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.

Larger Genomes Linked to Slower Development and Loss of Late-Developing Traits.

Genome size varies widely among organisms and is known to affect vertebrate development, morphology, and physiology. In amphibians, genome size is hypothesized to contribute to loss of late-forming structures, although this hypothesis has mainly been discussed in salamanders. Here we estimated genome size for 22 anuran species and combined this novel data set with existing genome size data for an additional 234 anuran species to determine whether larger genome size is associated with loss of a late-forming anuran sensory structure, the tympanic middle ear. We established that genome size is negatively correlated with development rate across 90 anuran species and found that genome size evolution is correlated with evolutionary loss of the middle ear bone (columella) among 241 species (224 eared and 17 earless). We further tested whether the development of the tympanic middle ear could be constrained by large cell sizes and small body sizes during key stages of tympanic middle ear development (metamorphosis). Together, our evidence suggests that larger genomes, slower development rate, and smaller body sizes at metamorphosis may contribute to the loss of the anuran tympanic middle ear. We conclude that increases in anuran genome size, although less drastic than those in salamanders, may affect development of late-forming traits.

Cricket tympanal organ revisited: morphology, development and possible functions of the adult-specific chitin core beneath the anterior tympanal membrane.

Vertebrates and insects are phylogenetically separated by millions of years but have commonly developed tympanal membranes for efficiently converting airborne sound to mechanical oscillation in hearing. The tympanal organ of the field cricket Gryllus bimaculatus, spanning 200 µm, is one of the smallest auditory organs among animals. It indirectly links to two tympana in the prothoracic tibia via tracheal vesicles. The anterior tympanal membrane is smaller and thicker than the posterior tympanal membrane and it is thought to have minor function as a sound receiver. Using differential labeling of sensory neurons/surrounding structures and three-dimensional reconstructions, we revealed that a shell-shaped chitin mass and associated tissues are hidden behind the anterior tympanal membrane. The mass, termed the epithelial core, is progressively enlarged by discharge of cylindrical chitin from epithelial cells that start to aggregate immediately after the final molt and it reaches a plateau in size after 6 days. The core, bridging between the anterior tracheal vesicle and the fluid-filled chamber containing sensory neurons, is supported by a taut membrane, suggesting the possibility that anterior displacements of the anterior tracheal vesicle are converted into fluid motion via a lever action of the core. The epithelial core did not exist in tympanal organ homologs of meso- and metathoracic legs or of nymphal legs. Taken together, the findings suggest that the epithelial core, a potential functional homolog to mammalian ossicles, underlies fine sound frequency discrimination required for adult-specific sound communications.

The adhesion molecule cadherin 11 is essential for acquisition of normal hearing ability through middle ear development in the mouse.

Cadherin 11 (Cdh11), a member of the cadherin adhesion molecule family, is expressed in various regions of the brain as well as the head and ear. To gain further insights into the roles of Cdh11 in the development of the ear, we performed behavioral tests using Cdh11 knockout (KO) mice. KO mice showed reduced acoustic startle responses and increased thresholds for auditory brainstem responses, indicating moderate hearing loss. The auditory bulla volume and ratio of air-filled to non-air-filled space in the middle ear cavity were reduced in KO mice, potentially causing conductive hearing loss. Furthermore, residual mesenchymal and inflammatory cells were observed in the middle ear cavity of KO mice. Cdh11 was expressed in developing mesenchymal cells just before the start of cavitation, indicating that Cdh11 may be directly involved in middle ear cavitation. Since the auditory bulla is derived from the neural crest, the regulation of neural crest-derived cells by Cdh11 may be responsible for structural development. This mutant mouse may be a promising animal model for elucidating the causes of conductive hearing loss and otitis media.

[The relationship between the parameters of evoked otoacoustical emission and tympanometry in the children born with the extremely low body weight]. / Vzaimosviaz' parametrov vyzvannoi otoakusticheskoi émissii i timpanometrii u detei, rozhdennykh s ékstremal'no nizkoi massoi tela.

