Math1: an essential gene for the generation of inner ear hair cells

The mammalian inner ear contains the cochlea and vestibular organs, which are responsible for hearing and balance, respectively. The epithelia of these sensory organs contain hair cells that function as mechanoreceptors to transduce sound and head motion. The molecular mechanisms underlying hair cell development and differentiation are poorly understood. Math1, a mouse homolog of the Drosophila proneural gene atonal, is expressed in inner ear sensory epithelia. Embryonic Math1-null mice failed to generate cochlear and vestibular hair cells. This gene is thus required for the genesis of hair cells.

Essential role of BETA2/NeuroD1 in development of the vestibular and auditory systems

BETA2/NeuroD1 is a bHLH transcription factor that is expressed during development in the mammalian pancreas and in many locations in the central and peripheral nervous systems. During inner ear ontogenesis, it is present in both sensory ganglion neurons and sensory epithelia. Although studies have shown that BETA2/NeuroD1 is important in the development of the hippocampal dentate gyrus and the cerebellum, its functions in the peripheral nervous system and in particular in the inner ear are unclear. Mice carrying a BETA2/NeuroD1 null mutation exhibit behavioral abnormalities suggestive of an inner ear defect, including lack of responsiveness to sound, hyperactivity, head tilting, and circling. Here we show that these defects can be explained by a severe reduction of sensory neurons in the cochlear-vestibular ganglion (CVG). A developmental study of CVG formation in the null demonstrates that BETA2/NeuroD1 does not play a primary role in the proliferation of neuroblast precursors or in their decision to become neuroblasts. Instead, the reduction in CVG neuron number is caused by a combination both of delayed or defective delamination of CVG neuroblast precursors from the otic vesicle epithelium and of enhanced apoptosis both in the otic epithelium and among those neurons that do delaminate to form the CVG. There are also defects in differentiation and patterning of the cochlear duct and sensory epithelium and loss of the dorsal cochlear nucleus. BETA2/NeuroD1 is, thus, the first gene to be shown to regulate neuronal and sensory cell development in both the cochlear and vestibular systems.

Math1 Expression Redefines the Rhombic Lip Derivatives and Reveals Novel Lineages within the Brainstem and Cerebellum

The rhombic lip (RL) is an embryonic proliferative neuroepithelium that generates several groups of hindbrain neurons. However, the precise boundaries and derivatives of the RL have never been genetically identified. We use beta-galactosidase expressed from the Math1 locus in Math1-heterozygous and Math1-null mice to track RL-derived cells and to evaluate their developmental requirements for Math1. We uncover a Math1-dependent rostral rhombic-lip migratory stream (RLS) that generates some neurons of the parabrachial, lateral lemniscal, and deep cerebellar nuclei, in addition to cerebellar granule neurons. A more caudal Math1-dependent cochlear extramural stream (CES) generates the ventral cochlear nucleus and cochlear granule neurons. Similarly, mossy-fiber precerebellar nuclei require Math1, whereas the inferior olive and locus coeruleus do not. We propose that Math1 expression delimits the extent of the rhombic lip and is required for the generation of the hindbrain superficial migratory streams, all of which contribute neurons to the proprioceptive/vestibular/auditory sensory network.

PTK7/CCK-4 is a novel regulator of planar cell polarity in vertebrates

In addition to the apical-basal polarity pathway operating in epithelial cells, a planar cell polarity (PCP) pathway establishes polarity within the plane of epithelial tissues and is conserved from Drosophila to mammals. In Drosophila, a 'core' group of PCP genes including frizzled (fz), flamingo/starry night, dishevelled (dsh), Van Gogh/strabismus and prickle, function to regulate wing hair, bristle and ommatidial polarity. In vertebrates, the PCP pathway regulates convergent extension movements and neural tube closure, as well as the orientation of stereociliary bundles of sensory hair cells in the inner ear. Here we show that a mutation in the mouse protein tyrosine kinase 7 (PTK7) gene, which encodes an evolutionarily conserved transmembrane protein with tyrosine kinase homology, disrupts neural tube closure and stereociliary bundle orientation, and shows genetic interactions with a mutation in the mouse Van Gogh homologue vangl2. We also show that PTK7 is dynamically localized during hair cell polarization, and that the Xenopus homologue of PTK7 is required for neural convergent extension and neural tube closure. These results identify PTK7 as a novel regulator of PCP in vertebrates.

