate the properties of cartilage stem/progenitor cells (CSPCs) derived from it. Two-dimensional (2D) systems are most commonly used to assess the chondrogenic potential of somatic stem cells in vitro, but limit cell interactions and differentiation. Hence here we investigated the behavior of microtic CSPCs in three-dimensional spheroid cultures. Remarkable similarities between human microtic cartilages from five patients, as compared to normal cartilage, were observed notwithstanding possibly different etiologies of the disease. Native microtic cartilage displayed poorly defined perichondrium and hyper-cellularity, an immature phenotype that resembled that of the normal developing human auricular cartilage we studied in parallel. Crucially, our analysis of microtic ears revealed for the first time that, unlike normal cartilage, microtic cartilages are vascularized. Importantly, CSPCs isolated from human microtic and normal ear cartilages were found to recapitulate many characteristics of pathological and healthy tissues, respectively, when allowed to differentiate as spheroids, but not in monolayer cultures. Noteworthily, starting from initially homogeneous cell pellets, CSPC spheroids spontaneously underwent a maturation process in culture, and formed two regions (inner and outer region) separated by a boundary, with distinct cell types that differed in chondrogenic commitment as indicated by expression of chondrogenic markers. Compared to normal ear-derived spheroids, microtic spheroids were asymmetric, hyper-cellularized and the inner and outer regions did not develop properly. Hence, their organization resembled that of native microtic cartilage. Together, our results identify novel features of microtic ears and highlight the importance of 3D self-organizing in vitro systems for better understanding somatic stem cell behavior and disease modeling. Our observations of ear-derived chondrogenic stem cell behavior have implications for choice of cells for tissue engineered reconstructive purposes and for modeling the etiopathogenesis of microtia.

Is There a Coexistence of Peritemporal Wormian Bones and Congenital Aural Atresia?

In this study, the percent of wormian bones in congenital aural atresia group was higher than healthy group, when tested by the Fisher exact test, there appeared to be no significant statistical difference. However, in congenital aural atresia surgery, surgical manipulation can dislodge these bones causing injury to the surrounding structures. Therefore, the radiologists should report the presence of wormian bones in the peritemporal area.

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.

Spliceosomopathies and neurocristopathies: Two sides of the same coin?

Mutations in core components of the spliceosome are responsible for a group of syndromes collectively known as spliceosomopathies. Patients exhibit microcephaly, micrognathia, malar hypoplasia, external ear anomalies, eye anomalies, psychomotor delay, intellectual disability, limb, and heart defects. Craniofacial malformations in these patients are predominantly found in neural crest cells-derived structures of the face and head. Mutations in eight genes SNRPB, RNU4ATAC, SF3B4, PUF60, EFTUD2, TXNL4, EIF4A3, and CWC27 are associated with craniofacial spliceosomopathies. In this review, we provide a brief description of the normal development of the head and the face and an overview of mutations identified in genes associated with craniofacial spliceosomopathies. We also describe a model to explain how and when these mutations are most likely to impact neural crest cells. We speculate that mutations in a subset of core splicing factors lead to disrupted splicing in neural crest cells because these cells have increased sensitivity to inefficient splicing. Hence, disruption in splicing likely activates a cellular stress response that includes increased skipping of regulatory exons in genes such as MDM2 and MDM4, key regulators of P53. This would result in P53-associated death of neural crest cells and consequently craniofacial malformations associated with spliceosomopathies.

CXCL12 is required for stirrup-shaped stapes formation during mammalian middle ear development

BACKGROUND

The mammalian middle ear comprises a chain of three ossicles-the malleus, incus, and stapes-each of which has a unique morphology for efficiently transmitting sound information. In particular, the stapes, which is attached to the inner ear, is stirrup-shaped with a head and base connected by two crural arches, forming the stapedial foramen, through which the stapedial artery passes. However, how the stapes acquires this critical stirrup shape for association with the stapedial artery during development is not clear.

RESULTS

C-X-C motif chemokine ligand 12 (CXCL12) is a chemoattractant essential for cellular movement and angiogenesis. In Cxcl12 ^-/- embryos, migration of neural crest cells into the prospective middle ear regions and their mesenchymal condensation to form the three ossicles proceed normally in correct alignment with each other and the inner ear. However, in the absence of CXCL12, the stapes loses its stirrup shape and instead exhibits a columnar shape lacking the crural arches and central hole. In addition, although the stapedial artery initially forms during early mesenchymal condensation of the stapes, it degenerates without CXCL12 function.

CONCLUSION

CXCL12 plays an essential role in establishing the stirrup-shaped architecture of the stapes, possibly by maintaining the stapedial foramen and stapedial artery throughout development.