The objective of the present study was to elucidate correlation between the parameters of evoked otoacoustical emission at the distorsion product otoacoustic emissionen frequency (DPOAE) and the results of tympoanometry performed at the probe tone frequencies of 226 Hz and 1 kHz in the children born with the extremely low body weight. The results of the study give evidence of the moderate correlation dependence between the strength of the cochlear acoustic response at DPOAE and the cohlear response amplitude at the frequencies of 2 kHz and 6 kHz from TTP (r=0.3; p=0.000) obtained at the tympanometry probe tone frequency of 1 kHz. The correlation between the magnitude of the acoustic response of the cochlea, the amplitude of this response at the frequencies of 2 kHz and 6 kHz, the width of the tympanograms, and their static compliance obtained in the studies at the tympanometry probe tone frequency of 1,000 Hz (r=0.3-0.5; p=0.001) was documented in the infants at the age of 6 months and 1 year.

Mammalian development does not recapitulate suspected key transformations in the evolutionary detachment of the mammalian middle ear.

The ectotympanic, malleus and incus of the developing mammalian middle ear (ME) are initially attached to the dentary via Meckel's cartilage, betraying their origins from the primary jaw joint of land vertebrates. This recapitulation has prompted mostly unquantified suggestions that several suspected--but similarly unquantified--key evolutionary transformations leading to the mammalian ME are recapitulated in development, through negative allometry and posterior/medial displacement of ME bones relative to the jaw joint. Here we show, using µCT reconstructions, that neither allometric nor topological change is quantifiable in the pre-detachment ME development of six marsupials and two monotremes. Also, differential ME positioning in the two monotreme species is not recapitulated. This challenges the developmental prerequisites of widely cited evolutionary scenarios of definitive mammalian middle ear (DMME) evolution, highlighting the requirement for further fossil evidence to test these hypotheses. Possible association between rear molar eruption, full ME ossification and ME detachment in marsupials suggests functional divergence between dentary and ME as a trigger for developmental, and possibly also evolutionary, ME detachment. The stable positioning of the dentary and ME supports suggestions that a 'partial mammalian middle ear' as found in many mammaliaforms--probably with a cartilaginous Meckel's cartilage--represents the only developmentally plausible evolutionary DMME precursor.

Better late than never: effective air-borne hearing of toads delayed by late maturation of the tympanic middle ear structures.

Most vertebrates have evolved a tympanic middle ear that enables effective hearing of airborne sound on land. Although inner ears develop during the tadpole stages of toads, tympanic middle ear structures are not complete until months after metamorphosis, potentially limiting the sensitivity of post-metamorphic juveniles to sounds in their environment. We tested the hearing of five species of toads to determine how delayed ear development impairs airborne auditory sensitivity. We performed auditory brainstem recordings to test the hearing of the toads and used micro-computed tomography and histology to relate the development of ear structures to hearing ability. We found a large (14-27 dB) increase in hearing sensitivity from 900 to 2500 Hz over the course of ear development. Thickening of the tympanic annulus cartilage and full ossification of the middle ear bone are associated with increased hearing ability in the final stages of ear maturation. Thus, juvenile toads are at a hearing disadvantage, at least in the high-frequency range, throughout much of their development, because late-forming ear elements are critical to middle ear function at these frequencies. We discuss the potential fitness consequences of late hearing development, although research directly addressing selective pressures on hearing sensitivity across ontogeny is lacking. Given that most vertebrate sensory systems function very early in life, toad tympanic hearing may be a sensory development anomaly.

Wideband acoustic immittance measures: developmental characteristics (0 to 12 months).

Rapid developmental changes of the peripheral auditory system in normal infants occur in the first year of life. Specifically, the postnatal development of the external and middle ear affects all measures of external and middle ear function including wideband acoustic immittance(WAI). This article provides an overview of WAI studies in newborns and infants from a developmental perspective. Normative WAI data in newborns are fairly consistent across studies. However, there are discrepancies in some WAI measures between studies, possibly due to differences in sampling, methodology, and instrumentation. Accuracy of WAI measurements is compromised when a good probe seal cannot be maintained during testing or an inaccurate estimate of the cross-sectional area of the ear canal of newborns occurs. Comparison of WAI data between age groups from 0 to 12 months reveals maturation effects. Additional age-specific longitudinal and cross-sectional normative WAI data for infants from birth to 12 months are required to validate and consolidate existing data.

Defects in middle ear cavitation cause conductive hearing loss in the Tcof1 mutant mouse.

