Development of the vertebrate inner ear

SOX2, JAGGED1, β-Catenin, and Vitamin D Receptor Expression Patterns during Early Development and Innervation of the Human Inner Ear

Sensorineural hearing loss can be caused by lesions to the inner ear during development. Understanding the events and signaling pathways that drive inner ear formation is crucial for determining the possible causes of congenital hearing loss. We have analyzed the innervation and expression of SOX2, JAGGED1, β-catenin (CTNNB1), and vitamin D receptor (VDR) in the inner ears of human conceptuses aged 5 to 10 weeks after fertilization (W) using immunohistochemistry. The prosensory domains of the human inner ear displayed SOX2 and JAGGED1 expression throughout the analyzed period, with SOX2 expression being more extensive in all the analyzed timepoints. Innervation of vestibular prosensory domains was present at 6 W and extensive at 10 W, while nerve fibers reached the base of the cochlear prosensory domain at 7-8 W. CTNNB1 and VDR expression was mostly membranous and present during all analyzed timepoints in the inner ear, being the strongest in the non-sensory epithelium. Their expression was stronger in the vestibular region compared to the cochlear duct. CTNNB1 and VDR expression displayed opposite expression trends during the analyzed period, but additional studies are needed to elucidate whether they interact during inner ear development.

The expression of p27 in the adult vestibular sensory organs and its possible roles

Vestibular hair cells (HCs) located in the inner ear are the receptors of vestibular sensory, which facilitates the human sense of balance. The detailed differentiation pattern and maturation process of the vestibular HCs are unclear now. p27, a cyclin/CDK inhibitor, plays a critical role in regulating the exit of cell cycle. We found that p27 was continuously expressed in the terminally differentiated and mature vestibular HCs using p27-P2A-iCreER/+; Rosa26-LSL-tdTomato/+ mice, suggesting p27 might have novel roles independent of its CDK inhibitory action. p27 is also reported to be associated with cell differentiation, cell migration and cell survival. We further explored the difference of p27 expression between two subtypes of vestibular HCs, and found that the proportion of p27-tdTomato positive type I vestibular HCs increased gradually along the subtype determination and maturation of vestibular HCs, suggesting that p27 might play a role in the HC subtype differentiation, maturation and function acquirement.

Mcrs1 is required for branchial arch and cranial cartilage development

Mcrs1 is a multifunctional protein that is critical for many cellular processes in a wide range of cell types. Previously, we showed that Mcrs1 binds to the Six1 transcription factor and reduces the ability of the Six1-Eya1 complex to upregulate transcription, and that Mcrs1 loss-of-function leads to the expansion of several neural plate genes, reduction of neural border and pre-placodal ectoderm (PPR) genes, and pleiotropic effects on various neural crest (NC) genes. Because the affected embryonic structures give rise to several of the cranial tissues affected in Branchio-otic/Branchio-oto-renal (BOR) syndrome, herein we tested whether these gene expression changes subsequently alter the development of the proximate precursors of BOR affected structures - the otic vesicles (OV) and branchial arches (BA). We found that Mcrs1 is required for the expression of several OV genes involved in inner ear formation, patterning and otic capsule cartilage formation. Mcrs1 knockdown also reduced the expression domains of many genes expressed in the larval BA, derived from either NC or PPR, except for emx2, which was expanded. Reduced Mcrs1 also diminished the length of the expression domain of tbx1 in BA1 and BA2 and interfered with cranial NC migration from the dorsal neural tube; this subsequently resulted in defects in the morphology of lower jaw cartilages derived from BA1 and BA2, including the infrarostral, Meckel's, and ceratohyal as well as the otic capsule. These results demonstrate that Mcrs1 plays an important role in processes that lead to the formation of craniofacial cartilages and its loss results in phenotypes consistent with reduced Six1 activity associated with BOR.