Characteristics of 43 multiple auricular deformity case families and auricle morphology in 463 microtia patients in South China

Background

Earlier studies have suggested that microtia is a genetic disease with a worldwide incidence of microtia is between 0.83/10,000 and 17.40/10,000. For microtia, auricle morphology is the most crucial characteristic. However, no studies have been performed to characterize the genetic similarity of microtia and auricle morphology similarity. For the sporadic patients, the relationship between the gestational age of parents and the incidence of microtia is unclear. To obtain the characteristics of auricular deformity multiple case family (AD-MCF) and clarify the relationship between genetic similarity and auricle morphology similarity in AD-MCF.

Methods

This study included 463 AD patients who were diagnosed by Sun Yat-sen Memorial Hospital, Sun Yat-sen University, from 2013 to 2019. Among these patients, 116 are from 43 MCF and the other 347 patients are sporadic. For the patients from families, the disease status of the four generations of immediate family members and the family tree map were collected to analyze the similarity of auricle shape in family members. A score evaluated the similarity of auricle shape according to the structure of the residual ear and the similarity in the morphology of each auricle. Moreover, the population distribution of AD and the gestational age of patients were further analyzed.

Results

From 2013 to 2019, a total of 463 patients were diagnosed as microtia in our hospital. There were 427 patients with unilateral disease and 36 patients with bilateral disease. Among them, 116 patients were from 34 families and 9 de novo families. The total scores of patients in different genetic difference levels were compared and were found significantly different (P<0.001). Moreover, 58.14% of families were consistent with the law of chromosomal recessive genetic diseases. Importantly, we found that the gestational age of father in microtia de novo families is 30.94±0.75, and mother in de novo is 28.39±0.73 that is significantly higher than the gestational ages of parents from microtia families with P value =0.0001.

Conclusions

The auricle similarity between family members is positively related to the genetic distance between family members. The microtia patients are potentially associated with the gestational ages of parents.

Congenital External Auditory Canal Cholesteatoma Resected by a Transcanal Endoscopic Approach

OBJECTIVE

To describe the first case of congenital external auditory canal cholesteatoma resected via a transcanal endoscopic approach and describe the possible pathologenesis of cholesteatoma.

PATIENT

A 2-year-old female patient presented with a cholesteatoma that extended from the floor of the external auditory bony canal to the inferior quadrant of her right tympanic membrane.

INTERVENTION

Otological examinations and computed tomography were performed. The cholesteatoma was resected using a transcanal endoscopic approach.

RESULTS

Pathologic examination confirmed the diagnosis of external auditory canal cholesteatoma. The patient's postoperative hearing was observed to be normal. Cholesteatoma did not recur during the 2-year follow-up period. Computed tomography scan revealed an isodense mass, partly surrounded by bony tissue, on the floor of the external auditory canal, outside of the inferior quadrant of an intact tympanic membrane. A minor malformation of the bony tissue, covering the inferiolateral surface of the cholesteatoma, was observed. The proposed mechanisms of pathogenesis are: 1) the embryonic developmental deficits of the meatal plug, during its differentiation into squamous epithelium, cause the arrest of ectodermal tissues, 2) a remnant of the squamous epithelium becomes trapped in the niche of the foramen tympanicum.

CONCLUSION

A rare case of congenital external auditory canal cholesteatoma, located on the inferior external auditory canal, was diagnosed and resected using a transcanal endoscopic approach. It was possibly caused by a minor anomaly of the first branchial cleft or by a remnant of the squamous epithelium trapped in the foramen tympanicum.

Middle ear congenital cholesteatoma: systematic review, meta-analysis and insights on its pathogenesis

PURPOSE

Congenital cholesteatoma (CC) presents as a white pearl-like lesion behind a normal tympanic membrane (TM), without a history of otorrhea, infection, perforation or previous otologic surgery. Several recent studies provided new data improving this pathology characterization. The aim of this paper is to expand the knowledge about CC and to provide new insights on its pathogenesis.

METHODS

The study consisted of two main research parts: (1) systematic review and meta-analysis; (2) medical literature review englobing anatomy, histology, embryology and congenital pathology of the ear.

RESULTS

The search strategy identified a total of 636 papers. Seventy retrospective studies were included. A total of 1497 cases were studied and the mean age was 6.58 years, with a male-female ratio of 3:1, 34% were asymptomatic, 26% had hearing loss and 2% had facial dysfunction/paralysis. The overall estimate for antero-superior quadrant involvement was 0.70 [95% confident interval (CI) 0.64-0.76], in the postero-superior quadrant was 0.60 (95% CI 0.52-0.69), in the antero-inferior quadrant was 0.32 (95% CI 0.23-0.41), in the postero-inferior quadrant was 0.38 (95% CI 0.29-0.47), in the attic was 0.53 (95% CI 0.43-0.63) and in the mastoid was 0.33 (95% CI 0.26-0.41). More advanced Potsic stages were present in older patients. The most likely inclusion place seems to be between the pars flaccida and the upper quadrants of the pars tensa.