Notch signalling pathway mediates hair cell development in mammalian cochlea

The mammalian cochlea contains an invariant mosaic of sensory hair cells and non-sensory supporting cells reminiscent of invertebrate structures such as the compound eye in Drosophila melanogaster. The sensory epithelium in the mammalian cochlea (the organ of Corti) contains four rows of mechanosensory hair cells: a single row of inner hair cells and three rows of outer hair cells. Each hair cell is separated from the next by an interceding supporting cell, forming an invariant and alternating mosaic that extends the length of the cochlear duct. Previous results suggest that determination of cell fates in the cochlear mosaic occurs via inhibitory interactions between adjacent progenitor cells (lateral inhibition). Cells populating the cochlear epithelium appear to constitute a developmental equivalence group in which developing hair cells suppress differentiation in their immediate neighbours through lateral inhibition. These interactions may be mediated through the Notch signalling pathway, a molecular mechanism that is involved in the determination of a variety of cell fates. Here we show that genes encoding the receptor protein Notch1 and its ligand, Jagged 2, are expressed in alternating cell types in the developing sensory epithelium. In addition, genetic deletion of Jag2 results in a significant increase in sensory hair cells, presumably as a result of a decrease in Notch activation. These results provide direct evidence for Notch-mediated lateral inhibition in a mammalian system and support a role for Notch in the development of the cochlear mosaic.

Math1 regulates development of the sensory epithelium in the mammalian cochlea

The transcription factor Math1 (encoded by the gene Atoh1, also called Math1) is required for the formation of mechanosensory hair cells in the inner ear; however, its specific molecular role is unknown. Here we show that absence of Math1 in mice results in a complete disruption of formation of the sensory epithelium of the cochlea, including the development of both hair cells and associated supporting cells. In addition, ectopic expression of Math1 in nonsensory regions of the cochlea is sufficient to induce the formation of sensory clusters that contain both hair cells and supporting cells. The formation of these clusters is dependent on inhibitory interactions mediated, most probably, through the Notch pathway, and on inductive interactions that recruit cells to develop as supporting cells through a pathway independent of Math1. These results show that Math1 functions in the developing cochlea to initiate both inductive and inhibitory signals that regulate the overall formation of the sensory epithelia.

Association of unconventional myosin MYO15 mutations with human nonsyndromic deafness DFNB3

DFNB3, a locus for nonsyndromic sensorineural recessive deafness, maps to a 3-centimorgan interval on human chromosome 17p11.2, a region that shows conserved synteny with mouse shaker-2. A human unconventional myosin gene, MYO15, was identified by combining functional and positional cloning approaches in searching for shaker-2 and DFNB3. MYO15 has at least 50 exons spanning 36 kilobases. Sequence analyses of these exons in affected individuals from three unrelated DFNB3 families revealed two missense mutations and one nonsense mutation that cosegregated with congenital recessive deafness.

TRP-2/DT, a new early melanoblast marker, shows that steel growth factor (c-kit ligand) is a survival factor

We have used a probe derived from TRP-2/DT to detect migratory melanoblasts shortly after they emerge from the neural crest, as early as 10 days post coitum (dpc). TRP-2/DT expression is otherwise restricted to the presumptive pigmented retinal epithelium, the developing telencephalon and the endolymphatic duct. The pattern of steel and c-kit hybridisation in the developing brain differed from that of TRP-2. TRP-1 and tyrosinase probes also detected melanoblasts but were both expressed later in development than TRP-2. We used the TRP-2/DT probe to investigate the way that the Steel-dickie (Sld) mutation interferes with melanocyte development, and found that the membrane-bound steel growth factor which is missing in Sld/Sld mutants is necessary for the survival of melanoblasts but not for their early migration and initial differentiation.

Murine dishevelled 3 functions in redundant pathways with dishevelled 1 and 2 in normal cardiac outflow tract, cochlea, and neural tube development