Sensing External and Self-Motion with Hair Cells: A Comparison of the Lateral Line and Vestibular Systems from a Developmental and Evolutionary Perspective

Detection of motion is a feature essential to any living animal. In vertebrates, mechanosensory hair cells organized into the lateral line and vestibular systems are used to detect external water or head/body motion, respectively. While the neuronal components to detect these physical attributes are similar between the two sensory systems, the organizational pattern of the receptors in the periphery and the distribution of hindbrain afferent and efferent projections are adapted to the specific functions of the respective system. Here we provide a concise review comparing the functional organization of the vestibular and lateral line systems from the development of the organs to the wiring from the periphery and the first processing stages. The goal of this review is to highlight the similarities and differences to demonstrate how evolution caused a common neuronal substrate to adapt to different functions, one for the detection of external water stimuli and the generation of sensory maps and the other for the detection of self-motion and the generation of motor commands for immediate behavioral reactions.

Semicircular canal-dependent developmental tuning of translational vestibulo-ocular reflexes in Xenopus laevis

Gaze stabilization during head/body movements is achieved to a large extent by vestibular-evoked compensatory eye movements. These reflexes derive from semicircular canal and otolith organs and depend on the transformation of the respective sensory signals into extraocular motor commands. To elicit directionally and dynamically appropriate compensatory eye movements, extraocular motoneurons require spatiotemporally specific inputs from semicircular canals and regions of the utricular epithelium with matching directional sensitivity. The ontogenetic establishment and maturation of the directional tuning of otolith inputs in extraocular motoneurons was studied in Xenopus laevis tadpoles. In young larvae at stage 46-48, superior oblique (SO) extraocular motoneurons receive omnidirectional utricular signals during horizontal translational motion, indicating an absence of spatial tuning. In contrast, in older larvae beyond stage 49 these motoneurons were activated by directionally more restricted otolith inputs with an increasingly enhanced spatial tuning until stage 53. This developmental process limited the origin of otolith signals to a utricular epithelial sector with a hair cell sensitivity that is coaligned with the pulling direction of the SO eye muscle. The maturation of the otolith response vector was abolished by enzymatic prevention of semicircular canal formation in postembryonic tadpoles at stage 44, suggesting that functionally intact semicircular canals are causally responsible for the observed directional tuning of utricular responses. A likely mechanism by which semicircular canals might influence the tuning of the otolith responses includes stabilization of coactivated and centrally converging sensory signals from semicircular canal and spatially aligned epithelial utricular regions during natural head/body motion.

Gene miles-apart is required for formation of otic vesicle and hair cells in zebrafish

Hearing loss is a serious burden to physical and mental health worldwide. Aberrant development and damage of hearing organs are recognized as the causes of hearing loss, the molecular mechanisms underlining these pathological processes remain elusive. Investigation of new molecular mechanisms involved in proliferation, differentiation, migration and maintenance of neuromast primordium and hair cells will contribute to better understanding of hearing loss pathology. This knowledge will enable the development of protective agents and mechanism study of drug ototoxicity. In this study, we demonstrate that the zebrafish gene miles-apart, a homolog of sphingosine-1-phosphate receptor 2 (s1pr2) in mammals, has an important role in the development of otic vesicle, neuromasts and survival of hair cells. Whole-mount in situ hybridization of embryos showed that miles-apart expression occurred mainly in the encephalic region and the somites at 24 h.p.f. (hour post fertilization), in the midbrain/hindbrain boundary, the brainstem and the pre-neuromast of lateral line at 48 h.p.f. in a strict spatiotemporal regulation. Both up- and downregulation of miles-apart led to abnormal otoliths and semicircular canals, excess or few hair cells and neuromasts, and their disarranged depositions in the lateral lines. Miles-apart (Mil) dysregulation also caused abnormal expression of hearing-associated genes, including hmx2, fgf3, fgf8a, foxi1, otop1, pax2.1 and tmieb during zebrafish organogenesis. Moreover, in larvae miles-apart gene knockdown significantly upregulated proapoptotic gene zBax2 and downregulated prosurvival gene zMcl1b; in contrast, the level of zBax2 was decreased and of zMcl1b enhanced by miles-apart overexpression. Collectively, Mil activity is linked to organization and number decision of hair cells within a neuromast, also to deposition of neuromasts and formation of otic vesicle during zebrafish organogenesis. At the larva stage, Mil as an upstream regulator of bcl-2 gene family has a role in protection of hair cells against apoptosis by promoting expression of prosurvival gene zMcl1b and suppressing proapoptotic gene zBax2.

