The development of the vertebrate inner ear

Regional expression of the ADAMs in developing chicken cochlea

The expression patterns of five members of the ADAM (a disintegrin and metalloprotease) family including ADAM9, ADAM10, ADAM17, ADAM22, and ADAM23 were analyzed in different anatomical structures of the developing chicken cochlea by in situ hybridization and immunohistochemistry. Results show that ADAM9, ADAM10, and ADAM17 are widely expressed in the sensory epithelium of the basilar papilla, by homogene cells, spindle-shaped cells, and acoustic ganglion cells, and in the tegmentum vasculosum, each with a different pattern. ADAM22 expression is restricted to spindle-shaped cells and acoustic ganglion cells, while ADAM23 is prominently expressed by hair cells and acoustic ganglion cells. Furthermore, ADAM10 protein is coexpressed with several members of the classic cadherins, including cadherin-7, N-cadherin, and R-cadherin in distinct anatomical regions of the cochlea except for acoustic ganglion cells. The expression of the ADAMs in the developing cochlea suggests a contribution of the ADAMs to the development of distinct cochlear structures.

Mammalian Otolin: a multimeric glycoprotein specific to the inner ear that interacts with otoconial matrix protein Otoconin-90 and Cerebellin-1

Background

The mammalian otoconial membrane is a dense extracellular matrix containing bio-mineralized otoconia. This structure provides the mechanical stimulus necessary for hair cells of the vestibular maculae to respond to linear accelerations and gravity. In teleosts, Otolin is required for the proper anchoring of otolith crystals to the sensory maculae. Otoconia detachment and subsequent entrapment in the semicircular canals can result in benign paroxysmal positional vertigo (BPPV), a common form of vertigo for which the molecular basis is unknown. Several cDNAs encoding protein components of the mammalian otoconia and otoconial membrane have recently been identified, and mutations in these genes result in abnormal otoconia formation and balance deficits.

Principal Findings

Here we describe the cloning and characterization of mammalian Otolin, a protein constituent of otoconia and the otoconial membrane. Otolin is a secreted glycoprotein of ∼70 kDa, with a C-terminal globular domain that is homologous to the immune complement C1q, and contains extensive posttranslational modifications including hydroxylated prolines and glycosylated lysines. Like all C1q/TNF family members, Otolin multimerizes into higher order oligomeric complexes. The expression of otolin mRNA is restricted to the inner ear, and immunohistochemical analysis identified Otolin protein in support cells of the vestibular maculae and semi-circular canal cristae. Additionally, Otolin forms protein complexes with Cerebellin-1 and Otoconin-90, two protein constituents of the otoconia, when expressed in vitro. Otolin was also found in subsets of support cells and non-sensory cells of the cochlea, suggesting that Otolin is also a component of the tectorial membrane.

Conclusion

Given the importance of Otolin in lower organisms, the molecular cloning and biochemical characterization of the mammalian Otolin protein may lead to a better understanding of otoconial development and vestibular dysfunction.

In vitro long-term development of cultured inner ear stem cells of newborn rat

The adult mammalian auditory receptor lacks any ability to repair and/or regenerate after injury. However, the late developing cochlea still contains some stem-cell-like elements that might be used to regenerate damaged neurons and/or cells of the organ of Corti. Before their use in any application, stem cell numbers need to be amplified because they are usually rare in late developing and adult tissues. The numerous re-explant cultures required for the progressive amplification process can result in a spontaneous differentiation process. This aspect has been implicated in the tumorigenicity of stem cells when transplanted into a tissue. The aim of this study has been to determine whether cochlear stem cells can proliferate and differentiate spontaneously in long-term cultures without the addition of any factor that might influence these processes. Cochlear stem cells, which express nestin protein, were cultured in monolayers and fed with DMEM containing 5% FBS. They quickly organized themselves into typical spheres exhibiting a high proliferation rate, self-renewal property, and differentiation ability. Secondary cultures of these stem cell spheres spontaneously differentiated into neuroectodermal-like cells. The expression of nestin, glial-fibrillary-acidic protein, vimentin, and neurofilaments was evaluated to identify early differentiation. Nestin expression appeared in primary and secondary cultures. Other markers were also identified in differentiating cells. Further research might demonstrate the spontaneous differentiation of cochlear stem cells and their teratogenic probability when they are used for transplantation.

A symphony of inner ear developmental control genes

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

From shared lineage to distinct functions: the development of the inner ear and epibranchial placodes

The inner ear and the epibranchial ganglia constitute much of the sensory system in the caudal vertebrate head. The inner ear consists of mechanosensory hair cells, their neurons, and structures necessary for sound and balance sensation. The epibranchial ganglia are knots of neurons that innervate and relay sensory signals from several visceral organs and the taste buds. Their development was once thought to be independent, in line with their independent functions. However, recent studies indicate that both systems arise from a morphologically distinct common precursor domain: the posterior placodal area. This review summarises recent studies into the induction, morphogenesis and innervation of these systems and discusses lineage restriction and cell specification in the context of their common origin.