Dishevelled (Dvl) proteins are important signaling components of both the canonical β-catenin/Wnt pathway, which controls cell proliferation and patterning, and the planar cell polarity (PCP) pathway, which coordinates cell polarity within a sheet of cells and also directs convergent extension cell (CE) movements that produce narrowing and elongation of the tissue. Three mammalian Dvl genes have been identified and the developmental roles of Dvl1 and Dvl2 were previously determined. Here, we identify the functions of Dvl3 in development and provide evidence of functional redundancy among the three murine Dvls. Dvl3^−/− mice died perinatally with cardiac outflow tract abnormalities, including double outlet right ventricle and persistent truncus arteriosis. These mutants also displayed a misorientated stereocilia in the organ of Corti, a phenotype that was enhanced with the additional loss of a single allele of the PCP component Vangl2/Ltap (LtapLp/+). Although neurulation appeared normal in both Dvl3^−/− and LtapLp/+ mutants, Dvl3^+/−;LtapLp/+ combined mutants displayed incomplete neural tube closure. Importantly, we show that many of the roles of Dvl3 are also shared by Dvl1 and Dvl2. More severe phenotypes were observed in Dvl3 mutants with the deficiency of another Dvl, and increasing Dvl dosage genetically with Dvl transgenes demonstrated the ability of Dvls to compensate for each other to enable normal development. Interestingly, global canonical Wnt signaling appeared largely unaffected in the double Dvl mutants, suggesting that low Dvl levels are sufficient for functional canonical Wnt signals. In summary, we demonstrate that Dvl3 is required for cardiac outflow tract development and describe its importance in the PCP pathway during neurulation and cochlea development. Finally, we establish several developmental processes in which the three Dvls are functionally redundant.

Multi-gene families, comprising a set of very similar genes with shared nucleotide sequences, are common in mammals. Individual family members may be expressed in different places and perform separate functions. Alternatively, the genes may have redundant functions, but distinct dosage requirements. Mammals share three Dishevelled (Dvl) family members and while the roles of Dvl1 and Dvl2 have been described previously, the functions of Dvl3 have remained elusive. Here, we show that the lack of Dvl3 in mice affects the formation of the heart, neural tube, and inner ear. We further show that the defects in these tissues are much more severe when the mice are deficient in more than one Dvl family member, indicating redundant functions for these genes. Congenital heart disease affects approximately 75 in every 1,000 live human births, and approximately 30% of these diseases are due to disruptions in the outflow tract, the region affected in mice lacking Dvl genes. Neural tube defects, similar to those observed in the Dvl mutants, are also common in humans. The animal models described here provide useful tools to elucidate the genetic mechanisms that underlie these abnormalities and may provide novel ways of treating these disorders in the future.

Developmental changes in the expression of potassium currents of embryonic, neonatal and mature mouse inner hair cells

Developmental changes in electrophysiological membrane properties of mouse cochlear inner hair cells (IHCs) were studied from just after terminal differentiation up to functional maturity. As early as embryonic day 14.5 (E14.5) newly differentiated IHCs express a very small outward K+ current that is largely insensitive to 4-aminopyridine (4-AP). One day later the inward rectifier, IK1, is first observed. These immature cells initially exhibit only slow graded voltage responses under current clamp. From E17.5 spontaneous action potentials occur. During the first week of postnatal development, the outward K+ current steadily increases in size and a progressively larger fraction of the current is sensitive to 4-AP. During the second postnatal week, the activation of the 4-AP-sensitive current, by now contributing about half of the outward K+ current, shifts in the hyperpolarizing direction. Together with an increase in size of IK1, this hyperpolarizes the cell, thus inhibiting the spontaneous spike activity, although spikes could still be evoked upon depolarizing current injection. Starting at about the onset of hearing (postnatal day 12, P12) immature IHCs make the final steps towards fully functional sensory receptors with fast graded voltage responses. This is achieved mainly by the expression of the large-conductance Ca2+-activated K+ current IK,f, but also of a current indistinguishable from the negatively activating IK,n previously described in mature outer hair cells (OHCs). The 4-AP-sensitive current continues to increase after the onset of hearing to form the major part of the mature delayed rectifier, IK,s. By P20 IHCs appear mature in terms of their complement of K+ conductances.

Functional auditory hair cells produced in the mammalian cochlea by in utero gene transfer

Sensory hair cells in the mammalian cochlea convert mechanical stimuli into electrical impulses that subserve audition. Loss of hair cells and their innervating neurons is the most frequent cause of hearing impairment. Atonal homologue 1 (encoded by Atoh1, also known as Math1) is a basic helix-loop-helix transcription factor required for hair-cell development, and its misexpression in vitro and in vivo generates hair-cell-like cells. Atoh1-based gene therapy to ameliorate auditory and vestibular dysfunction has been proposed. However, the biophysical properties of putative hair cells induced by Atoh1 misexpression have not been characterized. Here we show that in utero gene transfer of Atoh1 produces functional supernumerary hair cells in the mouse cochlea. The induced hair cells display stereociliary bundles, attract neuronal processes and express the ribbon synapse marker carboxy-terminal binding protein 2 (refs 12,13). Moreover, the hair cells are capable of mechanoelectrical transduction and show basolateral conductances with age-appropriate specializations. Our results demonstrate that manipulation of cell fate by transcription factor misexpression produces functional sensory cells in the postnatal mammalian cochlea. We expect that our in utero gene transfer paradigm will enable the design and validation of gene therapies to ameliorate hearing loss in mouse models of human deafness.