The mutation in Chd7 causes misexpression of Bmp4 and developmental defects in telencephalic midline

Mutations in chromosome-helicase-DNA-binding protein 7 (CHD7) are identified as the main cause for CHARGE syndrome (coloboma, heart anomaly, choanal atresia, retardation, genital and ear anomalies). Most patients (55% to 85%) with CHARGE syndrome display developmental defects in the central nervous system (CNS), of which pathology and molecular mechanisms remain unclear. In this study, we report a novel mutant mouse strain carrying a nonsense mutation, COA1, in exon4 of Chd7 gene. Chd7(COA1/+) mice phenocopied human CHARGE syndrome and displayed developmental defects in the telencephalic midline, including dilated third and lateral ventricles, reduced cerebral cortex, and corpus callosum crossing failure. Programed cell death in the telencephalic midline zone of Chd7(COA1/+) embryos was impaired, consistent with the incomplete telencephalic medial invagination in Chd7(COA1/+) embryos. Interestingly, expression of Bmp4, a signal well known to induce forebrain midline cell fate and apoptosis, was down-regulated and also expanded in the forebrain of Chd7(COA1/+) embryos. Furthermore, in vitro studies suggested that CHD7 may directly regulate Bmp4 expression by binding with an enhancer element downstream of the Bmp4 locus. These studies provide novel insight into pathogenesis of CNS anomalies in CHARGE syndrome.

N-myc controls proliferation, morphogenesis, and patterning of the inner ear

Myc family members play crucial roles in regulating cell proliferation, size, and differentiation during organogenesis. Both N-myc and c-myc are expressed throughout inner ear development. To address their function in the mouse inner ear, we generated mice with conditional deletions in either N-myc or c-myc. Loss of c-myc in the inner ear causes no apparent defects, whereas inactivation of N-myc results in reduced growth caused by a lack of proliferation. Reciprocally, the misexpression of N-myc in the inner ear increases proliferation. Morphogenesis of the inner ear in N-myc mouse mutants is severely disturbed, including loss of the lateral canal, fusion of the cochlea with the sacculus and utriculus, and stunted outgrowth of the cochlea. Mutant cochleas are characterized by an increased number of cells exiting the cell cycle that express the cyclin-dependent kinase inhibitor p27(Kip1) and lack cyclin D1, both of which control the postmitotic state of hair cells. Analysis of different molecular markers in N-myc mutant ears reveals the development of a rudimentary organ of Corti containing hair cells and the underlying supporting cells. Differentiated cells, however, fail to form the highly ordered structure characteristic for the organ of Corti but appear as rows or clusters with an excess number of hair cells. The Kölliker's organ, a transient structure neighboring the organ of Corti and a potential source of ectopic hair cells, is absent in the mutant ears. Collectively, our data suggest that N-myc regulates growth, morphogenesis, and pattern formation during the development of the inner ear.

