The development of the vertebrate inner ear

Ventrally emigrating neural tube cells migrate into the developing vestibulocochlear nerve and otic vesicle

Virtually all cell types in the inner ear develop from the cells of the otic vesicle. The otic vesicle is formed by the invagination of non-neural ectodermal cells known as the otic placode. We investigated whether a recently described cell population, originating from the ventral part of the hindbrain neural tube known as the ventrally emigrating neural tube (VENT) cells, also contributes cells to the otic vesicle. The ventral hindbrain neural tube cells were labeled with the fluorescent vital dye DiI or replication-deficient retroviruses containing the LacZ gene in chick embryos on embryonic day 2, after the emigration of neural crest from this region. One day later, the labeled cells were detected only in the hindbrain neural tube. Shortly thereafter, the labeled cells began to appear in the eighth (vestibulocochlear) cranial nerve and otic vesicle. From embryonic day 3.5-5, the labeled cells were detected in the major derivatives of the otic vesicle, i.e. the endolymphatic duct, semicircular canals, utricle, saccule, cochlea, and vestibulocochlear ganglion. That the emigrated cells originated from the ventral part of the hindbrain neural tube was confirmed by focal application of DiI impregnated filter paper and with quail chimeras. It is concluded that, in addition to the otic placode cells, the otic vesicle also contains the ventrally emigrating neural tube cells, and that both cell populations contribute to the structures and cell types in the inner ear. It is well known that inductive signals from the hindbrain are required for the morphogenesis of the inner ear. The migration of the hindbrain neural tube cells into the otic vesicle raises the possibility that the inductive effect of the hindbrain might be mediated, at least in part, by the ventrally emigrating neural tube cells and that, therefore, a mechanism exists that involves cells rather than diffusible molecules only.

Fgf3 and Fgf8 dependent and independent transcription factors are required for otic placode specification

The vertebrate inner ear develops from the otic placode, an ectodermal thickening that forms adjacent to the presumptive hindbrain. Previous studies have suggested that competent ectodermal cells respond to signals from adjacent tissues to form the placode. Members of the Fgf family of growth factors and the Dlx family of transcription factors have been implicated in this signal-response pathway. We show that compromising Fgf3 and Fgf8 signaling blocks ear development; only a few scattered otic cells form. Removal of dlx3b, dlx4b and sox9a genes together also blocks ear development, although a few residual cells form an otic epithelium. These cells fail to form if sox9b function is also blocked. Combined loss of Fgf signaling and the three transcription factor genes, dlx3b, dlx4b and sox9a, also completely eliminates all indications of otic cells. Expression of sox9a but not dlx3b, dlx4b or sox9b requires Fgf3 and Fgf8. Our results provide evidence for Fgf3- and Fgf8-dependent and -independent genetic pathways for otic specification and support the notion that Fgf3 and Fgf8 function to induce both the otic placode and the epithelial organization of the otic vesicle.

Expression zones of three novel genes abut the developing anterior neural plate of Xenopus embryo

We identified three novel genes that were expressed within the anterior non-neural ectoderm of Xenopus early neurula embryos. The expression of these genes was observed in the different areas complementary to the expression zone of a homeodomain gene Xanf-1 in the anterior neural plate. One of these genes, a Ras-like GTP-ase Ras-dva, marked the anterior placodal ectoderm area; a second, an Agr family homologous gene, XAgr2, was expressed in the anterior-most ectoderm in the cement gland primordium, and a third, novel gene Nlo was expressed in the lateral neural folds. The genes were transiently expressed in the developing cement and hatching gland primordia, and repressed in the mature cement and hatching glands. XAgr2 and Nlo were also expressed in the otic vesicles, and Ras-dva was expressed in the dorso-lateral column of the neural tube.

Zebrafish foxi1 mediates otic placode formation and jaw development

The otic placode is a transient embryonic structure that gives rise to the inner ear. Although inductive signals for otic placode formation have been characterized, less is known about the molecules that respond to these signals within otic primordia. Here, we identify a mutation in zebrafish, hearsay, which disrupts the initiation of placode formation. We show that hearsay disrupts foxi1, a forkhead domain-containing gene, which is expressed in otic precursor cells before placodes become visible; foxi1 appears to be the earliest marker known for the otic anlage. We provide evidence that foxi1 regulates expression of pax8, indicating a very early role for this gene in placode formation. In addition, foxi1 is expressed in the developing branchial arches, and jaw formation is disrupted in hearsay mutant embryos.

Programmed cell death in the developing inner ear is balanced by nerve growth factor and insulin-like growth factor I

Nerve growth factor induces cell death in organotypic cultures of otic vesicle explants. This cell death has a restricted pattern that reproduces the in vivo pattern of apoptosis occurring during inner ear development. In this study, we show that binding of nerve growth factor to its low affinity p75 neurotrophin receptor is essential to achieve the apoptotic response. Blockage of binding to p75 receptor neutralized nerve-growth-factor-induced cell death, as measured by immunoassays detecting the presence of cytosolic oligonucleosomes and by TUNEL assay to visualize DNA fragmentation. Nerve growth factor also induced a number of cell-death-related intracellular events including ceramide generation, caspase activation and poly-(ADP ribose) polymerase cleavage. Again, p75 receptor blockade completely abolished all of these effects. Concerning the intracellular pathway, ceramide increase depended on initiator caspases, whereas its actions depended on both initiator and effector caspases, as shown by using site-specific caspase inhibitors. Conversely, insulin-like growth factor I, which promotes cell growth and survival in the inner ear, abolished apoptosis induced by nerve growth factor. Insulin-like growth factor cytoprotective actions were accomplished, at least in part, by decreasing endogenous ceramide levels and activating Akt. Taken together, these results strongly suggest that regulation of nerve-growth-factor-induced apoptosis in the otocysts occurs via p75 receptor binding and is strictly controlled by the interaction with survival signalling pathways.

Molecular conservation and novelties in vertebrate ear development

Evolution shaped the vertebrate ear into a complicated three-dimensional structure and positioned the sensory epithelia so that they can extract specific aspects of mechanical stimuli to govern vestibular and hearing-related responses of the whole organism. This information is conducted from the ear via specific neuronal connections to distinct areas of the hindbrain for proper processing. During development, the otic placode, a simple sheet of epidermal cells, transforms into a complicated system of ducts and recesses. This placode also generates the mechanoelectrical transducers, the hair cells, and sensory neurons of the vestibular and cochlear (spiral) ganglia of the ear. We argue that ear development can be broken down into dynamic processes that use a number of known and unknown genes to govern the formation of the three-dimensional labyrinth in an interactive fashion. Embedded in this process, but in large part independent of it, is an evolutionary conserved process that induces early the development of the neurosensory component of the ear. We present molecular data suggesting that this later process is, in its basic aspects, related to the mechanosensory cell formation across phyla and is extremely conserved at the molecular level. We suggest that sensory neuron development and maintenance are vertebrate or possibly chordate novelties and present the molecular data to support this notion.