The zinc finger transcription factor Gfi1, implicated in lymphomagenesis, is required for inner ear hair cell differentiation and survival

Gfi1 was first identified as causing interleukin 2-independent growth in T cells and lymphomagenesis in mice. Much work has shown that Gfi1 and Gfi1b, a second mouse homolog, play pivotal roles in blood cell lineage differentiation. However, neither Gfi1 nor Gfi1b has been implicated in nervous system development, even though their invertebrate homologues, senseless in Drosophila and pag-3 in C. elegans are expressed and required in the nervous system. We show that Gfi1 mRNA is expressed in many areas that give rise to neuronal cells during embryonic development in mouse, and that Gfi1 protein has a more restricted expression pattern. By E12.5 Gfi1 mRNA is expressed in both the CNS and PNS as well as in many sensory epithelia including the developing inner ear epithelia. At later developmental stages, Gfi1 expression in the ear is refined to the hair cells and neurons throughout the inner ear. Gfi1 protein is expressed in a more restricted pattern in specialized sensory cells of the PNS, including the eye, presumptive Merkel cells, the lung and hair cells of the inner ear. Gfi1 mutant mice display behavioral defects that are consistent with inner ear anomalies, as they are ataxic, circle, display head tilting behavior and do not respond to noise. They have a unique inner ear phenotype in that the vestibular and cochlear hair cells are differentially affected. Although Gfi1-deficient mice initially specify inner ear hair cells, these hair cells are disorganized in both the vestibule and cochlea. The outer hair cells of the cochlea are improperly innervated and express neuronal markers that are not normally expressed in these cells. Furthermore, Gfi1 mutant mice lose all cochlear hair cells just prior to and soon after birth through apoptosis. Finally, by five months of age there is also a dramatic reduction in the number of cochlear neurons. Hence, Gfi1 is expressed in the developing nervous system, is required for inner ear hair cell differentiation, and its loss causes programmed cell death.

Hes1 is a negative regulator of inner ear hair cell differentiation

Hair cell fate determination in the inner ear has been shown to be controlled by specific genes. Recent loss-of-function and gain-of-function experiments have demonstrated that Math1, a mouse homolog of the Drosophila gene atonal, is essential for the production of hair cells. To identify genes that may interact with Math1 and inhibit hair cell differentiation, we have focused on Hes1, a mammalian hairy and enhancer of split homolog, which is a negative regulator of neurogenesis. We report here that targeted deletion of Hes1 leads to formation of supernumerary hair cells in the cochlea and utricle of the inner ear. RT-PCR analysis shows that Hes1 is expressed in inner ear during hair cell differentiation and its expression is maintained in adulthood. In situ hybridization with late embryonic inner ear tissue reveals that Hes1 is expressed in supporting cells, but not hair cells, of the vestibular sensory epithelium. In the cochlea, Hes1 is selectively expressed in the greater epithelial ridge and lesser epithelial ridge regions which are adjacent to inner and outer hair cells. Co-transfection experiments in postnatal rat explant cultures show that overexpression of Hes1 prevents hair cell differentiation induced by Math1. Therefore Hes1 can negatively regulate hair cell differentiation by antagonizing Math1. These results suggest that a balance between Math1 and negative regulators such as Hes1 is crucial for the production of an appropriate number of inner ear hair cells.

Proliferation of Functional Hair Cells in Vivo in the Absence of the Retinoblastoma Protein

In mammals, hair cell loss causes irreversible hearing and balance impairment because hair cells are terminally differentiated and do not regenerate spontaneously. By profiling gene expression in developing mouse vestibular organs, we identified the retinoblastoma protein (pRb) as a candidate regulator of cell cycle exit in hair cells. Differentiated and functional mouse hair cells with a targeted deletion of Rb1 undergo mitosis, divide, and cycle, yet continue to become highly differentiated and functional. Moreover, acute loss of Rb1 in postnatal hair cells caused cell cycle reentry. Manipulation of the pRb pathway may ultimately lead to mammalian hair cell regeneration.