CONCLUSIONS

During the last decades, a substantial improvement in CC diagnosis and management had been achieved. The presented mechanism seems to explain most of middle ear CC.

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

The 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.

Use of Zebrafish in Drug Discovery Toxicology

Unpredicted human safety events in clinical trials for new drugs are costly in terms of human health and money. The drug discovery industry attempts to minimize those events with diligent preclinical safety testing. Current standard practices are good at preventing toxic compounds from being tested in the clinic; however, false negative preclinical toxicity results are still a reality. Continual improvement must be pursued in the preclinical realm. Higher-quality therapies can be brought forward with more information about potential toxicities and associated mechanisms. The zebrafish model is a bridge between in vitro assays and mammalian in vivo studies. This model is powerful in its breadth of application and tractability for research. In the past two decades, our understanding of disease biology and drug toxicity has grown significantly owing to thousands of studies on this tiny vertebrate. This Review summarizes challenges and strengths of the model, discusses the 3Rs value that it can deliver, highlights translatable and untranslatable biology, and brings together reports from recent studies with zebrafish focusing on new drug discovery toxicology.

Early evolution of the ossicular chain in Cetacea: into the middle ear gears of a semi-aquatic protocetid whale

Modifications of the morphology and acoustic properties of the ossicular chain are among the major changes that accompanied the adaptation of Cetacea to the aquatic environment. Thus, data on the middle ear ossicles of early whales are crucial clues to understand the first steps of the emblematic terrestrial/aquatic transition that occurred in that group. Yet, the delicate nature and very small size of these bones make their preservation in the fossil record extremely rare. Due to the scarcity of available data, major questions remain concerning the sound transmission pathways in early non-fully aquatic whales. Virtual reconstruction of a partially complete ossicular chain of an Eocene protocetid whale documents for the first time the three ossicles of a semi-aquatic archaeocete. Contrary to previous hypotheses, these ossicles present different evolutionary patterns, showing that the ossicular chain does not act as a single morphological module. Functional analyses of the different middle ear units highlight a mosaic pattern of terrestrial and aquatic signatures. This integrative anatomical and functional study brings strong evidence that protocetids were adapted to their dual acoustic environment with efficient hearing in both air and water.

Embryonic and fetal development of the collared peccary (Pecari tajacu)

The relative timing of the main fetal development events in species determine the extent of fetal development at birth, which range along a gradient of having altricial and precocial traits. The results from this study allow for description of important fetal developments in collared peccary (Pecari tajacu) using data from 118 embryo/fetuses from 68 pregnant peccaries obtained over a period of 15 years through collaborative methods with local hunters in the Amazon. The chronological order of emergence of external characteristics in relation to the total dorsal length (TDL) was: differentiated genitalia, limbs and eyelid buds (TDL ≥ 4.5 cm), fusioned eyelids and outer ear (TDL ≥ 5.6 cm), dorsal gland (TDL ≥ 7.3 cm), skin (TDL ≥ 9.2 cm), tactile pelage (TDL ≥ 12.9 cm), coverage pelage (TDL ≥ 17.0 cm), opened eyelids (TDL ≥ 21.5 cm) and tooth eruption (TDL ≥ 24.5 cm). The formula of fetal age was ∛W = 0.079 (t - 27.6), with a linear relationship between TDL and gestational age. The relative weight of tubular gastrointestinal organs, lungs, spleen and thymus increased during fetal development. In contrast, the relative weight of kidneys and liver consistently decreased during the fetal development period. Results of this study indicate the collared peccary is a precocial species and that changes during fetal development are very similar to those in other Suiform species.

Histochemical and Ultrastructural Study of Developing Gonial Bone With Reference to Initial Ossification of the Malleus and Reduction of Meckel's Cartilage in Mice

Development of mouse gonial bone and initial ossification process of malleus were investigated. Before the formation of the gonial bone, the osteogenic area expressing alkaline phosphatase and Runx2 mRNA was widely recognized inferior to Meckel's cartilage. The gonial bone was first formed within the perichondrium at E16.0 via intramembranous ossification, surrounded the lower part of Meckel's cartilage, and then continued to extend anteriorly and medially until postnatal day (P) 3.0. At P0, multinucleated chondroclasts started to resorb the mineralized cartilage matrix with ruffled borders at the initial ossification site of the malleus (most posterior part of Meckel's cartilage). Almost all CD31-positive capillaries did not run through the gonial bone but entered the cartilage through the site where the gonial bone was not attached, indicating the forms of the initial ossification site of the malleus are similar to those at the secondary ossification center rather than the primary ossification center in the long bone. Then, the reducing process of the posterior part of Meckel's cartilage with extending gonial bone was investigated. Numerous tartrate-resistant acid phosphatase-positive mononuclear cells invaded the reducing Meckel's cartilage, and the continuity between the malleus and Meckel's cartilage was completely lost by P3.5. Both the cartilage matrix and the perichondrium were degraded, and they seemed to be incorporated into the periosteum of the gonial bone. The tensor tympani and tensor veli palatini muscles were attached to the ligament extending from the gonial bone. These findings indicated that the gonial bone has multiple functions and plays important roles in cranial formation. Anat Rec, 302:1916-1933, 2019. © 2019 American Association for Anatomy.