Determination of the embryonic inner ear

Inner ear induction, like induction of other tissues examined in recent years, is likely to be comprised of several stages. The process begins during gastrulation when the ectoderm is competent to respond to induction. It appears that a signal from the endomesoderm underlying the otic area during gastrulation initiates induction complemented by a signal from presumptive neural tissue. By the neural plate stage, a region of ectoderm outside the neural plate is "biased" toward ear formation; this process may be part of a more general "placodal" bias shared by several sensory tissues. Induction continues during neurulation when a signal from neural tissue (possibly augmented by mesoderm underlying the otic area) results in ectoderm committed to otic vesicle formation at the time of neural tube closure. Studies on several gene families implicate them in the ear determination process. Fibroblast Growth Factor (FGF) family members are clearly involved in induction: FGFs are appropriately expressed for such a role, and have been shown to be essential for inner ear development. FGFs also have inductive activity, although it is not clear if they are sufficient for ear induction. Activation of transcription factors in the otic ectoderm, for example, by Pax gene family members, provides evidence for important changes in the responding ectoderm beginning during gastrulation and continuing through specification at the end of neurulation, although few functional tests have defined the role of these genes in determination. The challenge remains to merge embryologic data with gene function studies to develop a clear model for the molecular basis of inner ear induction.

Fgf8 and Fgf3 are required for zebrafish ear placode induction, maintenance and inner ear patterning

The vertebrate inner ear develops from initially 'simple' ectodermal placode and vesicle stages into the complex three-dimensional structure which is necessary for the senses of hearing and equilibrium. Although the main morphological events in vertebrate inner ear development are known, the genetic mechanisms controlling them are scarcely understood. Previous studies have suggested that the otic placode is induced by signals from the chordamesoderm and the hindbrain, notably by fibroblast growth factors (Fgfs) and Wnt proteins. Here we study the role of Fgf8 as a bona-fide hindbrain-derived signal that acts in conjunction with Fgf3 during placode induction, maintenance and otic vesicle patterning. Acerebellar (ace) is a mutant in the fgf8 gene that results in a non-functional Fgf8 product. Homozygous mutants for acerebellar (ace) have smaller ears that typically have only one otolith, abnormal semi-circular canals, and behavioral defects. Using gene expression markers for the otic placode, we find that ace/fgf8 and Fgf-signaling are required for normal otic placode formation and maintenance. Conversely, misexpression of fgf8 or Fgf8-coated beads implanted into the vicinity of the otic placode can increase ear size and marker gene expression, although competence to respond to the induction appears restricted. Cell transplantation experiments and expression analysis suggest that Fgf8 is required in the hindbrain in the rhombomere 4-6 area to restore normal placode development in ace mutants, in close neighbourhood to the forming placode, but not in mesodermal tissues. Fgf3 and Fgf8 are expressed in hindbrain rhombomere 4 during the stages that are critical for placode induction. Joint inactivation of Fgf3 and Fgf8 by mutation or antisense-morpholino injection causes failure of placode formation and results in ear-less embryos, mimicking the phenotype we observe after pharmacological inhibition of Fgf-signaling. Fgf8 and Fgf3 together therefore act during induction and differentiation of the ear placode. In addition to the early requirement for Fgf signaling, the abnormal differentiation of inner ear structures and mechanosensory hair cells in ace mutants, pharmacological inhibition of Fgf signaling, and the expression of fgf8 and fgf3 in the otic vesicle demonstrate independent Fgf function(s) during later development of the otic vesicle and lateral line organ. We furthermore addressed a potential role of endomesomerm by studying mzoep mutant embryos that are depleted of head endomesodermal tissue, including chordamesoderm, due to a lack of Nodal-pathway signaling. In these embryos, early placode induction proceeds largely normally, but the ear placode extends abnormally to midline levels at later stages, suggesting a role for the midline in restricting placode development to dorsolateral levels. We suggest a model of zebrafish inner ear development with several discrete steps that utilize sequential Fgf signals during otic placode induction and vesicle patterning.

A compendium of mouse knockouts with inner ear defects

Genetically engineered strains of mice, modified by gene targeting (knockouts), are increasingly being employed as alternative effective research tools in elucidating the genetic basis of human deafness. An impressive array of auditory and vestibular mouse knockouts is already available as a valuable resource for studying the ontogenesis, morphogenesis and function of the mammalian inner ear. This article provides a current catalog of mouse knockouts with inner ear morphogenetic malformations and hearing or balance deficits resulting from ablation of genes that are regionally expressed in the inner ear and/or within surrounding tissues, such as the hindbrain, neural crest and mesenchyme.

Extensive cell movements accompany formation of the otic placode

During development, the vertebrate inner ear arises from the otic placode, a thickened portion of the ectoderm next to the hindbrain. Here, the first detailed fate maps of this region in the chick embryo are presented. At head process stages, placode precursors are scattered throughout a large region of the embryonic ectoderm, where they intermingle with future neural, neural crest, epidermal, and other placode cells. Within the next few hours, dramatic cell movements shift the future otic placode cells toward the midline and ultimately result in convergence to their final position next to rhombomeres 5-6. Individual cells and small cell groups undergo constant cell rearrangements and appear to sort out from nonotic cells. While the major portion of the otic placode is derived from the nonneural ectoderm, the neural folds also contribute cells to the placode at least until the four-somite stage. Comparison of these fate maps with gene expression patterns at equivalent stages reveals molecular heterogeneity of otic precursor cells in terms of their expression of dlx5, msx1, Six4, and ERNI. Although Pax2 expression coincides with the region where otic precursors are found from stage 8, not all Pax2-positive cells will ultimately contribute to the otic placode.

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.

The Dlx5 homeobox gene is essential for vestibular morphogenesis in the mouse embryo through a BMP4-mediated pathway

In the mouse embryo, Dlx5 is expressed in the otic placode and vesicle, and later in the semicircular canals of the inner ear. In mice homozygous for a null Dlx5/LacZ allele, a severe dysmorphogenesis of the vestibular region is observed, characterized by the absence of semicircular canals and the shortening of the endolymphatic duct. Minor defects are observed in the cochlea, although Dlx5 is not expressed in this region. Cristae formation is severely impaired; however, sensory epithelial cells, recognized by calretinin immunostaining, are present in the vestibular epithelium of Dlx5(-/-) mice. The maculae of utricle and saccule are present but cells appear sparse and misplaced. The abnormal morphogenesis of the semicircular canals is accompanied by an altered distribution of proliferating and apoptotic cells. In the Dlx5(-/-) embryos, no changes in expression of Nkx5.1(Hmx3), Pax2, and Lfng have been seen, while expression of bone morphogenetic protein-4 (Bmp4) was drastically reduced. Notably, BMP4 has been shown to play a fundamental role in vestibular morphogenesis of the chick embryo. We propose that development of the semicircular canals and the vestibular inner ear requires the independent control of several homeobox genes, which appear to exert their function via tight regulation of BPM4 expression and the regional organization of cell differentiation, proliferation, and apoptosis.

Concerted action of twodlxparalogs in sensory placode formation

Sensory placodes are ectodermal thickenings that give rise to elements of the vertebrate cranial sensory nervous system, including the inner ear and nose. Although mutations have been described in humans, mice and zebrafish that perturb ear and nose development, no mutation is known to prevent sensory placode formation. Thus, it has been postulated that a functional redundancy exists in the genetic mechanisms that govern sensory placode development. We describe a zebrafish deletion mutation, b380, which results in a lack of both otic and olfactory placodes.