A symphony of inner ear developmental control genes

The inner ear is one of the most complex and detailed organs in the vertebrate body and provides us with the priceless ability to hear and perceive linear and angular acceleration (hence maintain balance). The development and morphogenesis of the inner ear from an ectodermal thickening into distinct auditory and vestibular components depends upon precise temporally and spatially coordinated gene expression patterns and well orchestrated signaling cascades within the otic vesicle and upon cellular movements and interactions with surrounding tissues. Gene loss of function analysis in mice has identified homeobox genes along with other transcription and secreted factors as crucial regulators of inner ear morphogenesis and development. While otic induction seems dependent upon fibroblast growth factors, morphogenesis of the otic vesicle into the distinct vestibular and auditory components appears to be clearly dependent upon the activities of a number of homeobox transcription factors. The Pax2 paired-homeobox gene is crucial for the specification of the ventral otic vesicle derived auditory structures and the Dlx5 and Dlx6 homeobox genes play a major role in specification of the dorsally derived vestibular structures. Some Micro RNAs have also been recently identified which play a crucial role in the inner ear formation.

Ontogenetic rules and constraints of vestibulo-ocular reflex development

Vestibulo-ocular reflexes (VOR) assist retinal image stabilization during vertebrate locomotion thereby ensuring accurate visual perception. The importance of this motor behavior for animal survival requires that the underlying circuitry and all individual components are fully developed and functional as soon as post-embryonic animals initiate self-motion. Recent progress on the genetic, molecular, and activity-dependent regulation of placode development, vestibular sensory organ formation, circuit assembly, and acquisition of neuronal properties revealed rules and restrictions that give insight into how hindbrain VOR neuronal networks are assembled and become functional during ontogeny. Major crucial steps that correlate with early/delayed functional VOR onsets concern the maturation of cellular properties (precocial/altricial species) and the acquisition of minimal semicircular canal dimensions (small-sized vertebrates).

Ablation studies on the developing inner ear reveal a propensity for mirror duplications

The inner ear develops from a simple ectodermal thickening known as the otic placode. Classic embryological manipulations rotating the prospective placode tissue found that the anteroposterior axis was determined before the dorsoventral axis. A small percentage of such rotations also resulted in the formation of mirror duplicated ears, or enantiomorphs. We demonstrate a different embryological manipulation in the frog Xenopus: the physical removal or ablation of either the anterior or posterior half of the placode, which results in an even higher percentage of mirror image ears. Removal of the posterior half results in mirror anterior duplications, whereas removal of the anterior half results in mirror posterior duplications. In contrast, complete extirpation results in more variable phenotypes but never mirror duplications. By the time the otocyst separates from the surface ectoderm, complete extirpation results in no regeneration. To test for a dosage response, differing amounts of the placode or otocyst were ablated. Removal of one third of the placode resulted in normal ears, whereas two-thirds ablations resulted in abnormal ears, including mirror duplications. Recent studies in zebrafish have demonstrated a role for the hedgehog (Hh) signaling pathway in anteroposterior patterning of the developing ear. We have used overexpression of Hedgehog interacting protein (Hip) to block Hh signaling and find that this strategy resulted in mirror duplications of anterior structures, consistent with the results in zebrafish.

Rebuilding lost hearing using cell transplantation

OBJECTIVE

The peripheral auditory nervous system (cochlea and auditory nerve) has a complex anatomy, and it has traditionally been thought that once the sensorineural structures are damaged, restoration of hearing is impossible. In the past decade, however, the potential to restore lost hearing has been intensively investigated using molecular and cell biological techniques, and we can now part with such a pessimistic view. In this review, we examine an important field in hearing restoration research: cell transplantation.

METHODS

Most efforts in this field have been directed to the replacement of hair cells by transplantation to the cochlea. Here, we focus on transplantation to the auditory nerve, from the side of the cerebellopontine angle rather than the cochlea.

RESULTS

Delivery of cells to the cochlea is potentially damaging, and nerve cells transplanted distally to the Schwann-glial transitional zone (cochlear side) may become inhibited when they reach the transitional zone. The auditory nerve is probably the most suitable route for cell transplantation.

CONCLUSION

The auditory nerve occupies an important position not only in neurosurgery but also in various diseases in other disciplines, and several lines of recent evidence indicate that it is a key target for hearing restoration. It is familiar to most neurosurgeons, and the recent advances in the molecular and cell biology of inner-ear development are of direct importance to neurorestorative medicine. In this article, we review the anatomy, development, and molecular biology of the auditory nerve and cochlea, with emphasis on the advances in cell transplantation.