Correlation Between Mandible and External Ear in Patients with Treacher-Collins Syndrome

BACKGROUND

Patients with Treacher-Collins syndrome (TCS) are frequently affected by congenital ear deformities. The external ear in patients with TCS tends to have both abnormal morphology and reduced overall volume. Previous studies considered a correlation exists between TCS mandibular skeletal features and external ear volume. The purpose of this study was to assess the external ear volume in patients with TCS 3-dimensionally. Furthermore, this study evaluated the relationship between mandibular morphology, external ear profile, and external ear volume.

METHODS

A total of 36 nonoperated patients with TCS were compared to 39 age- and gender-matched normal controls. Morphologic variables of the mandible and the external ear were compared between TCS group and controls by 3-dimensional cephalometrics. The external ear volume and morphologic variables were analyzed with independent sample T-tests and Pearson correlation coefficient analyses (level of evidence: level III).

RESULTS

The external ear volume was reduced by approximately 50% in patients with TCS compared to controls (P < 0.001). External ear length and width were positively correlated with external ear volume (length: r = 0.809, P < 0.001 on left and r = 0.732, P < 0.001 on right; width: r = 0.518, P = 0.001 on left and r = 0.447, P < 0.010 on right). A negative correlation of right ear inclination angle and external ear volume was shown in patients with TCS (r = -0.396, P = 0.027). However, no correlation was shown for the mandibular anatomic variables.

CONCLUSIONS

Three-dimensional analysis confirmed that external ear volume is significantly reduced in patients with TCS. The external ear dimensions and orientation correlated significantly with ear volume. There was no intrinsic association between the severity of mandibular deformity and external ear volume.

Pars tensa and tympanicomalleal joint: proposal for a new anatomic classification

PURPOSE

The tympanic membrane (TM) belongs to the ear. Despite its place in the ear anatomy, can we give it also a different anatomic classification? The main objective is to clarify the nature of TM, tympanic bone and malleus to propose a new anatomic classification.

METHODS

This cadaveric study was performed in two human heads and six fresh temporal bones. A study of the temporomandibular joint, external acoustic meatus (EAM), TM and middle ear structures was conducted. A medical literature review englobing anatomy, embryology, histology and phylogeny of the ear was performed and the results were compared with the results of the dissection.

RESULTS

The external ear is constituted by the auricle and the EAM. This last segment is made by a cartilaginous and an osseous portion. The osseous portion of the EAM is constituted mainly by tympanic bone. The external ear is separated from the middle ear by the TM. Inside the middle ear, there are three ossicles: malleus, incus and stapes, which allow the conduction of sound to the cochlea. Based on the anatomic dissection and medical literature review of the tympanic bone, malleus and TM, we propose that these structures are interconnected like a joint, and named it "Tympanicomalleal joint".

CONCLUSIONS

It seems that the TM can be part of a joint that evolved to improve sound transmission and middle ear protection. Thinking TM has part of a joint may help in the development of more efficient reconstructive surgical techniques.

An Integrative Model Accounting for the Symptom Cluster Triggered After an Acoustic Shock

Acoustic shocks and traumas sometimes result in a cluster of debilitating symptoms, including tinnitus, hyperacusis, ear fullness and tension, dizziness, and pain in and outside the ear. The mechanisms underlying this large variety of symptoms remain elusive. In this article, we elaborate on the hypothesis that the tensor tympani muscle (TTM), the trigeminal nerve (TGN), and the trigeminal cervical complex (TCC) play a central role in generating these symptoms. We argue that TTM overuse (due to the acoustic shock), TTM overload (due to muscle tension), and ultimately, TTM injury (due to hypoxia and "energy crisis") lead to inflammation, thereby activating the TGN, TCC, and cortex. The TCC is a crossroad structure integrating sensory inputs coming from the head-neck complex (including the middle ear) and projecting back to it. The multimodal integration of the TCC may then account for referred pain outside the ear when the middle ear is inflamed and activates the TGN. We believe that our model proposes a synthetic and explanatory framework to explain the phenomena occurring postacoustic shock and potentially also after other nonauditory causes. Indeed, due to the bidirectional properties of the TCC, musculoskeletal disorders in the region of the head-neck complex, including neck injury due to whiplash or temporomandibular disorders, may impact the middle ear, thereby leading to otic symptoms. This previously unavailable model type is experimentally testable and must be taken as a starting point for identifying the mechanisms responsible for this particular subtype of tinnitus and its associated symptoms.