The b380 deletion removes several known genes and expressed sequence tags, including dlx3 and dlx7, two transcription factors that share a homoeobox domain similar in sequence to the Drosophila Distal-less gene. dlx3 and dlx7 are expressed in an overlapping pattern in the regions that produce the otic and olfactory placodes in zebrafish. We present evidence suggesting that it is specifically the removal of these two genes that leads to the otic and olfactory phenotype of b380 mutants. Using morpholinos, antisense oligonucleotides that effectively block translation of target genes, we find that functional reduction of both dlx genes contributes to placode loss. Expression patterns of the otic marker pax2.1, olfactory marker anxV and eya1, a marker of both placodes, in morpholino-injected embryos recapitulate the reduced expression of these genes seen in b380 mutants. We also examine expression of dlx3 and dlx7 in the morpholino-injected embryos and present evidence for existence of auto- and cross-regulatory control of expression among these genes.

We demonstrate that dlx3 is necessary and sufficient for proper otic and olfactory placode development. However, our results indicate that dlx3 and dlx7 act in concert and their importance in placode formation is only revealed by inactivating both paralogs.

A subtracted cDNA library from the zebrafish (Danio rerio) embryonic inner ear

A database was built that consists of 4694 sequence contigs of approximately 18,000 reads of cDNAs isolated from the microdissected otocysts of zebrafish embryos at 20-30 hour postfertilization, following subtraction with a pool of liver cDNAs from adult fish. These sequences were compared with those of public databanks. Significant similarity were recorded and organized in a relational database at http://www.genoscope.cns.fr/zie. A first group of 2067 sequences correspond to 1428 known zebrafish genes or ESTs present in the Danio rerio section of UniGene. A second group of 302 sequences encode putative proteins that showed significant similarity (50%-100%) with 302 nonzebrafish proteins in the nr databank, a public databank containing an exhaustive nonredundant collection of protein sequences from different species (ftp://ftp.ncbi.nlm.nih.gov/blast/db/nr). The remaining 2325 (49.5%) sequence contigs or singletons showed no significant similarity with sequences available in public databanks. Several genes known to be expressed in the developing inner ear were represented in the present database, in particular genes involved in hair cell differentiation or innervation The occurrence of these genes validates the outcome of this study as the first collection of ESTs preferentially expressed in the zebrafish inner ear during the period of hair cell differentiation and neuroblast delamination from the otic vesicle epithelium. Novel zebrafish genes also involved in these processes are thus likely to be represented among the sequences obtained herein, for which no homology was found in the D. rerio section of UniGene. [The sequence data from this study have been submitted to EMBL under accession nos. AL714032-AL731531].

Wnt6 marks sites of epithelial transformations in the chick embryo

In a screen for Wnt genes executing the patterning function of the vertebrate surface ectoderm, we have isolated a novel chick Wnt gene, chick Wnt6. This gene encodes the first pan-epidermal Wnt signalling molecule. Further sites of expression are the boundary of the early neural plate and surface ectoderm, the roof of mesencephalon, pretectum and dorsal thalamus, the differentiating heart, and the otic vesicle. The precise sites of Wnt6 expression coincide with crucial changes in tissue architecture, namely epithelial remodelling and epithelial-mesenchymal transformation (EMT). Moreover, the expression of Wnt6 is closely associated with areas of Bmp signalling.

Pseudoexfoliation and sensorineural hearing loss

AIMS

There is increasing evidence that pseudoexfoliation (PXF) not only affects ocular anterior segment structures, but may also be a systemic disease. This study was undertaken to assess the relationship between PXF and sensorineural hearing loss.

METHODS

Patients with PXF were identified from hospital records and underwent complete ocular examination. The sum of pure-tone hearing thresholds measured at 1, 2 and 3 kHz (HTL1,2,3) in each ear was compared with the ISO 7029 standard sex-matched, median age-associated hearing loss summed over the same frequencies (AAHL1,2,3). The proportion of ears with thresholds higher than the ISO 7029 median AAHL1,2,3 on the same side as eyes without PXF was compared with the proportion of ears ipsilateral to eyes with PXF but without glaucoma and similarly the proportion of ears on the same side as eyes with PXF and glaucoma.

RESULTS

In total, 69 patients were studied, of whom 39 were male (56.5%). The mean age of the male patients was 75.8 years, while that of the female group was 75.1 years. All patients had PXF affecting at least one eye. Overall 101 ears (73.7%) had a higher HTL1,2,3 than the ISO 7029 median AAHL1,2,3 which included 56 ears of 78 in the male group (71.8%) and 45 ears of 59 in the female group (76.3%). There was no significant difference between the proportion of ears with HTL1,2,3 higher than the ISO 7029 median AAHL1,2,3 on the same side as eyes without PXF, with PXF but not glaucoma and with PXF and glaucoma, in either the male or female groups.

CONCLUSIONS

A large proportion of patients with PXF have sensorineural hearing loss in comparison to age-matched controls, regardless of whether or not there is associated glaucoma. This finding supports the theory that PXF may be a systemic condition.

Fgf3 and Fgf8 are required together for formation of the otic placode and vesicle

Fgf3 has long been implicated in otic placode induction and early development of the otocyst; however, the results of experiments in mouse and chick embryos to determine its function have proved to be conflicting. In this study, we determined fgf3 expression in relation to otic development in the zebrafish and used antisense morpholino oligonucleotides to inhibit Fgf3 translation. Successful knockdown of Fgf3 protein was demonstrated and this resulted in a reduction of otocyst size together with reduction in expression of early markers of the otic placode.

fgf3 is co-expressed with fgf8 in the hindbrain prior to otic induction and, strikingly, when Fgf3 morpholinos were co-injected together with Fgf8 morpholinos, a significant number of embryos failed to form otocysts. These effects were made manifest at early stages of otic development by an absence of early placode markers (pax2.1 and dlx3) but were not accompanied by effects on cell division or death. The temporal requirement for Fgf signalling was established as being between 60% epiboly and tailbud stages using the Fgf receptor inhibitor SU5402. However, the earliest molecular event in induction of the otic territory, pax8 expression, did not require Fgf signalling, indicating an inductive event upstream of signalling by Fgf3 and Fgf8. We propose that Fgf3 and Fgf8 are required together for formation of the otic placode and act during the earliest stages of its induction.