A novel locus for autosomal recessive nonsyndromic hearing impairment, DFNB63, maps to chromosome 11q13.2-q13.4

Hereditary hearing impairment is a genetically heterogeneous disorder. To date, 49 autosomal recessive nonsyndromic hearing impairment (ARNSHI) loci have been described, and there are more than 16 additional loci announced. In 25 of the known loci, causative genes have been identified. A genome scan and fine mapping revealed a novel locus for ARNSHI (DFNB63) on chromosome 11q13.2-q13.4 in a five-generation Turkish family (TR57). The homozygous linkage interval is flanked by the markers D11S1337 and D11S2371 and spans a 5.3-Mb interval. A maximum two-point log of odds score of 6.27 at a recombination fraction of theta = 0.0 was calculated for the marker D11S4139. DFNB63 represents the eighth ARNSHI locus mapped to chromosome 11, and about 3.33 Mb separate the DFNB63 region from MYO7A (DFNB2/DFNB11). Sequencing of coding regions and exon-intron boundaries of 13 candidate genes, namely SHANK2, CTTN, TPCN2, FGF3, FGF4, FGF19, FCHSD2, PHR1, TMEM16A, RAB6A, MYEOV, P2RY2 and KIAA0280, in genomic DNA from an affected individual of family TR57 revealed no disease-causing mutations.

Dlx5 and Dlx6 homeobox genes are required for specification of the mammalian vestibular apparatus

The mammalian inner ear is a complex organ that develops from a surface ectoderm into distinct auditory and vestibular components. Congenital malformation of these two components resulting from single or multiple gene defects is a common clinical occurrence and is observed in patients with split hand/split foot malformation, a malformation which is phenocopied by Dlx5/6 null mice. Analysis of mice lacking Dlx5 and Dlx6 homeobox genes identified their restricted and combined expression in the otic epithelium as a crucial regulator of vestibular cell fates. Otic induction initiates without incident in Dlx5/6(-/-) embryos, but dorsal otic derivatives including the semicircular ducts, utricle, saccule, and endolymphatic duct fail to form. Dlx5 and Dlx6 seem to influence vestibular cell fates by restricting Pax2 and activating Gbx2 and Bmp4 expression domains. Given their proximity to the disease locus and the observed phenotype in Dlx5/6 null mice, Dlx5/6 are likely candidates to mediate the inner ear defects observed in patients with split hand/split foot malformation.

OC29 is preferentially expressed in the presumptive sensory organ region of the otocyst

The mammalian inner ear derives from the otocyst. Molecular mechanisms underlying inner ear development are largely unknown. We have isolated a secreted molecule, OC29, from a rat otocyst cDNA library by the signal sequence trap method. OC29 was revealed to be a rat homologue of human WFIKKN. OC29 is preferentially expressed in the developing inner ear and the dorsal neural tube. In the inner ear, the expression of OC29 is first detectable at embryonic day 11.5 (E11.5), broadly in the dorsolateral region of the otocyst, which gives rise to the vestibular organ. At E12.5, the expression of OC29 becomes restricted to the presumptive sensory region, mainly to the BMP4-positive presumptive cristae, and expression becomes reduced at later stages. These results suggest that OC29 may have a role in the early development of the inner ear sensory organ, particularly in the formation of the cristae of the semicircular canals.