Conductive hearing loss (CHL) is one of the most common forms of human deafness. Despite this observation, a surprising gap in our understanding of the mechanisms underlying CHL remains, particularly with respect to the molecular mechanisms underlying middle ear development and disease. Treacher Collins syndrome (TCS) is an autosomal dominant disorder of facial development that results from mutations in the gene TCOF1. CHL is a common feature of TCS but the causes of the hearing defect have not been studied. In this study, we have utilized Tcof1 mutant mice to dissect the developmental mechanisms underlying CHL. Our results demonstrate that effective cavitation of the middle ear is intimately linked to growth of the auditory bulla, the neural crest cell-derived structure that encapsulates all middle ear components, and that defects in these processes have a profoundly detrimental effect on hearing. This research provides important insights into a poorly characterized cause of human deafness, and provides the first mouse model for the study of middle ear cavity defects, while also being of direct relevance to a human genetic disorder.

[Specific surgical features of the tympanic cavity in the children under the age of 3 years].

UNLABELLED: The objective of the present study was to obtain new data of practical significance from the comparative analysis of the anatomical structure of the tympanic cavity in newborn infants and young children. MATERIALS AND METHODS: A total of 23 and 24 temporal bone macropreparations obtained from the newborn infants and young children (between 1 to 3 years of age) respectively were available for the examination that included sequential dissection, measurement, and macrophotographing of the specimens. RESULTS: The study has revealed a number of distinctive features of the anatomical structure of the tympanic cavity in newborn infants and young children.

[Clinical and anatomical features of the middle ear in the premature infants at different gestational time].

The objective of the present study was to analyse anatomical features of the middle ear in the premature infants of different gestational age. Materials from 100 still-born and live-born babies (200 temporal bones) were available for the investigation. The study has revealed a number of distinctive clinical and morphological peculiarities in the structure of tympanic membranes in both the prematurely born infants depending on the gestational age and in the full-term babies. The fluid from the tympanic cavity was found to contain human beta-chorionic gonadotropin.

Can you hear me now? A genetic model of otitis media with effusion.

Otitis media with effusion (OME) is characterized by the occurrence of fluid in the middle-ear cavity in the absence of any signs of acute ear infection and occurs most frequently in children with auditory or eustachian tube dysfunction. Its chronic form is an important clinical issue for pediatricians and otologists alike. The study by Depreux et al. in this issue of the JCI shows that absence of the transcriptional activator Eya4 in knockout mice results in abnormal structuring of the eustachian tube, thus predisposing these animals to OME (see the related article beginning on page 651). The development of this genetics-based animal model is an important advance for understanding OME and for exploring new avenues of treatment.

The efficiency of the single- versus multiple-stimulus auditory steady state responses in infants.