Hair cell development in vivo and in vitro: analysis by using a monoclonal antibody specific to hair cells in the chick inner ear

The purpose of this study was to establish a hair cell-specific marker and a convenient explant culture system for developing chick otocysts to facilitate in vivo and in vitro studies focusing on hair cell genesis in the inner ear. To achieve this, a hair cell-specific monoclonal antibody, 2A7, was generated by immunizing chick inner ear tissues to a mouse. Through the use of immunofluorescence and immunoelectron microscopy, it was shown that 2A7 immunoreactivity (2A7-IR) was primarily restricted to the apical region of inner ear hair cells, including stereocilia, kinocilia, apical membrane amongst the extending cilia, and superficial layer of the cuticular plate. Although the 2A7 antibody immunolabeled basically all of the hair cells in the posthatch chick inner ear, two different patterns of 2A7-IR were observed; hair cells located in the striolar region of the utricular macula, which consist of two distinct cell types identifiable on the basis of the type of nerve ending, Type I and II hair cells, showed labeling restricted to the basal end of the hair bundles. On the other hand, hair cells in the extrastriolar region, which are exclusively of Type II, showed labeling extending over virtually the entire length of the bundles. These findings raised the possibility that chick vestibular Type II hair cells, characterized by their bouton-type afferent nerve endings, can be divided into two subpopulations. Analysis of developing inner ear by using the 2A7 antibody revealed that this antibody also recognizes newly differentiated immature hair cells. Thus, the 2A7 antibody is able to recognize both immature and mature hair cells in vivo. The developmental potential of embryonic otocysts in vitro was then assessed by using explant cultures as a model. In this study, conventional otocyst explant cultures were modified by placing the tissues on floating polycarbonate filters on culture media, thereby allowing the easy manipulation of explants. In these cultures, 2A7-positive hair cells were differentiated from dividing precursor cells in vitro on the same schedule as in vivo. Furthermore, it was found that hair cells with both types of 2A7-IR were generated in culture as in vivo, indicating that a maturational process of hair cells also occurred. All these results as presented here suggest that the 2A7 monoclonal antibody as a hair cell-specific marker together with the culture system could be a potential tool in analysis of mechanisms underlying hair cell development.

Vestibular dysgenesis in mice lacking Abr and Bcr Cdc42/RacGAPs

The inner ear develops from a simple epithelium (otic placode) into the complex structures specialized for balance (vestibule) and sound (cochlea) detection. Abnormal vestibular and cochlear development is associated with many birth defects. During recent years, considerable progress has been made in understanding the molecular bases of these conditions. To determine the biological function of two closely related GTPase activating proteins for the Cdc42/Rac GTPases, Abr and Bcr, we generated a mouse strain deficient in both of these proteins. Double null mutant mice exhibit hyperactivity, persistent circling, and are unable to swim. These phenotypes are typically found in mice with vestibular defects. Indeed, adult double null mutants display abnormal dysmorphic structures of both the saccule and utricle. Moreover, a total loss of otoconia can be seen in the utricle, whereas in the saccule, otoconia are either missing or their number is drastically decreased and they are abnormally large. Interestingly, both the cochlea and semicircular canals are normal and the double null mutant mice are not deaf. These data demonstrate that Abr and Bcr play important complementary roles during vestibular morphogenesis and that a function of Cdc42/RacGAPs and, therefore, that of the small Rho-related GTPases is critically important for balance and motor coordination.

Differential expression of Otx2, Gbx2, Pax2, and Fgf8 in the developing vestibular and auditory sensory organs

The vertebrate inner ear is a complex organ with vestibular and auditory sensory functions, which derives from a single ectoderm structure, the otic placode. The development and regional patterning of the otic primordium is determined by the restricted expression of several genes. Here, we show the expression pattern of three transcription factors (Otx2, Gbx2, Pax2) and of a member of the fibroblast growth factor family (Fgf8) in the developing chick inner ear, and we correlate these patterns with the developing sensory and nonsensory elements.

The nuclear receptor Nor-1 is essential for proliferation of the semicircular canals of the mouse inner ear

Nor-1 belongs to the nur subfamily of nuclear receptor transcription factors. The precise role of Nor-1 in mammalian development has not been established. However, recent studies indicate a function for this transcription factor in oncogenesis and apoptosis. To examine the spatiotemporal expression pattern of Nor-1 and the developmental and physiological consequences of Nor-1 ablation, Nor-1-null mice were generated by insertion of the lacZ gene into the Nor-1 genomic locus. Disruption of the Nor-1 gene results in inner ear defects and partial bidirectional circling behavior. During early otic development, Nor-1 is expressed exclusively in the semicircular canal forming fusion plates. After formation of the membranous labyrinth, Nor-1 expression in the vestibule is limited to nonsensory epithelial cells localized at the inner edge of the semicircular canals and to the ampullary and utricular walls. In the absence of Nor-1, the vestibular walls fuse together as normal; however, the endolymphatic fluid space in the semicircular canals is diminished and the roof of the ampulla appears flattened due to defective continual proliferative growth of the semicircular canals.

Expression of the transcription factors GATA3 and Pax2 during development of the mammalian inner ear

The transcription factors GATA3 and Pax2 are expressed throughout development of the mouse inner ear. We have used antibodies to study their temporal and spatial expression patterns from embryonic days E8-E16.5. The two factors show reciprocal relationships in the regional patterning of the early otocyst and cellular patterning within the sensory epithelia. GATA3 is expressed in the whole otic placode at E8. In the otocyst at E9.5-10.5, the distribution is lateral and complementary to the medial expression pattern of Pax2. Only Pax2 is expressed in the endolymphatic duct, but both factors are expressed in the cochlea. At E11.5-13.5, GATA3 is expressed strongly in the cochlea, but in the dorsal, vestibular region it is downregulated. In all sensory epithelia, downregulation coincides with sensory innervation. Pax2 is expressed in all sensory and some nonsensory epithelia, but within sensory epithelia at E16.5 it is restricted to hair cells. GATA3 is expressed throughout key periods of cell proliferation, fate determination, and differentiation and is not specifically associated with any of these processes. Expression persists most strongly in the main components of the developing auditory system. These include the auditory sensory epithelium, the afferent and efferent nerves, and the mesenchymal and ectodermal cells in regions that form key parts of the middle and outer ear. GATA3 is thus expressed in functionally distinct groups of cells that integrate to form a complete sensory system. The results suggest that both factors may be involved in tissue compartmentalisation, morphogenesis, and cell signalling.

Molecular genetics of hearing loss

Hereditary isolated hearing loss is genetically highly heterogeneous. Over 100 genes are predicted to cause this disorder in humans. Sixty loci have been reported and 24 genes underlying 28 deafness forms have been identified. The present epistemic stage in the realm consists in a preliminary characterization of the encoded proteins and the associated defective biological processes. Since for several of the deafness forms we still only have fuzzy notions of their pathogenesis, we here adopt a presentation of the various deafness forms based on the site of the primary defect: hair cell defects, nonsensory cell defects, and tectorial membrane anomalies. The various deafness forms so far studied appear as monogenic disorders. They are all rare with the exception of one, caused by mutations in the gene encoding the gap junction protein connexin26, which accounts for between one third to one half of the cases of prelingual inherited deafness in Caucasian populations.

Development of the sensory organs

The sensory organs--the eye, ear, and nose- are formed, in part, from ectodermal thickenings: placodes. Their development is distinct from that of other regions of the developing body and they are essential for the development of other structures. For example, the olfactory placode which gives rise to the nose is essential for the functional development of the reproductive organs and hence fertility. Recently much progress has been made in the understanding of placode development, at both a molecular and embryological level. This is important as abnormal development of placodes occurs in a number of human syndromes. Furthermore, knowledge of placode development will give insight into therapeutic strategies to prevent degenerative change such as deafness. This review highlights the current knowledge of placode development and the future challenges in unravelling the cascades of signalling interactions that control development of these unique structures.