Hedgehog signaling governs the development of otic sensory epithelium and its associated innervation in zebrafish

The inner ear is responsible for the perception of motion and sound in vertebrates. Its functional unit, the sensory patch, contains mechanosensory hair cells innervated by sensory neurons from the statoacoustic ganglion (SAG) that project to the corresponding nuclei in the brainstem. How hair cells develop at specific positions, and how otic neurons are sorted to specifically innervate each endorgan and to convey the extracted information to the hindbrain is not completely understood. In this work, we study the generation of macular sensory patches and investigate the role of Hedgehog (Hh) signaling in the production of their neurosensory elements. Using zebrafish transgenic lines to visualize the dynamics of hair cell and neuron production, we show that the development of the anterior and posterior maculae is asynchronic, suggesting they are independently regulated. Tracing experiments demonstrate the SAG is topologically organized in two different neuronal subpopulations, which are spatially segregated and innervate specifically each macula. Functional experiments identify the Hh pathway as crucial in coordinating the production of hair cells in the posterior macula, and the formation of its specific innervation. Finally, gene expression analyses suggest that Hh influences the balance between different SAG neuronal subpopulations. These results lead to a model in which Hh orients functionally the development of inner ear towards an auditory fate in all vertebrate species.

Hoxa1 lineage tracing indicates a direct role for Hoxa1 in the development of the inner ear, the heart, and the third rhombomere

Loss of Hoxa1 function results in severe defects of the brainstem, inner ear, and cranial ganglia in humans and mice as well as cardiovascular abnormalities in humans. Because Hoxa1 is expressed very transiently during an early embryonic stage, it has been difficult to determine whether Hoxa1 plays a direct role in the precursors of the affected organs or if all defects result from indirect effects due to mispatterning of the hindbrain. In this study we use a Hoxa1-IRES-Cre mouse to genetically label the early Hoxa1-expressing cells and determine their contribution to each of the affected organs, allowing us to conclude in which precursor tissue Hoxa1 is expressed. We found Hoxa1 lineage-labeled cells in all tissues expected to be derived from the Hoxa1 domain, such as the facial and abducens nuclei and nerves as well as r4 neural crest cells. In addition, we detected the lineage in derivatives that were not thought to have expressed Hoxa1 during development. In the brainstem, the anterior border of the lineage was found to be in r3, which is more anterior than previously reported. We also observed an interesting pattern of the lineage in the inner ear, namely a strong contribution to the otic epithelium with the exception of sensory patches. Moreover, lineage-labeled cells were detected in the atria and outflow tract of the developing heart. In conclusion, Hoxa1 lineage tracing uncovered new domains of Hoxa1 expression in rhombomere 3, the otic epithelium, and cardiac precursors, suggesting a more direct role for Hoxa1 in development of these tissues than previously believed.

Retinoic acid receptors exhibit cell-autonomous functions in cranial neural crest cells

Previous work has emphasized the crucial role of retinoic acid (RA) in the ontogenesis of the vast majority of mesenchymal structures derived from the neural crest cells (NCC), which migrate through, or populate, the frontonasal process and branchial arches. Using somatic mutagenesis in the mouse, we have selectively ablated two or three retinoic acid receptors (i.e., RARalpha/RARbeta, RARalpha/RARgamma and RARalpha/RARbeta/RARgamma) in NCC. By rigorously analyzing these mutant mice, we found that survival and migration of NCC is normal until gestational day 10.5, suggesting that RAR-dependent signaling is not intrinsically required for the early steps of NCC development. However, ablation of Rara and Rarg genes in NCC yields an agenesis of the median portion of the face, demonstrating that RARalpha and RARgamma act cell-autonomously in postmigratory NCC to control the development of structures derived from the frontonasal process. In contrast, ablation of the three Rar genes in NCC leads to less severe defects of the branchial arches derived structures compared with Rar compound null mutants. Therefore, RARs exert a function in the NCC as well as in a separated cell population. This work demonstrates that RARs use distinct mechanisms to pattern cranial NCC.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Requirement for Lmo4 in the vestibular morphogenesis of mouse inner ear