Region-specific endodermal signals direct neural crest cells to form the three middle ear ossicles

Defects in the middle ear ossicles - malleus, incus and stapes - can lead to conductive hearing loss. During development, neural crest cells (NCCs) migrate from the dorsal hindbrain to specific locations in pharyngeal arch (PA) 1 and 2, to form the malleus-incus and stapes, respectively. It is unclear how migratory NCCs reach their proper destination in the PA and initiate mesenchymal condensation to form specific ossicles. We show that secreted molecules sonic hedgehog (SHH) and bone morphogenetic protein 4 (BMP4) emanating from the pharyngeal endoderm are important in instructing region-specific NCC condensation to form malleus-incus and stapes, respectively, in mouse. Tissue-specific knockout of Shh in the pharyngeal endoderm or Smo (a transducer of SHH signaling) in NCCs causes the loss of malleus-incus condensation in PA1 but only affects the maintenance of stapes condensation in PA2. By contrast, knockout of Bmp4 in the pharyngeal endoderm or Smad4 (a transducer of TGFβ/BMP signaling) in the NCCs disrupts NCC migration into the stapes region in PA2, affecting stapes formation. These results indicate that region-specific endodermal signals direct formation of specific middle ear ossicles.

Copy number variation in the MSRB3 gene enlarges porcine ear size through a mechanism involving miR-584-5p

Background

The size and type of ears are important conformation characteristics that distinguish pig breeds. A significant quantitative trait locus (QTL) for ear size has been identified on SSC5 (SSC for Sus scrofa chromosome) but the underlying causative gene and mutation remain unknown. Thus, our aim was to identify the gene responsible for enlarged ears in pig.

Results

First, we narrowed down the QTL region on SSC5 to a 137.85-kb interval that harbors only the methionine sulfoxide reductase B3 (MSRB3) gene. Then, we identified a 38.7-kb copy number variation (CNV) that affects the last two exons of MSRB3 and could be the candidate causative mutation for this QTL. This CNV showed complete concordance with genotype at the QTL of the founder animals in a white Duroc × Erhualian F₂ intercross and was found only in pigs from six Chinese indigenous breeds with large ears and from the Landrace breed with half-floppy ears. Moreover, it accounted for the significant association with ear size on SSC5 across the five pig populations tested. eQTL mapping revealed that this CNV was significantly associated with the expression of the microRNA (miRNA) miR-584-5p, which interacts with MSRB3, one of its target genes. In vivo and in vitro experiments confirmed that miR-584-5p inhibits the translation of MSRB3 mRNA. Taken together, these results led us to conclude that presence of the 38.7-kb CNV in the genome of some pig breeds affects ear size by altering the expression of miR-584-5p, which consequently hinders the expression of one of its target genes (e.g. MSRB3).

Conclusions

Our findings shed insight into the underlying mechanism of development of external ears in mammals and contribute to a better understanding of how the presence of CNV can regulate gene expression.

Electronic supplementary material

The online version of this article (10.1186/s12711-018-0442-6) contains supplementary material, which is available to authorized users.

Hox-A2 protein expression in mouse embryo middle ear ossicles

The origin of the mammalian middle ear ossicles from mandibular and hyoid pharyngeal arches remains controversial and discussed. Two adverse theories are proposed. The first claims that malleus and incus derive from the Meckel's cartilage of the mandibular arch, and stapes from Reichert's cartilage of the hyoid arch. The second postulates that handle of malleus and long process of the incus are derived from the second arch as well as the stapes. Contradictory analyses support alternatively each theory without any experimental evidence. In order to bring new data, we analyzed by immunohistochemistry the expression of Hox-A2 protein in ossicular anlagen in E11 to 13 mouse embryos. HOXA2 gene is known to be expressed in second arch cells and to be absent from mandibular arch derivatives. Surprisingly, Hox-A2 protein was present in all ossicular primordia, as well in Reichert's cartilage. Meckel's cartilage was free of staining. Unlabeled cells were also present in ossicular blastemata. These results suggest that ossicular condensations could arise from mixed cell populations originated in both mandibular and hyoid pharyngeal arches. However, we cannot exclude that diffuse Hox-A2 immunoreactivity could correspond to a secondary expression in craniofacial mesenchyme independently from the branchial origin of cells.