Hmx2 homeobox gene control of murine vestibular morphogenesis

Development of the vertebrate inner ear is characterized by a series of genetically programmed events involving induction of surface ectoderm, preliminary morphogenesis, specification and commitment of sensory, nonsensory and neuronal cells, as well as outgrowth and restructuring of the otocyst to form a complex labyrinth. Hmx2, a member of the Hmx homeobox gene family, is coexpressed with Hmx3 in the dorsolateral otic epithelium. Targeted disruption of Hmx2 in mice demonstrates the temporal and spatial involvement of Hmx2 in the embryonic transition of the dorsal portion (pars superior) of the otocyst to a fully developed vestibular system. In Hmx2 null embryos, a perturbation in cell fate determination in the lateral aspect of the otic epithelium results in reduced cell proliferation in epithelial cells, which includes the vestibular sensory patches and semicircular duct fusion plates, as well as in the adjacent mesenchyme. Consequently, enlargement and morphogenesis of the pars superior of the otocyst to form a complex labyrinth of cavities and ducts is blocked, as indicated by the lack of any distinguishable semicircular ducts, persistence of the primordial vestibular diverticula, significant loss in the three cristae and the macula utriculus, and a fused utriculosaccular chamber. The developmental regulators Bmp4, Dlx5 and Pax2 all play a critical role in inner ear ontogeny, and the expression of each of these genes is affected in the Hmx2 null otocyst suggesting a complex regulatory role for Hmx2 in this genetic cascade. Both Hmx2 and Hmx3 transcripts are coexpressed in the developing central nervous system including the neural tube and hypothalamus. A lack of defects in the CNS, coupled with the fact that not all of the Hmx2-positive regions in developing inner ear are impaired in the Hmx2 null mice, suggest that Hmx2 and Hmx3 have both unique and overlapping functions during embryogenesis.

FGFs control the patterning of the inner ear but are not able to induce the full ear program

FGF2 or FGF8 applied ectopically, close to the developing otic placode enhances transcription of a subset of ear marker genes such as Nkx5-1, SOHo1 and Pax2. Other ear expressed genes (Dlx5 and BMP4) are not up-regulated by FGFs. Ectopic FGFs lead to an increase in size of the vestibulo-cochlear ganglion. This phenotypic change is due to an increased recruitment of epithelial cells to the neuronal fate rather than to an enhanced proliferation. We also observed an induction of additional, vesicle-like structures upon ectopic FGF treatment, but this induction never led to enrolment of a full ear program. We further demonstrate that FGF8 is expressed in two separate, short waves, first at the otic placode stage and later at the vesicle stage. Both activities correspond to critical morphogenetic events in ear development. We propose that FGF8 is an important regulator of otocyst patterning.

Increased cell death in the developing vestibulocochlear ganglion complex of the mouse after prenatal ethanol exposure

BACKGROUND

Previous studies have demonstrated that excessive prenatal alcohol exposure can damage the auditory and vestibular systems, in particular, cochlear hair cells. However, the direct effect of ethanol on the peripheral neurons in these pathways has not been examined. To study the effects of prenatal ethanol exposure on the developing vestibulocochlear ganglion (VCG) complex and the peripheral sensory organs, we exposed pregnant mice to ethanol and examined the levels of cell death in the inner ear.

METHODS

Pregnant C57BL/6J mice were administered one of three doses of either ethanol (3.0, 4.5, and 5.5 g/kg) or isocaloric maltose/dextrin via intragastric intubation on gestational day (GD) 12.5. Embryos were dissected out of the uterus 8 hr after the intubation. Dying cells in the inner ear were stained with Nissl stain and labeled by in situ terminal dUTP nick-end labeling (TUNEL), and the percentage of dying cells was quantified.

RESULTS

Ethanol exposure produced region-specific effects, with ethanol-exposed embryos exhibiting enhanced cell death only in the VCG complex, and not in the primitive saccule, cochlea, semicircular canal, or endolymphatic sac. The effects of ethanol on cell death in the VCG are dose dependent, with a significant increase in the level of cell death found only at the higher doses.

CONCLUSIONS

Ethanol has a selective cytotoxic dose-dependent effect on the VCG at GD 12.5 suggesting that loss of VCG neurons may contribute to hearing and /or vestibular abnormalities in FAS children. Furthermore, the presence of TUNEL-positive cells and DNA laddering is consistent with the cells undergoing apoptotic cell death.

Remarks on the inner ear of elasmobranchs and its interpretation from skeletal labyrinth morphology

The structure and function of the craniate inner ear is reviewed, with 33 apomorphic characters of the membranous labyrinth and associated structures identified in craniates, gnathostomes, and elasmobranchs. Elasmobranchs are capable of low-frequency semi-directional phonoreception, even in the absence of any pressure-to-displacement transducer such as ear ossicles. The endolymphatic (parietal) fossa, semicircular canals, and crista (macula) neglecta are all adapted toward phonoreception. Some (but not all) of the morphological features associated with phonoreception can be inferred from the elasmobranch skeletal labyrinth. Endocranial spaces such as the skeletal labyrinth also provide suites of morphological characters that may be incorporated into phylogenetic analyses, irrespective of how closely these spaces reflect underlying soft anatomy. The skeletal labyrinths of Squalus and Notorynchus are compared using silicone endocasts and high-resolution CT-scanning. The latter procedure offers several advantages over other techniques; it is more informative, nondestructive, preserves relationships of surrounding structures, and it can be applied both to modern and fossil material.

The ectodermal placodes: a dysfunctional family

The ectodermal placodes are focal thickenings of the cranial embryonic ectoderm that contribute extensively to the cranial sensory systems of the vertebrates. The ectodermal placodes have long been thought of as representing a coherent group, which share a developmental and evolutionary history. However, it is now becoming clear that there are substantial differences between the placodes with respect to their early development, their induction and their evolution. Indeed, it is now hard to consider the ectodermal placodes as a single entity. Rather, they fall into a number of distinct classes and it is within each of these that the members share a common development and evolution.

Development of the vertebrate inner ear

The inner ear, also called the membranous labyrinth, contains the cochlea, which is responsible for the sense of hearing, and the vestibular apparatus, which is necessary for the sense of balance and gravity. The inner ear arises in the embryo from placodes, which are epithelial thickenings of the cranial ectoderm symmetrically located on either side of hindbrain rhombomeres 5 and 6. Placode formation in mice is first visible at the 12-somite stage and is controlled by surrounding tissues, the paraxial mesoderm and neural ectoderm. Diffusible molecules such as growth factors play an important role in this process. The activity of several genes confers the identity to the placodal cells. Subsequent cellular proliferation processes under influences from the adjacent hindbrain cause the inner ear epithelium to invaginate and form a vesicle called the otocyst. Combinatorial expression of several genes and diffusible factors secreted from the vesicle epithelium and hindbrain control specification of distinct inner ear compartments. Transplantation studies and inner ear in vitro cultures show that each of these compartments is already committed to develop unique inner ear structures. Later developmental periods are principally characterized by intrinsic differentiation processes. In particular, sensory patches differentiate into fully functional sensory epithelia, and the semicircular canals along with the cochlear duct are elaborated and ossified.