Expression and function of FGF10 in mammalian inner ear development

We have investigated the expression of FGF10 during ear development and the effect of an FGF10 null mutation on ear development. Our in situ hybridization data reveal expression of FGF10 in all three canal crista sensory epithelia and the cochlea anlage as well as all sensory neurons at embryonic day 11.5 (E11.5). Older embryos (E18.5) displayed strong graded expression in all sensory epithelia. FGF10 null mutants show complete agenesis of the posterior canal crista and the posterior canal. The posterior canal sensory neurons form initially and project rather normally by E11.5, but they disappear within 2 days. FGF10 null mutants have no posterior canal system at E18.5. In addition, these mutants have deformations of the anterior and horizontal cristae, reduced formation of the anterior and horizontal canals, as well as altered position of the remaining sensory epithelia with respect to the utricle. Hair cells form but some have defects in their cilia formation. No defects were detected in the organ of Corti at the cellular level. Together these data suggest that FGF10 plays a major role in ear morphogenesis. Most of these data are consistent with earlier findings on a null mutation in FGFR2b, one of FGF10's main receptors.

Specification of the mammalian cochlea is dependent on Sonic hedgehog

Organization of the inner ear into auditory and vestibular components is dependent on localized patterns of gene expression within the otic vesicle. Surrounding tissues are known to influence compartmentalization of the otic vesicle, yet the participating signals remain unclear. This study identifies Sonic hedgehog (Shh) secreted by the notochord and/or floor plate as a primary regulator of auditory cell fates within the mouse inner ear. Whereas otic induction proceeds normally in Shh(-/-) embryos, morphogenesis of the inner ear is greatly perturbed by midgestation. Ventral otic derivatives including the cochlear duct and cochleovestibular ganglia failed to develop in the absence of Shh. The origin of the inner ear defects in Shh(-/-) embryos could be traced back to alterations in the expression of a number of genes involved in cell fate specification including Pax2, Otx1, Otx2, Tbx1, and Ngn1. We further show that several of these genes are targets of Shh signaling given their ectopic activation in transgenic mice that misexpress Shh in the inner ear. Taken together, our data support a model whereby auditory cell fates in the otic vesicle are established by the direct action of Shh.

Development and properties of stereociliary link types in hair cells of the mouse cochlea

The hair bundles of outer hair cells in the mature mouse cochlea possess three distinct cell-surface specializations: tip links, horizontal top connectors, and tectorial membrane attachment crowns. Electron microscopy was used to study the appearance and maturation of these link types and examine additional structures transiently associated with the developing hair bundle. At embryonic day 17.5 (E17.5), the stereocilia are interconnected by fine lateral links and have punctate elements distributed over their surface. Oblique tip links are also seen at this stage. By postnatal day 2 (P2), outer hair cell bundles have a dense cell coat, but have lost many of the lateral links seen at E17.5. At P2, ankle links appear around the base of the bundle and tectorial membrane attachment crowns are seen at the stereociliary tips. Ankle links become less apparent by P9 and are completely lost by P12. The appearance of horizontal top connectors, which persist into adulthood, occurs concomitant with this loss of ankle links. Treatment with the calcium chelator BAPTA or the protease subtilisin enabled these links to be further distinguished. Ankle links are susceptible to both treatments, tip links are only sensitive to BAPTA, and tectorial membrane attachment crowns are removed by subtilisin but not BAPTA. The cell-coat material is partially sensitive to subtilisin alone, while horizontal top connectors resist both treatments. These results indicate there is a rich, rapidly changing array of different links covering the developing hair bundle that becomes progressively refined to generate the mature complement by P19.

Expression of Delta1 and Serrate1 (Jagged1) in the mouse inner ear

The Notch signalling pathway is thought to play a key part in controlling the production of sensory hair cells in the vertebrate inner ear via lateral inhibition; but there is disagreement as to which Notch ligands are expressed in hair cells as they develop. We show, using a mouse Delta1:LacZ knock-in as a reporter, that nascent hair cells, but not their neighbours, express Delta1. Expression of Serrate1 (Jagged1), meanwhile becomes restricted to the supporting cells of each sensory patch. Delta1 is also expressed: (a) at early stages, at the site of otic neurogenesis; and (b) in scattered cells of the endolymphatic sac, as is Serrate1.

Mutations in the transcriptional activator EYA4 cause late-onset deafness at the DFNA10 locus

We identified Eyes absent 4 (EYA4), a member of the vertebrate Eya family of transcriptional activators, as the causative gene of postlingual, progressive, autosomal dominant hearing loss at the DFNA10 locus. In two unrelated families from Belgium and the USA segregating for deafness at this locus, we found different mutations in EYA4, both of which create premature stop codons. Although EYA proteins interact with members of the SIX and DACH protein families in a conserved network that regulates early embryonic development, this finding shows that EYA4 is also important post-developmentally for continued function of the mature organ of Corti.