A requirement for Fgfr2 in middle ear development

The skeletal structure of the mammalian middle ear, which is composed of three endochondral ossicles suspended within a membranous air-filled capsule, plays a critical role in conducting sound. Gene mutations that alter skeletal development in the middle ear result in auditory impairment. Mutations in fibroblast growth factor receptor 2 (FGFR2), an important regulator of endochondral and intramembranous bone formation, cause a spectrum of congenital skeletal disorders featuring conductive hearing loss. Although the middle ear malformations in multiple FGFR2 gain-of-function disorders are clinically characterized, those in the FGFR2 loss-of-function disorder lacrimo-auriculo-dento-digital (LADD) syndrome are relatively undescribed. To better understand conductive hearing loss in LADD, we examined the middle ear skeleton of mice with conditional loss of Fgfr2. We find that decreased auditory function in Fgfr2 mutant mice correlates with hypoplasia of the auditory bulla and ectopic bone growth at sites of tendon/ligament attachment. We show that ectopic bone associated with the intra-articular ligaments of the incudomalleal joint is derived from Scx-expressing cells and preceded by decreased expression of the joint progenitor marker Gdf5. Together, these results identify a role for Fgfr2 in development of the middle ear skeletal tissues and suggest potential causes for conductive hearing loss in LADD syndrome.

Stem cells in middle ear cholesteatoma contribute to its pathogenesis

Cholesteatoma is a potentially life-threatening middle ear lesion due to the formation of an inflamed ectopic mass of keratinizing squamous epithelium. Surgical removal remains the only treatment option, emphasizing the need to gain a better understanding of this severe disease. We show for the first time that stem cells residing in cholesteatoma tissue contribute to disease progression. Cells expressing the “stemness” markers Nestin and S100B were detected in middle ear cholesteatoma and auditory canal skin. Isolated Nestin + /S100B + -cells showed the capability for self-renewal, neurosphere formation and differentiation into mesodermal and ectodermal cell types. Compared to auditory canal skin stem cells middle ear cholesteatoma-derived stem cells displayed an enhanced susceptibility to inflammatory stimuli, and this suggested a possible contribution to the inflammatory environment in cholesteatoma tissue. Cholesteatoma derived stem cells were able to differentiate into keratinocyte-like cells using factors mimicking the microenvironment of cholesteatoma. Our findings demonstrate a new perspective on the pathogenesis of cholesteatoma and may lead to new treatment strategies for this severe middle ear lesion.

Structural and Mechanical Comparison of Human Ear, Alar, and Septal Cartilage

Supplemental Digital Content is available in the text.

Background:

In the human ear and nose, cartilage plays a key role in establishing its form and function. Interestingly, there is a noticeable paucity on biochemical, structural, and mechanical studies focused on facial cartilage. Such studies are needed to provide elementary knowledge that is fundamental to tissue engineering of cartilage. Therefore, in this study, a comparison is made of the biochemical, structural, and mechanical differences between ear, ala nasi, and septum on the extracellular matrix (ECM) level.

Methods:

Cartilage samples were harvested from 10 cadaveric donors. Each sample was indented 10 times with a nanoindenter to determine the effective Young’s modulus. Structural information of the cartilage was obtained by multiple-photon laser scanning microscopy capable of revealing matrix components at subcellular resolution. Biochemistry was performed to measure glycosaminoglycan (GAG), DNA, elastin, and collagen content.

Results:

Significant differences were seen in stiffness between ear and septal cartilage (P = 0.011) and between ala nasi and septal cartilage (P = 0.005). Elastin content was significantly higher in ear cartilage. Per cartilage subtype, effective Young’s modulus was not significantly correlated with cell density, GAG, or collagen content. However, in septal cartilage, low elastin content was associated with higher stiffness. Laser microscopy showed a distinct difference between ear cartilage and cartilage of nasal origin.

Conclusion:

Proposed methods to investigate cartilage on the ECM level provided good results. Significant differences were seen not only between ear and nasal cartilage but also between the ala nasi and septal cartilage. Albeit its structural similarity to septal cartilage, the ala nasi has a matrix stiffness comparable to ear cartilage.

Low-set ears and associated anomalies in human foetuses

OBJECTIVES

To determine the prevalence of low-set ears (LSE) in a group of human foetuses, to analyse the associated anomalies, and to review the development mechanisms possibly involved.

METHODS

A total of 1759 human foetuses from spontaneous abortion were evaluated. Foetuses were obtained from the Foetuses and Embryos Collection of the Embryology Department of the Faculty of Medicine of the Autonomous University of Nuevo León. The Ethics Committee gave its approval for this study (EH-230-16). The position of the auricles was determined according to the standards recommended by the Elements of Morphology. Two study groups were created: foetuses with LSE and foetuses with normal ears. In both groups, a detailed examination of the external morphology was performed, followed by thoraco-abdominal micro dissection. Statistical analysis was performed.