Tight transcriptional control of the ETS domain factors Erm and Pea3 by Fgf signaling during early zebrafish development

Several molecules of the Fibroblast growth factor family have been implicated in the development of the vertebrate brain, but the effectors of these molecules remain largely unknown. Here we study Erm and Pea3, two ETS domain transcription factors, and show that their expression correlates closely with the domains of fgf8 and fgf3 expression. In situ hybridization analysis in wild-type and acerebellar (ace) mutant embryos defective for fgf8 demonstrates a requirement of Fgf8 for normal expression levels of erm and pea3 transcripts in and close to various domains of Fgf8 action, including the prospective midbrain-hindbrain region, the somites, the neural crest, the forebrain, and developing eyes. Morpholino-oligomer-assisted gene knock-down experiments targeted against fgf8 and fgf3 suggest that Fgf3 and Fgf8 are co-regulators of these genes in the early forebrain anlage. Furthermore, inhibition of Fgf signaling by overexpression of sprouty4 or application of the Fgf inhibitor SU5402 leads to a loss of all erm and pea3 expression domains. Conversely, ectopically provided fgf3 mRNA or implanted beads coated with Fgf8 elicit ectopic transcription of erm and pea3. Both activation and loss of transcripts can be observed within short time frames. We conclude that both the transcriptional onset and maintenance of these factors are tightly coupled to Fgf signaling and propose that erm and pea3 transcription is a direct readout of cells to Fgf levels. Given the knowledge that has accumulated on the posttranslational control of ETS domain factors and their combinatorial interactions with other transcription factors, we suggest that the close coupling of erm and pea3 transcription to Fgf signaling might serve to integrate Fgf signaling with other signals to establish refined patterns in embryonic development.

In early development of the rat mRNA for the major myelin protein P(0) is expressed in nonsensory areas of the embryonic inner ear, notochord, enteric nervous system, and olfactory ensheathing cells

The myelin protein P(0) has a major structural role in Schwann cell myelin, and the expression of P(0) protein and mRNA in the Schwann cell lineage has been extensively documented. We show here, using in situ hybridization, that the P(0) gene is also activated in a number of other tissues during embryonic development. P(0) mRNA is first detectable in 10-day-old embryos (E10) and is at this time seen only in cells in the cephalic neural crest and in the otic placode/pit. P(0) expression continues in the otic vesicle and at E12 P(0) expression in this structure largely overlaps with expression of another myelin gene, proteolipid protein. In the developing ear at E14, P(0) expression is complementary to expression of serrate and c-ret mRNAs, which later are expressed in sensory areas of the inner ear, while expression of bone morphogenetic protein (BMP)-4 and P(0), though largely complementary, shows small areas of overlap. P(0) mRNA and protein are detectable in the notochord from E10 to at least E13. In addition to P(0) expression in a subpopulation of trunk crest cells at E11/E12 and in Schwann cell precursors thereafter, P(0) mRNA is also present transiently in a subpopulation of cells migrating in the enteric neural crest pathway, but is down-regulated in these cells at E14 and thereafter. P(0) is also detected in the placode-derived olfactory ensheathing cells from E13 and is maintained in the adult. No signal is seen in cells in the melanocyte migration pathway or in TUJ1 positive neuronal cells in tissue sections. The activation of the P(0) gene in specific tissues outside the nervous system was unexpected. It remains to be determined whether this is functionally significant, or whether it is an evolutionary relic, perhaps reflecting ancestral use of P(0) as an adhesion molecule.

Retinoic acid rescues inner ear defects in Hoxa1 deficient mice

Little is known about the genetic pathways involved in the early steps of inner ear morphogenesis. Hoxa1 is transiently expressed in the developing hindbrain; its targeted inactivation in mice results in severe abnormalities of the otic capsule and membranous labyrinth. Here we show that a single maternal administration of a low dose of the vitamin A metabolite retinoic acid is sufficient to compensate the requirement for Hoxa1 function. It rescues cochlear and vestibular defects in mutant fetuses without affecting the development of the wildtype fetuses. These results identify a temporal window of susceptibility to retinoids that is critical for mammalian inner ear specification, and provide the first evidence that a subteratogenic dose of vitamin A derivative can be effective in rescuing a congenital defect in the mammalian embryo.

Expression and phylogeny of claudins in vertebrate primordia

Claudins, the major transmembrane proteins of tight junctions, are members of the tetraspanin superfamily of proteins that mediate cellular adhesion and migration. Their functional importance is demonstrated by mutations in claudin genes that eliminate tight junctions in myelin and the testis, abolish Mg(2+) resorption in the kidney, and cause autosomal recessive deafness. Here we report that two paralogs among 15 claudin genes in the zebrafish, Danio rerio, are expressed in the otic and lateral-line placodes at their earliest stages of development. Related claudins in amphibians and mammals are expressed in a similar manner in vertebrate primordia such as sensory placodes, branchial arches, and limb buds. We also show that the claudin gene family may have expanded along the chordate stem lineage from urochordates to gnathostomes, in parallel with the elaboration of vertebrate characters. We propose that tight junctions not only form barriers in mature epithelia, but also participate in vertebrate morphogenesis.

Mechanisms that regulate mechanosensory hair cell differentiation

Hair cells of the vertebrate inner ear are mechanosensors that detect sound, gravity and acceleration. They have a specialized cytoskeleton optimized for the transmission of mechanical force. Hair cell defects are a major cause of deafness. The cloning of disease genes and studies of model organisms have provided insights into the mechanisms that regulate the differentiation of hair cells and their cytoskeleton. The studies have also provided new insights into the function of receptors such as integrins and protocadherins, and cytoplasmic proteins such as Rho-type GTPases and unconventional myosins, in organizing the actin cytoskeleton.

Hes1 and Hes5 activities are required for the normal development of the hair cells in the mammalian inner ear

The mammalian inner ear contains two sensory organs, the cochlea and vestibule. Their sensory neuroepithelia are characterized by a mosaic of hair cells and supporting cells. Cochlear hair cells differentiate in four rows: a single row of inner hair cells (IHCs) and three rows of outer hair cells (OHCs). Recent studies have shown that Math1, a mammalian homolog of Drosophila atonal is a positive regulator of hair cell differentiation. The basic helix-loop-helix (bHLH) genes Hes1 and Hes5 (mammalian hairy and Enhancer-of-split homologs) can influence cell fate determination by acting as negative regulators to inhibit the action of bHLH-positive regulators. We show by using reverse transcription-PCR analysis that Hes1, Hes5, and Math1 are expressed in the developing mouse cochleae. In situ hybridization revealed a widespread expression of Hes1 in the greater epithelial ridge (GER) and in lesser epithelial ridge (LER) regions. Hes5 is predominantly expressed in the LER, in supporting cells, and in a narrow band of cells within the GER. Examination of cochleae from Hes1(-/-) mice showed a significant increase in the number of IHCs, whereas cochleae from Hes5(-/-) mice showed a significant increase in the number of OHCs. In the vestibular system, targeted deletion of Hes1 and to a lesser extent Hes5 lead to formation of supernumerary hair cells in the saccule and utricle. The supernumerary hair cells in the mutant mice showed an upregulation of Math1. These data indicate that Hes1 and Hes5 participate together for the control of inner ear hair cell production, likely through the negative regulation of Math1.