Isolation of novel and known genes from a human fetal cochlear cDNA library using subtractive hybridization and differential screening

We used a combination of subtractive hybridization and differential screening strategies to identify genes that may function normally in hearing and, when mutated, result in deafness. A human fetal cochlear (membranous labyrinth) cDNA library was subtracted against total human fetal brain RNAs by an avidin-biotin-based procedure to enrich for cochlear transcripts. Subtracted cochlear clones were differentially screened with 32P-labeled total cochlear and total brain cDNA probes. Sequence analysis of clones that hybridized more intensely with cochlear than with brain cDNA probes revealed some previously characterized genes, including mitochondrial sequences, collagen type I alpha-2 (COL1A2), collagen type II alpha-1 (COL2A1), collagen type III alpha-1 (COL3A1), spermidine/spermine N1-acetyltransferase (SAT), osteonectin (SPARC), and peripheral myelin protein 22 (PMP22). Also identified were clones that are potential novel cochlear genes. Northern blots of cochlear and brain RNAs probed with COL1A2, COL2A1, COL3A1, SAT, SPARC, PMP22, and a novel sequence, designated Coch-5B2, confirm results of the subtractive procedure by showing preferential cochlear expression. A number of these genes serve structural or regulatory functions in extracellular matrix or neural conduction; defects in some of these genes are associated with disorders involving hearing loss. Partial sequence analysis of Coch-5B2 reveals a von Willebrand factor type A-like domain in this cDNA. To assess the cochlear specificity of Coch-5B2, a Northern blot panel of 14 human fetal tissue RNAs was probed with Coch-5B2, showing differential expression of this novel gene in the cochlea.

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.

Expression of Prox1 during mouse cochlear development

We carried out an analysis of the expression of Prox1, a homeo-domain transcription factor, during mouse inner ear development with particular emphasis on the auditory system. Prox1 is expressed in the otocyst beginning at embryonic day (E)11, in the developing vestibular sensory patches. Expression is down regulated in maturing (myosin VIIA immunoreactive) vestibular hair cells and subsequently in the underlying support cell layer by E16.5. In the auditory sensory epithelium, Prox1 is initially expressed at embryonic day 14.5 in a narrow stripe of cells at the base of the cochlea. This stripe encompasses the full thickness of the sensory epithelium, including developing hair cells and support cells. Over the next several days the stripe of expression extends to the apex, and as the sensory epithelium differentiates Prox1 becomes restricted to a subset of support cells. Double labeling for Prox1 and cell-type-specific markers revealed that the outer hair cells transiently express Prox1. After E18, Prox1 protein is no longer detectable in hair cells, but it continues to be expressed in support cells for the rest of embryogenesis and into the second postnatal week. During this time, Prox1 is not expressed in all support cell types in the organ of Corti, but is restricted to developing Deiters' and pillar cells. The expression is maintained in these cells into the second week of postnatal life, at which time Prox1 is dynamically down regulated. These studies form a baseline from which we can analyze the role of Prox1 in vertebrate sensory development.

Development and evolution of inner ear sensory epithelia and their innervation

The development and evolution of the inner ear sensory patches and their innervation is reviewed. Recent molecular developmental data suggest that development of these sensory patches is a developmental recapitulation of the evolutionary history. These data suggest that the ear generates multiple, functionally diverse sensory epithelia by dividing a single sensory primordium. Those epithelia will establish distinct identities through the overlapping expression of genes of which only a few are currently known. One of these distinctions is the unique pattern of hair cell polarity. A hypothesis is presented on how the hair cell polarity may relate to the progressive segregation of the six sensory epithelia. Besides being markers for sensory epithelia development, neurotrophins are also expressed in delaminating cells that migrate toward the developing vestibular and cochlear ganglia. These delaminating cells originate from multiple sites at or near the developing sensory epithelia and some also express neuronal markers such as NeuroD. The differential origin of precursors raises the possibility that some sensory neurons acquire positional information before they delaminate the ear. Such an identity of these delaminating sensory neurons may be used both to navigate their dendrites to the area they delaminated from, as well as to help them navigate to their central target. The navigational properties of sensory neurons as well as the acquisition of discrete sensory patch phenotypes implies a much more sophisticated subdivision of the developing otocyst than the few available gene expression studies suggest.