Development of the inner ear in Splotch mutant mice

The Splotch mouse, a Pax 3 mutation, represents a model of Waardenburg syndrome I. We show that the homozygous Splotch mutation (Sp(2H)) is associated with severe defects that prevent the formation of the cochlea and vestibulo-cochlear ganglion. To clarify the role of Pax 3 in inner ear formation, we examined the expression of polysialic acid (PSA) associated with neural cell adhesion molecule (NCAM). In accordance with the occurrence of phenotypic abnormalities, PSA NCAM was expressed early in otocyst development in the otic epithelium and the vestibulo-cochlear anlage. During the period of vestibular and cochlear ganglia formation, PSA NCAM expression was decreased. In the late phase of embryonic development, the expression of calcium binding proteins (S100) in the vestibulo-cochlear ganglion was also decreased. Minor differences in S100 immunostaining were found postnatally between the cochleas of heterozygous and wild type animals.

Identification ofcis-element regulating expression of the mouseFgf10 gene during inner ear development

Fibroblast growth factor (FGF) signaling is crucial for the induction and growth of the ear, a sensory organ that involves intimate tissue interactions. Here, we report the abnormality of Fgf10 null ear and the identification of a cis-regulatory element directing otic expression of Fgf10. In Fgf10 null inner ears, we found that the initial development of semicircular, vestibular, and cochlear divisions is roughly normal, after which there are abnormalities of semicircular canal/cristae and vestibular development. The mutant semicircular disks remain without canal formation by the perinatal stage. To elucidate regulation of the Fgf10 expression during inner ear development, we isolated a 6.6-kb fragment of its 5'-upstream region and examined its transcriptional activity with transgenic mice, using a lacZ-reporter system. From comparison of the mouse sequences of the 6.6-kb fragment with corresponding sequences of the human and chicken Fgf10, we identified a 0.4-kb enhancer sequence that drives Fgf10 expression in the developing inner ear. The enhancer sequences have motifs for many homeodomain-containing proteins (e.g., Prx, Hox, Nkx), in addition to POU-domain factors (e.g., Brn3), zinc-finger transcription factors (e.g., GATA-binding factors), TCF/LEF-1, and a SMAD-interacting protein. Thus, FGF10 signaling is dispensable for specification of otic compartment identity but is required for hollowing the semicircular disk. Furthermore, the analysis of a putative inner ear enhancer of Fgf10 has disclosed a complicated regulation of Fgf10 during inner ear development by numerous transcription factors and signaling pathways.

Correlation of Pax-2 expression with cell proliferation in the developing chicken inner ear

In vertebrates, the paired-box transcription factor Pax-2 is one of the earliest markers of the developing inner ear and is robustly expressed in the otic placode and the otic vesicle. Mutations in the Pax-2 gene result in developmental defects of the vestibular and auditory apparatus. We set out to investigate whether regions of Pax-2 expression in the developing otic vesicle correlate with areas of cell proliferation or cell death, which would indicate a possible role of Pax-2 in these processes. Regionalized proliferation and local apoptosis are the principal mechanisms that lead to the complex morphogenesis of the highly compartmentalized inner ear starting from a simple vesicle. We found a high correlation of Pax-2 expression with proliferating cells in the walls of the early otic vesicle. Apoptotic cells were mostly localized outside of the Pax-2-expressing regions. At later stages, we found the highest intensity of proliferating and Pax-2-positive cells in areas of the developing sensory epithelia. When hair cells begin to differentiate, they maintain a lower level of Pax-2 expression than neighboring cells for a brief period, before they completely down-regulate expression of this transcription factor. We conclude that a significant proportion of proliferating cells in the developing otocyst express Pax-2, in particular in regions that include developing sensory patches. This implicates Pax-2 as a marker for proliferating hair and supporting cell progenitors. Furthermore, the likelihood that Pax-2-expressing cells in the otocyst die by apoptosis is much lower when compared with cells residing in Pax-2-negative regions.