The Wheels mutation in the mouse causes vascular, hindbrain, and inner ear defects

In a screen for mouse mutations with dominant behavioral anomalies, we identified Wheels, a mutation associated with circling and hyperactivity in heterozygotes and embryonic lethality in homozygotes. Mutant Wheels embryos die at E10.5-E11.5 and exhibit a host of morphological anomalies which include growth retardation and anomalies in vascular and hindbrain development. The latter includes perturbation of rhombomeric boundaries as detected by Krox20 and Hoxb1. PECAM-1 staining of embryos revealed normal formation of the primary vascular plexus. However, subsequent stages of branching and remodeling do not proceed normally in the yolk sac and in the embryo proper. To obtain insights into the circling behavior, we examined development of the inner ear by paint-filling of membranous labyrinths of Whl/+ embryos. This analysis revealed smaller posterior and lateral semicircular canal primordia and a delay in the canal fusion process at E12.5. By E13.5, the lateral canal was truncated and the posterior canal was small or absent altogether. Marker analysis revealed an early molecular phenotype in heterozygous embryos characterized by perturbed expression of Bmp4 and Msx1 in prospective lateral and posterior cristae at E11.5. We have constructed a genetic and radiation hybrid map of the centromeric portion of mouse Chromosome 4 across the Wheels region and refined the position of the Wheels locus to the approximately 1.1-cM region between D4Mit104 and D4Mit181. We have placed the locus encoding Epha7, in the Wheels candidate region; however, further analysis showed no mutations in the Epha7-coding region and no detectable changes in mRNA expression pattern. In summary, our findings indicate that Wheels, a gene which is essential for the survival of the embryo, may link diverse processes involved in vascular, hindbrain, and inner ear development.

The cell adhesion molecule BEN defines a prosensory patch in the developing avian otocyst

The distribution of the cell adhesion molecule BEN in the developing chick inner ear is described. BEN is first detected in the otic placode at stage 11. As the placode begins to invaginate, BEN becomes concentrated in a ventromedial region extending from the anterior to the posterior end of the otic pit. BEN expression levels increase in this region as the pit closes to form the otocyst, and distinct boundaries become defined along the dorsal and ventral edges of the ventromedial band of BEN expression. BEN expression also becomes concentrated dorsally within the otic epithelium as the pit closes and is observed in the condensing otic ganglion. By stage 22, the ventromedial band of BEN expression splits into two distinct regions, a small caudal patch within which the posterior crista will develop, and a larger anterior patch. By stage 26, this larger anterior patch of cells expressing BEN becomes subdivided into five separate areas corresponding to the regions within which the anterior crista, the lateral crista, the utricle, the saccule, and both the basilar papilla and lagenar macula form. Hair cells only develop within these regions defined by BEN distribution. The data suggest that the ventromedial patch of BEN expression observed from stage 11 onwards defines a single sensory competent zone from which all sensory organs of the inner ear develop. BEN immunoreactivity in the inner ear declines after stage 38. In response to noise exposure, upregulation of BEN expression is mainly detected in regions of the posthatch papilla where the damage is severe and regenerating hair cells are not observed. The regenerating hair and supporting cells do not express BEN, highlighting a molecular difference between the processes of development and regeneration.

Lateral Line Placodes Are Induced during Neurulation in the Axolotl

In order to determine the time window for induction of lateral line placodes in the axolotl, we performed two series of heterotopic and isochronic transplantations from pigmented to albino embryos at different stages of embryogenesis and assessed the distribution of pigmented neuromasts in the hosts at later stages. First, ectoderm from the prospective placodal region was transplanted to the belly between early neurula and mid tailbud stages (stages 13-27). Whereas grafts from early neurulae typically differentiated only into epidermis, grafts from late neural fold stages on reliably resulted in differentiation of ectopic pigmented neuromasts. Second, belly ectoderm was transplanted to the prospective placodal region between early neurula and tailbud stages (stages 13-35). Normal lateral lines containing pigmented neuromasts formed in most embryos when grafts were performed prior to early tailbud stages (stage 24) but not when they were performed later. Our findings indicate that lateral line placodes, from which neuromasts originate, are already determined at late neural fold stages (first series of grafts) but are inducible until early tailbud stages (second series of grafts). A further series of heterochronic transplantations demonstrated that the decline of inducibility at mid tailbud stages is mainly due to the loss of ectodermal competence.

Adult rat otic placode-derived neurons and sensory epithelium express all four erbB receptors: a role in regulating vestibular ganglion neuron viability

The erbB receptor family consists of erbB1/epidermal growth factor receptor, erbB2/neu, erbB3, and erbB4, all of which have been implicated in cell proliferation, differentiation, and survival in several tissues. In the nervous system, these family members can function in a trophic capacity for certain subpopulations of neurons and some types of non-neuronal cells. Vestibular sensory epithelial cells and vestibular ganglion neurons are derived from ectodermal otic placode and are essential components of the peripheral vestibular system, the sensory system for balance. Recent studies in mammals suggest that certain ligands of the epidermal growth factor receptor can induce proliferation of vestibular sensory epithelial cells. We now show that vestibular ganglion neurons and vestibular sensory epithelial cells express all four erbB receptors in adult rats. Cultured vestibular ganglion neurons also expressed all four erbB family members and were therefore used to analyze the effects of modulating erbB signaling on differentiated vestibular ganglion neurons. Transforming growth factor-alpha (a ligand for epidermal growth factor receptor) and sensory and motor neuron-derived factor (a ligand for erbB3 and erbB4) promoted vestibular ganglion neuron viability, whereas epidermal growth factor (another ligand for epidermal growth factor receptor) did not. Glial growth factor 2 (another ligand for erbB3 and erbB4) and an antibody that blocks erbB2/neu-mediated signaling inhibited vestibular ganglion neuron viability. Collectively, these observations indicate that erbB signaling regulates the viability of differentiated otic placode-derived cells in mammals and suggest that exogenous modulation of erbB signaling in peripheral vestibular tissues may prove therapeutically useful in peripheral vestibular disorders.

Otxgenes in the development and evolution of the vertebrate brain

Most of the gene candidates for the control of developmental programmes that underlie brain morphogenesis in vertebrates are the orthologues of Drosophila genes coding for signalling molecules or transcription factors. Among these, the orthodenticle group, including the Drosophila orthodenticle (otd) and the vertebrate Otx1 and Otx2 genes, is mostly involved in fundamental processes of anterior neural patterning. In mouse, Drosophila and intermediate species otd/Otx genes have shown a remarkable similarity in expression pattern suggesting that they could be part of a conserved control system operating in the brain and different from that coded by the HOX complexes controlling the hindbrain and spinal cord. In order to verify this hypothesis, a series of mouse models have been generated in which the functions of the murine Otx genes were: (i) fully inactivated, (ii) replaced with each other, and (iii) replaced with the Drosophila otd gene. The data obtained highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system and lead to hypothesize that modification of their regulatory control may have influenced the morphogenesis and evolution of the brain.