OBJECTIVES: Multiple auditory steady state responses (ASSRs) will likely be included in the diagnostic test battery for estimating infant auditory thresholds in the near future; however, the effects of single- versus multiple-stimulus presentation in infants has never been investigated. In adults, there are no interactions (reduced amplitudes) between responses to multiple simultaneous stimuli presented at 60 dB SPL or lower. Maturational differences, however, may lead to greater interactions in infants; thus, it is unknown whether the single-stimulus technique or the multiple-stimulus technique is more efficient for testing infants. Two studies were carried out to address this issue. DESIGN: All infants in study A participated in three stimulus conditions, which differed in the number of stimuli presented simultaneously. The monotic single (MS) condition consisted of 500, 1000, 2000, and 4000 Hz tones, which were presented singly to one ear. The monotic multiple (MM) condition was composed of four tones (500, 1000, 2000, and 4000 Hz) presented to one ear simultaneously. The dichotic multiple (DM) condition consisted of eight tones presented simultaneously to both ears (four tones to each ear). ASSR amplitudes were obtained from 15 normal infants (mean age: 23.1 wks) in response to multiple (MM, DM) and single (MS) air conduction amplitude-modulated (AM) tones (77 to 105 Hz modulation rates; 60 dB SPL). In study B, ASSR thresholds were determined for 500-Hz stimuli in the single- and DM-stimulus conditions (14 infants; mean age: 20.2 wks). RESULTS: Mean single-stimulus ASSR amplitudes for 500, 1000, 2000, and 4000 Hz were 30, 39, 45 and 43 nV, respectively. Presentation of multiple AM tones (i.e., four octave-spaced frequencies) to one ear resulted in ASSR amplitudes that were 97%, 87%, 82%, and 70% (for 500, 1000, 2000, and 4000 Hz, respectively) of the single-stimulus ASSR amplitudes. Results for the dichotic presentation of eight AM tones show ASSR amplitudes that were 70%, 77%, 67%, and 67% relative to the MS condition. Although decreases in amplitude occurred using multiple stimuli in infants, the multiple ASSR remained more efficient than the single-stimulus ASSR (i.e., multiple-stimulus amplitudes were greater than single-stimulus amplitudes divided by √K, where K is the number of stimuli). Results from study B indicate that ASSR thresholds for 500 Hz presented in the DM condition were elevated 3 dB compared with that obtained in the 500-Hz single-stimulus condition. This statistically nonsignificant difference is within the range of acceptable test-retest variability and is thus not of clinical significance. CONCLUSIONS: The amplitude reductions seen in the multiple-stimulus conditions in infants, not seen in adults, could be related to maturational differences in the ear canal, middle ear, cochlea, and/or brain stem. Because greater interactions occur in the DM-stimulus condition compared with the monotic multiple-stimulus condition and baseline single-stimulus condition, brain stem origins of these interactions are likely. Study B revealed statistically nonsignificant differences between threshold for 500 Hz when presented in the single- and DM-stimulus conditions. In summary, as with adults, multiple-stimulus presentation in infants is more efficient than single AM tones, at least for 60 dB SPL stimuli.

Distortion-product otoacoustic-emission suppression tuning in human infants and adults using absorbed sound power.

The greatest difference in distortion product otoacoustic emission (DPOAE) suppression tuning curves (STCs) in infant and adult ears occurs at a stimulus frequency of 6 kHz. These infant and adult STCs are much more similar when constructed using the absorbed power level of the stimulus and suppressor tones rather than using sound pressure level. This procedure incorporates age-related differences in forward and reverse transmission of sound power through the ear canal and middle ear. These results support the theory that the cochlear mechanics underlying DPOAE suppression are substantially mature in full-term infants.

[The middle ear in the ontogenesis of mammals].

The middle ear in mammals is characterized by structural variations and a broad spectrum of adaptive transformations related to peculiarities of species ecology, but it preserves the general basic principle of structure in most mammals. In species remote from a phylogenetic point of view but close in ecologic specialization, features of parallelism are observed concerning the development of separate elements of auditory ossicles as well as the way of their interconnection and attachment to the tympanic cavity. Along the way to the adaptation to the water lifestyle in semi-aqueous and aqueous species, new additional structures, not intrinsic to initial terrestrial forms, have been formed. The use of ecological and morphological approaches to research the peripheral division of the auditory system of mammals with different ecological specialization in the ontogenesis permitted us to reveal that peculiarities of its structure in different groups of mammals are preconditioned by the animals' adaptation to specific acoustic properties of their environment. Morphofunctional adaptations of the peripheral auditory system aimed at optimizing auditory sensitivity in the environments differing in physical properties are of great importance in evolution. Adaptive specific features in the structure of the middle ear in aqueous species appear at early stages of development in spite of intrauterine growth without the direct influence of environmental conditions.

History of studies on mammalian middle ear evolution: a comparative morphological and developmental biology perspective.

The mammalian middle ear represents one of the most fundamental morphological features that define this class of vertebrates. Its skeletal pattern differs conspicuously from those of other amniotes and has attracted the attention of comparative zoologists for about 200 years. To reconcile this morphological inconsistency, early comparative morphologists suggested that the mammalian middle ear was derived from elements of the jaw joint of nonmammalian amniotes. Fossils of mammalian ancestors also implied a transition in skeletal morphology that resulted in the mammalian state. During the latter half of the 20th century, developmental mechanisms controlling the formation of the jaw skeleton became the subject of studies in developmental biology and molecular genetics. Mammalian middle ear evolution can now be interpreted as a series of changes in the developmental program of the pharyngeal arches. In this review, we summarize the history of middle ear research, highlight some of the remaining problems, and suggest possible future directions. We propose that to understand mammalian middle ear evolution, it is essential to identify the critical developmental events underlying the particular mammalian anatomy and to describe the evolutionary sequence of changes in developmental and molecular terms. We also discuss the degree of consistency between the developmental explanation of the mammalian middle ear based on molecular biology and morphological changes in the fossil record.