RESULTS

Two hundred two of the foetuses presented LSE (1148 per 10,000). In this group, 68.8% did not present associated anomalies, while 31.2% had an associated anomaly. The most frequently affected organ was the heart (53.6%), followed by the digestive tract (23.9%), urinary system (16.9%), head and neck (4.2%), and limbs (1.4%). In the group of foetuses with normal ears, only 7.4% of the specimens had associated anomalies, which was a significant difference compared with the LSE group.

CONCLUSIONS

Based on the obtained results, we consider that LSE can be used as a sensitive indicator of major anomalies. It is recommended to include a systematic assessment of the position of the auricles in the initial clinical evaluation of any newborn.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

Cilia distribution and polarity in the epithelial lining of the mouse middle ear cavity

The middle ear conducts sound to the cochlea for hearing. Otitis media (OM) is the most common illness in childhood. Moreover, chronic OM with effusion (COME) is the leading cause of conductive hearing loss. Clinically, COME is highly associated with Primary Ciliary Dyskinesia, implicating significant contributions of cilia dysfunction to COME. The understanding of middle ear cilia properties that are critical to OM susceptibility, however, is limited. Here, we confirmed the presence of a ciliated region near the Eustachian tube orifice at the ventral region of the middle ear cavity, consisting mostly of a lumen layer of multi-ciliated and a layer of Keratin-5-positive basal cells. We also found that the motile cilia are polarized coordinately and display a planar cell polarity. Surprisingly, we also found a region of multi-ciliated cells that line the posterior dorsal pole of the middle ear cavity which was previously thought to contain only non-ciliated cells. Our study provided a more complete understanding of cilia distribution and revealed for the first time coordinated polarity of cilia in the epithelium of the mammalian middle ear, thus illustrating novel structural features that are likely critical for middle ear functions and related to OM susceptibility.

On the development of the chondrocranium and the histological anatomy of the head in perinatal stages of marsupial mammals

An overview of the literature on the chondrocranium of marsupial mammals reveals a relative conservatism in shape and structures. We document the histological cranial anatomy of individuals representing Monodelphis domestica, Dromiciops gliroides, Perameles sp. and Macropus eugenii. The marsupial chondrocranium is generally characterized by the great breadth of the lamina basalis, absence of pila metoptica and large otic capsules. Its most anterior portion (cupula nasi anterior) is robust, and anterior to it there are well-developed tactile sensory structures, functionally important in the neonate. Investigations of ossification centers at and around the nasal septum are needed to trace the presence of certain bones (e.g., mesethmoid, parasphenoid) across marsupial taxa. In many adult marsupials, the tympanic floor is formed by at least three bones: alisphenoid (alisphenoid tympanic process), ectotympanic and petrosal (rostral and caudal tympanic processes); the squamosal also contributes in some diprotodontians. The presence of an entotympanic in marsupials has not been convincingly demonstrated. The tubal element surrounding the auditory tube in most marsupials is fibrous connective tissue rather than cartilage; the latter is the case in most placentals recorded to date. However, we detected fibrocartilage in a late juvenile of Dromiciops, and a similar tissue has been reported for Tarsipes. Contradictory reports on the presence of the tegmen tympani can be found in the literature. We describe a small tegmen tympani in Macropus. Several heterochronic shifts in the timing of development of the chondocranium and associated structures (e.g., nerves, muscles) and in the ossification sequence have been interpreted as largely being influenced by functional requirements related to the altriciality of the newborn marsupial during early postnatal life. Comparative studies of chondocranial development of mammals can benefit from a solid phylogenetic framework, research on non-classical model organisms, and integration with imaging and sectional data derived from computer-tomography.

Dynamic property changes in stapedial annular ligament associated with acute otitis media in the chinchilla

Located at the end of the ossicular chain, the stapedial annular ligament (SAL) serves as a closed yet mobile boundary between the cochlear fluid and stapes footplate. It is unclear how SAL properties change with acute otitis media (AOM). This paper reports the measurements of SAL dynamic properties in chinchilla AOM model using dynamic mechanical analyzer (DMA) and frequency-temperature superposition (FTS) principle. AOM was analyzed in two infection groups: 4 days (4D) and 8 days (8D) post induction. SAL specimens were measured using DMA at three temperatures: 5, 25, and 37°C. To extend the testing frequencies to higher levels, FTS principle was employed. Then generalized Maxwell model was utilized to define the constitutive equations of the SAL. The complex shear moduli were obtained from seven samples of control, 4D, and 8D groups. Results show that the storage and loss shear moduli of SALs decreased due to AOM. The storage moduli for 4D and 8D ears were similar below 100Hz, and the loss modulus for 4D was significantly larger than 8D across the entire frequency range. This study reports data that contributes to ear biomechanics and improves understanding on the effects of AOM in middle ear tissues.