Prenatal growth and development of the modern human labyrinth

The modern human bony labyrinth is morphologically distinct from that of all other primates, showing derived features linked with vestibular function and the overall shape of the cranial base. However, little is known of how this unique morphology emerges prenatally. This study examines in detail the developing fetal human labyrinth, both to document this basic aspect of cranial biology, and more specifically, to gain insight into the ontogenetic basis of its phylogenetically derived morphology. Forty-one post-mortem human fetuses, ranging from 9 to 29 weeks gestation, were investigated with high-resolution magnetic resonance imaging. Quantitative analyses of the labyrinthine morphology revealed a number of interesting age-related trends. In addition, our findings show that: (1) the prenatal labyrinth attains an adult equivalent size between 17 and 19 weeks gestation; (2) within the period investigated, shape changes to all or most of the labyrinth cease after the 17-19-week size maturation point or after the otic capsule ossifies; (3) fetal cochlea development correlates with the surrounding petrosal morphology, but not with the midline basicranium; (4) gestational age-related rotations of the ampullae and cochlea relative to the lateral canal, and posterior canal torsion are similar to documented phylogenetic trends whereas other trends remain distinct. Findings are discussed in terms of the ontogenetic processes and mechanisms that most likely led, in part, to the emergence of the phylogenetically derived adult modern human labyrinth.

Requirements for FGF3 and FGF10 during inner ear formation

Members of the fibroblast growth factor (FGF) gene family control formation of the body plan and organogenesis in vertebrates. FGF3 is expressed in the developing hindbrain and has been shown to be involved in inner ear development of different vertebrate species, including zebrafish, Xenopus, chick and mouse. In the mouse, insertion of a neomycin resistance gene into the Fgf3 gene via homologous recombination results in severe developmental defects during differentiation of the otic vesicle. We have addressed the precise roles of FGF3 and other FGF family members during formation of the murine inner ear using both loss- and gain-of-function experiments. We generated a new mutant allele lacking the entire FGF3-coding region but surprisingly found no evidence for severe defects either during inner ear development or in the mature sensory organ, suggesting the functional involvement of other FGF family members during its formation. Ectopic expression of FGF10 in the developing hindbrain of transgenic mice leads to the formation of ectopic vesicles, expressing some otic marker genes and thus indicating a role for FGF10 during otic vesicle formation. Expression analysis of FGF10 during mouse embryogenesis reveals a highly dynamic pattern of expression in the developing hindbrain, partially overlapping with FGF3 expression and coinciding with formation of the inner ear. However, FGF10 mutant mice have been reported to display only mild defects during inner ear differentiation. We thus created double mutant mice for FGF3 and FGF10, which form severely reduced otic vesicles, suggesting redundant roles of these FGFs, acting in combination as neural signals for otic vesicle formation.

Fgf3 and Fgf10 are required for mouse otic placode induction

The inner ear, which contains the sensory organs specialised for audition and balance, develops from an ectodermal placode adjacent to the developing hindbrain. Tissue grafting and recombination experiments suggest that placodal development is directed by signals arising from the underlying mesoderm and adjacent neurectoderm. In mice, Fgf3 is expressed in the neurectoderm prior to and concomitant with placode induction and otic vesicle formation, but its absence affects only the later stages of otic vesicle morphogenesis. We show here that mouse Fgf10 is expressed in the mesenchyme underlying the prospective otic placode. Embryos lacking both Fgf3 and Fgf10 fail to form otic vesicles and have aberrant patterns of otic marker gene expression, suggesting that FGF signals are required for otic placode induction and that these signals emanate from both the hindbrain and mesenchyme. These signals are likely to act directly on the ectoderm, as double mutant embryos showed normal patterns of gene expression in the hindbrain. Cell proliferation and survival were not markedly affected in double mutant embryos, suggesting that the major role of FGF signals in otic induction is to establish normal patterns of gene expression in the prospective placode. Finally, examination of embryos carrying three out of the four mutant Fgf alleles revealed intermediate phenotypes, suggesting a quantitative requirement for FGF signalling in otic vesicle formation.