Normative reflectance and transmittance measurements on healthy newborn and 1-month-old infants.

OBJECTIVE: Ear-canal-based wideband reflectance (WBR) measurements may provide objective measures to assess and monitor middle-ear status in young babies. This work presents WBR measurements of power reflectance and transmittance on populations of healthy newborn babies (3 to 5 days) and healthy 1-mo-old babies (28 to 34 days). Thus, this work determines how power reflectance and transmittance vary between newborn and 1-mo-old babies and characterizes the range of these measures in normal populations. DESIGN: Power reflectance and transmittance were calculated from pressure measurements made in the ear canals of seven newborn (12 ears) and eleven 1-mo-old (19 ears) babies. Permutation tests, t tests, and regression (random effects) models were used to test the effects of age (newborn versus 1 mo), gender, and ear side (right versus left). RESULTS: The power reflectance and transmittance did not differ significantly for the age comparison (newborn versus 1 mo), although the results suggest a possible difference between newborn and 1-mo-old ears near 2000 Hz. There were no differences between the male and female ears. There are small but significant differences between left and right ears in three frequency bands encompassing 500 to 4000 Hz, where the predicted power reflectance mean for the left ear differs from the right ear by 0.02 to -0.07 depending on the frequency band. CONCLUSIONS: At most frequencies, power reflectance and transmittance are indistinguishable for newborn and 1-mo-old healthy babies, with limited or no differences between the two age groups and the males and females. There were small differences in some frequency bands for left and right ears. The measurements made here are similar to other published results in some frequency ranges but differ in other frequency ranges; differences among other studies from neonatal intensive care unit babies, healthy newborn babies, and healthy 1-mo-old babies are discussed.

[Peculiarities of the structure of the labyrinthine wall of the tympanic cavity in young children].

The authors report results of topographic measurement of the labyrinthine wall of the tympanic cavity using preparations of temporal bones from young children. Serial saw-cuts of the bones were made in different planes to obtain material for comprehensive assessment. It is recommended to take into consideration the results of the study for the purpose of surgical intervention on the middle ear of young children.

Coevolution of the mammalian middle ear and neocortex.

Phylogenetic analysis with x-ray computed tomography of fossilized and recent crania implicates differential growth of the neocortex in the evolution and development of the mammalian middle ear. In premammalian tetrapods, the middle ear evolved as a chain of bones attached to the mandible and cranium, but in adult mammals the chain is detached from the mandible and lies behind it. The neocortex evolved concurrently with detachment of the chain. In mammalian development the auditory chain arises connected to the mandible but later detaches, recapitulating the phylogenetic transformation. In modern didelphid development, the auditory chain reaches mature size by the third week after birth and is then separated from the jaw and displaced caudally as the neocortex grows for another 9 weeks.

Effects of maturation on tympanometric wideband acoustic transfer functions in human infants.

Wideband acoustic transfer function (ATF) measurements of energy reflectance (ER) and admittance magnitude (|Y|) were obtained at varying static ear-canal pressures in 4-, 12-, and 27-week-old infants and young adults. Developmental changes in wideband ATF measurements varied as a function of frequency. For frequencies from 0.25 to 0.75 kHz there was as much as a 30% change in mean ER and mid |Y| with changes in static ear-canal pressure between 4 and 24 weeks of age. From 0.75 to 2 kHz, the effects of pressure produced a small number of significant differences in ER and mid |Y| with age, suggestive of a developmentally stable frequency range. Between 2 and 6 kHz, there were differential effects of pressure for the youngest infants; negative pressures caused increased ER and mid |Y| and positive pressures caused decreased ER and mid |Y|; the magnitude of this effect decreased with age. Findings from this study demonstrate developmental differences in wideband tympanometric ATF measurements in 4-, 12- and 24-week-old infants and provide additional insight on the effects of static ear-canal pressure in the young infant's ear. The maturational effects shown in the experimental data are discussed in light of known age-related anatomical changes in the developing outer and middle ear.