Origins of inner ear sensory organs revealed by fate map and time-lapse analyses

The inner ear develops from a simple ectodermal thickening called the otic placode into a labyrinth of chambers which house sensory organs that sense sound and are used to maintain balance. Although the morphology and function of the sensory organs are well characterized, their origins and lineage relationships are virtually unknown. In this study, we generated a fate map of Xenopus laevis inner ear at otic placode and otocyst stages to determine the developmental origins of the sensory organs. Our lineage analysis shows that all regions of the otic placode and otocyst can give rise to the sensory organs of the inner ear, though there were differences between labeled quadrants in the range of derivatives formed. A given region often gives rise to cells in multiple sensory organs, including cells that apparently dispersed from anterior to posterior poles and vice versa. These results suggest that a single sensory organ arises from cells in different parts of the placode or otocyst and that cell mixing plays a large role in ear development. Time-lapse videomicroscopy provides further evidence that cells from opposite regions of the inner ear mix during the development of the inner ear, and this mixing begins at placode stages. Lastly, bone morphogenetic protein 4 (BMP-4), a member of the transforming growth factor beta (TGF-beta) family, is expressed in all sensory organs of the frog inner ear, as it is in the developing chicken ear. Inner ear fate maps provide a context for interpreting gene expression patterns and embryological manipulations.

Otx genes in brain morphogenesis

Most of the gene candidates for the control of developmental programmes that underlie brain morphogenesis in vertebrates are the homologues of Drosophila genes coding for signalling molecules or transcription factors. Among these, the orthodenticle group includes the Drosophila orthodenticle (otd) and the vertebrate Otx1 and Otx2 genes, which are mostly involved in fundamental processes of anterior neural patterning. These genes encode transcription factors that recognise specific target sequences through the DNA binding properties of the homeodomain. In Drosophila, mutations of otd cause the loss of the anteriormost head neuromere where the gene is transcribed, suggesting that it may act as a segmentation "gap" gene. In mouse embryos, the expression patterns of Otx1 and Otx2 have shown a remarkable similarity with the Drosophila counterpart. This suggested that they could be part of a conserved control system operating in the brain and different from that coded by the HOX complexes controlling the hindbrain and spinal cord. To verify this hypothesis a series of mouse models have been generated in which the functions of the murine genes were: (i) fully inactivated, (ii) replaced with each others, (iii) replaced with the Drosophila otd gene. Otx1-/- mutants suffer from epilepsy and are affected by neurological, hormonal, and sense organ defects. Otx2-/- mice are embryonically lethal, they show gastrulation impairments and fail in specifying anterior neural plate. Analysis of the Otx1-/-; Otx2+/- double mutants has shown that a minimal threshold level of the proteins they encode is required for the correct positioning of the midbrain-hindbrain boundary (MHB). In vivo otd/Otx reciprocal gene replacement experiments have provided evidence of a general functional equivalence among otd, Otx1 and Otx2 in fly and mouse. Altogether these data highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system (CNS) and lead to hypothesize that modification of their regulatory control may have influenced morphogenesis and evolution of the brain.

Vertebrate cranial placodes I. Embryonic induction

Cranial placodes are focal regions of thickened ectoderm in the head of vertebrate embryos that give rise to a wide variety of cell types, including elements of the paired sense organs and neurons in cranial sensory ganglia. They are essential for the formation of much of the cranial sensory nervous system. Although relatively neglected today, interest in placodes has recently been reawakened with the isolation of molecular markers for different stages in their development. This has enabled a more finely tuned approach to the understanding of placode induction and development and in some cases has resulted in the isolation of inducing molecules for particular placodes. Both morphological and molecular data support the existence of a preplacodal domain within the cranial neural plate border region. Nonetheless, multiple tissues and molecules (where known) are involved in placode induction, and each individual placode is induced at different times by a different combination of these tissues, consistent with their diverse fates. Spatiotemporal changes in competence are also important in placode induction. Here, we have tried to provide a comprehensive review that synthesises the highlights of a century of classical experimental research, together with more modern evidence for the tissues and molecules involved in the induction of each placode.

Transcription factor GATA-3 alters pathway selection of olivocochlear neurons and affects morphogenesis of the ear

Patterning the vertebrate ear requires the coordinated expression of genes that are involved in morphogenesis, neurogenesis, and hair cell formation. The zinc finger gene GATA-3 is expressed both in the inner ear and in afferent and efferent auditory neurons. Specifically, GATA-3 is expressed in a population of neurons in rhombomere 4 that extend their axons across the floor plate of rhombomere 4 (r4) at embryonic day 10 (E10) and reach the sensory epithelia of the ear by E13.5. The distribution of their cell bodies corresponds to that of the cell bodies of the cochlear and vestibular efferent neurons as revealed by labeling with tracers. Both GATA-3 heterozygous and GATA-3 null mutant mice show unusual axonal projections, such as misrouted crossing fibers and fibers in the facial nerve, that are absent in wild-type littermates. This suggests that GATA-3 is involved in the pathfinding of efferent neuron axons that navigate to the ear. In the ear, GATA-3 is expressed inside the otocyst and the surrounding periotic mesenchyme. The latter expression is in areas of branching of the developing ear leading to the formation of semicircular canals. Ears of GATA-3 null mutants remain cystic, with a single extension of the endolymphatic duct and no formation of semicircular canals or saccular and utricular recesses. Thus, both the distribution of GATA-3 and the effects of null mutations on the ear suggest involvement of GATA-3 in morphogenesis of the ear. This study shows for the first time that a zinc finger factor is involved in axonal navigation of the inner ear efferent neurons and, simultaneously, in the morphogenesis of the inner ear.

Origin of the vertebrate inner ear: evolution and induction of the otic placode

The vertebrate inner ear forms a highly complex sensory structure responsible for the detection of sound and balance. Some new aspects on the evolutionary and developmental origin of the inner ear are summarised here. Recent molecular data have challenged the longstanding view that special sense organs such as the inner ear have evolved with the appearance of vertebrates. In addition, it has remained unclear whether the ear originally arose through a modification of the amphibian mechanosensory lateral line system or whether both evolved independently. A comparison of the developmental mechanisms giving rise to both sensory systems in different species should help to clarify some of these controversies. During embryonic development, the inner ear arises from a simple epithelium adjacent to the hindbrain, the otic placode, that is specified through inductive interactions with surrounding tissues. This review summarises the embryological evidence showing that the induction of the otic placode is a multistep process which requires sequential interaction of different tissues with the future otic ectoderm and the recent progress that has been made to identify some of the molecular players involved. Finally, the hypothesis is discussed that induction of all sensory placodes initially shares a common molecular pathway, which may have been responsible to generate an 'ancestral placode' during evolution.

Identification of synergistic signals initiating inner ear development

Tissue manipulation experiments in amphibians more than 50 years ago showed that induction of the inner ear requires two signals: a mesodermal signal followed by a neural signal. However, the molecules mediating this process have remained elusive. We present evidence for mesodermal initiation of otic development in higher vertebrates and show that the mesoderm can direct terminal differentiation of the inner ear in rostral ectoderm. Furthermore, we demonstrate the synergistic interactions of the extracellular polypeptide ligands FGF-19 and Wnt-8c as mediators of mesodermal and neural signals, respectively, initiating inner ear development.