Establishment and characterization of rat progenitor hair cell lines

Cochlear progenitor hair cell lines are useful for studies of cellular specification, gene expression features, and signal transduction involved in the development of hair cells. To obtain embryonic and postnatal cochlear progenitor hair cell lines, we immortalized primary cultures of sensorineural epithelial cells from otocysts on embryonic day 12 (E12) and explants of the organ of Corti tissues on postnatal day 5 (P5). Primary cultures and explants were then transduced by the E6/E7 genes of human papilloma virus type 16. Transduced cells were passed for >50 passages and partial clonal cells were isolated from the above P5 organ of Corti explants by limiting dilution. The expression of neuronal, neural, epithelial, hair cell markers, and important transcription factors were then examined in these cell clones. Clones that express the above markers were considered as being progenitor hair cells. At least two representative cell lines, one from a mixed culture of otocyst epithelial cells and the other from the organ of Corti cells, ultimately expressed hair cell markers and neuronal/neural cell markers. The former only expressed the early hair cell marker oncomodulin and myosin VIIa, whereas the latter expressed oncomodulin, calretinin, myosin VIIa and Brn 3.1. These cell lines may represent progenitor hair cells at the different stages of cochlear development.

Cell adhesion molecules during inner ear and hair cell development, including notch and its ligands

Cellular adhesion plays a key role in a number of unique developmental events, including proliferation, cell fate, morphogenesis, neurite outgrowth, fasciculation, and synaptogensis. The number of families of molecules that can mediate cell adhesion and the number of members of each of those families has continued to increase over time. Moreover, the potential for the formation of different pairs of heterodimers with different binding specificities, and for both homo- and hetero-dimeric interactions suggest that a vast number of specific signaling events can be mediated through the expression of different combinations of adhesion factors at different developmental time points. By comparison with the number of known adhesion molecules and their potential effects, our understanding of the role of adhesion in ear development is extremely limited. The patterns of expression for some adhesion molecules have been determined for some aspects of inner ear development. Similarly, with a few exceptions, functional data to indicate the roles of these adhesion molecules are also lacking. However, a consideration of even the limited existing data must lead to the conclusion that adhesion molecules play key roles in all aspects of the development of the auditory system. Unique expression domains for different groups of adhesion molecules within the developing otocyst and ear strongly suggest a role in the determination of different cellular domains. Similarly, the specific expression of adhesion molecules on developing neurites and their target hair cells, suggests a key role for adhesion in the establishment of neuronal connections and possible the development of tonotopy. Finally, the recent demonstration that Cdh23 and Pcdh15 play specific roles in the formation of the hair cell stereociliary bundle provides compelling evidence for the importance of adhesion molecules in the development of stereocilia. With the imminent completion of the mouse genome, it seems likely that the number of adhesion molecules can soon be fixed and that it will then be possible to generate a more comprehensive map of expression of these molecules within the developing inner ear. At the same time, the generation of new transgenic and molecular technologies promises to provide researchers with new tools to examine the specific effects of different adhesion molecules during inner ear development.

A review of inner ear fate maps and cell lineage studies

A renewed interest in the development of the inner ear has provided more data on the fate and cell lineage relationships of the tissues making up this complex structure. The inner ear develops from a simple ectodermal thickening of the head called the otic placode, which undergoes a great deal of growth and differentiation to form a multichambered nonsensory epithelium that houses the six to nine sensory organs of the inner ear. Despite a large number of studies examining otic development, there have been surprisingly few fate maps generated. The published fate maps encompass four species and range from preotic to otocyst stages. Although some of these studies were consistent with a compartment and boundary model, other studies reveal extensive cell mixing during development. Cell lineage studies have been done in fewer species. At the single cell level the resulting clones in both chicks and frogs appear somewhat restricted in terms of distribution. We conclude that up until late placode stages there are no clear lineage restriction boundaries, meaning that cells seem to mix extensively at these early stages. At late placode stages, when the otic cup has formed, there are at least two boundaries located dorsally in the forming otocyst but none ventrally. These conclusions are consistent with all the fate maps and reconciles the chick and frog data. These results suggest that genes involved in patterning the inner ear may have dynamic and complex expression patterns.