Finite-Element Modelling of the Acoustic Input Admittance of the Newborn Ear Canal and Middle Ear

Admittance measurement is a promising tool for evaluating the status of the middle ear in newborns. However, the newborn ear is anatomically very different from the adult one, and the acoustic input admittance is different than in adults. To aid in understanding the differences, a finite-element model of the newborn ear canal and middle ear was developed and its behaviour was studied for frequencies up to 2000 Hz. Material properties were taken from previous measurements and estimates. The simulation results were within the range of clinical admittance measurements made in newborns. Sensitivity analyses of the material properties show that in the canal model, the maximum admittance and the frequency at which that maximum admittance occurs are affected mainly by the stiffness parameter; in the middle-ear model, the damping is as important as the stiffness in influencing the maximum admittance magnitude but its effect on the corresponding frequency is negligible. Scaling up the geometries increases the admittance magnitude and shifts the resonances to lower frequencies. The results suggest that admittance measurements can provide more information about the condition of the middle ear when made at multiple frequencies around its resonance.

Early development of the malleus and incus in humans

It is widely accepted by developmental biologists that the malleus and incus of the mammalian middle ear are first pharyngeal arch derivatives, a contention based originally on classical embryology that has now been backed up by molecular evidence from rodent models. However, it has been claimed in several studies of human ossicular development that the manubrium of the malleus and long process of the incus are actually derived from the second arch. This 'dual-arch' interpretation is commonly presented in otolaryngology textbooks, and it has been used by clinicians to explain the aetiology of certain congenital abnormalities of the human middle ear. In order to re-examine the origins of the human malleus and incus, we made three-dimensional reconstructions of the pharyngeal region of human embryos from 7 to 28 mm crown-rump length, based on serial histological sections from the Boyd Collection. We considered the positions of the developing ossicles relative to the pharyngeal pouches and clefts, and the facial and chorda tympani nerves. Confirming observations from previous studies, the primary union between first pharyngeal pouch and first cleft found in our youngest specimens was later lost, the external meatus developing rostroventral to this position. The mesenchyme of the first and second arches in these early embryos seemed to be continuous, but the boundaries of the developing ossicles proved to be very hard to determine at this stage. When first distinguishable, the indications were that both the manubrium of the malleus and the long process of the incus were emerging within the first pharyngeal arch. We therefore conclude that the histological evidence, on balance, favours the 'classical' notion that the human malleus and incus are first-arch structures. The embryological basis of congenital ossicular abnormalities should be reconsidered in this light.

The taming of the neural crest: a developmental perspective on the origins of morphological covariation in domesticated mammals

Studies on domestication are blooming, but the developmental bases for the generation of domestication traits and breed diversity remain largely unexplored. Some phenotypic patterns of human neurocristopathies are suggestive of those reported for domesticated mammals and disrupting neural crest developmental programmes have been argued to be the source of traits deemed the 'domestication syndrome'. These character changes span multiple organ systems and morphological structures. But an in-depth examination within the phylogenetic framework of mammals including domesticated forms reveals that the distribution of such traits is not universal, with canids being the only group showing a large set of predicted features. Modularity of traits tied to phylogeny characterizes domesticated mammals: through selective breeding, individual behavioural and morphological traits can be reordered, truncated, augmented or deleted. Similarly, mammalian evolution on islands has resulted in suites of phenotypic changes like those of some domesticated forms. Many domesticated mammals can serve as valuable models for conducting comparative studies on the evolutionary developmental biology of the neural crest, given that series of their embryos are readily available and that their phylogenetic histories and genomes are well characterized.

The Nervous System Orchestrates and Integrates Craniofacial Development: A Review

Development of a head is a dazzlingly complex process: a number of distinct cellular sources including cranial ecto- and endoderm, mesoderm and neural crest contribute to facial and other structures. In the head, an extremely fine-tuned developmental coordination of CNS, peripheral neural components, sensory organs and a musculo-skeletal apparatus occurs, which provides protection and functional integration. The face can to a large extent be considered as an assembly of sensory systems encased and functionally fused with appendages represented by jaws. Here we review how the developing brain, neurogenic placodes and peripheral nerves influence the morphogenesis of surrounding tissues as a part of various general integrative processes in the head. The mechanisms of this impact, as we understand it now, span from the targeted release of the morphogens necessary for shaping to providing a niche for cellular sources required in later development. In this review we also discuss the most recent findings and ideas related to how peripheral nerves and nerve-associated cells contribute to craniofacial development, including teeth, during the post- neural crest period and potentially in regeneration.