During development, compartmentalization of an early embryonic structure produces blocks of cells with distinct properties and developmental potentials. The auditory and vestibular components of vertebrate inner ears are derived from defined compartments within the otocyst during embryogenesis. The vestibular apparatus, including three semicircular canals, saccule, utricle, and their associated sensory organs, detects angular and linear acceleration of the head and relays the information through vestibular neurons to vestibular nuclei in the brainstem. How the early developmental events manifest vestibular structures at the molecular level is largely unknown. Here, we show that LMO4, a LIM-domain-only transcriptional regulator, is required for the formation of semicircular canals and their associated sensory cristae. Targeted disruption of Lmo4 resulted in the dysmorphogenesis of the vestibule and in the absence of three semicircular canals, anterior and posterior cristae. In Lmo4-null otocysts, canal outpouches failed to form and cell proliferation was reduced in the dorsolateral region. Expression analysis of the known otic markers showed that Lmo4 is essential for the normal expression of Bmp4, Fgf10, Msx1, Isl1, Gata3, and Dlx5 in the dorsolateral domain of the otocyst, whereas the initial compartmentalization of the otocyst remains unaffected. Our results demonstrate that Lmo4 controls the development of the dorsolateral otocyst into semicircular canals and cristae through two distinct mechanisms: regulating the expression of otic specific genes and stimulating the proliferation of the dorsolateral part of the otocyst.

Expression patterns of miR-96, miR-182 and miR-183 in the development inner ear

MicroRNAs (miRNAs) constitute a class of small non-coding endogenous RNAs that downregulate gene expression by binding to 3' untranslated region (UTR) of target messenger RNAs. Although they have been found to regulate developmental and physiological processes in several organs and tissues, their role in the regulation of the inner ear transcriptome remains unknown. In this report, we have performed systematic in situ hybridization to analyze the temporal and spatial distribution of three miRNAs (miR-96, mR-182, and mR-183) that are likely to arise from a single precursor RNA during the development and the maturation of the cochlea. Strikingly we found that the expression of mR-96, mR-182 and mR-183 was highly dynamic during the development of the cochlea, from the patterning to the differentiation of the main cochlear structures.

Sox10 promotes the survival of cochlear progenitors during the establishment of the organ of Corti

Transcription factors of the SoxE family are critical players that underlie various embryological processes. However, little is known about their function during inner ear development. Here, we show that Sox10 is initially expressed throughout the otic vesicle epithelium and becomes later restricted to supporting cells as cell differentiation proceeds in the organ of Corti. Morphological analyses of Sox10 mutant mice reveal a significant shortening of the cochlear duct likely resulting from the progressive depletion of cochlear progenitors. While Sox10 appears dispensable for the differentiation and patterning of the inner ear prosensory progenitors, our data support a critical role for this transcription factor in the promotion of their survival. We provide genetic evidences that Sox10, in a concentration-dependant manner, could play a role in the regulation of Jagged1, a gene known to be important for inner ear prosensory development. Together, our results demonstrate that Sox10 regulates the biology of early cochlear progenitors during inner ear development, but, in contrast to neural crest-derived cells, this transcription factor is dispensable for their differentiation. Evidence also suggests that this effect occurs via the activation of the Jagged1 gene.

Building the world's best hearing aid; regulation of cell fate in the cochlea

In mammals, auditory perception is initially mediated through sensory cells located in a rigorously patterned mosaic of unique cell types located within the coiled cochlea. Identification of the factors that direct multipotent progenitor cells to develop as each of these specialized cell types has the potential to enhance our understanding of the development of the auditory system and to identify potential targets for regenerative therapies. Recent results have identified specific signaling molecules and pathways, including Notch, Hedgehog, Sox2 and Fgfs, that guide progenitor cells to develop first as a sensory precursor, referred to as a prosensory cell, and subsequently as one of the specialized cell types within the sensory mosaic.

Gene therapy in the inner ear using adenovirus vectors

Therapies for the protection and regeneration of auditory hair cells are of great interest given the significant monetary and lifestyle impact of hearing loss. The past decade has seen tremendous advances in the use of adenoviral vectors to achieve these aims. Preliminary data demonstrated the functional capacity of this technique as adenoviral-induced expression of neurotrophic and growth factors protected hair cells and spiral ganglion neurons from ototoxic insults. Subsequent efforts confirmed the feasibility of adenoviral transfection of cells in the auditory neuroepithelium via cochleostomy into the scala media. Most recently, efforts have focused on regeneration of depleted hair cells. Mammalian hearing loss is generally considered a permanent insult as the auditory epithelium lacks a basal layer capable of producing new hair cells. Recently, the transcription factor Atoh1 has been found to play a critical role in hair cell differentiation. Adenoviral-mediated overexpression of Atoh1 in culture and in vivo have shown the ability to regenerate auditory and vestibular hair cells by causing transdifferentiation of neighboring epithelial-supporting cells. Functional recovery of both the auditory and vestibular systems has been documented following adenoviral induced Atoh1 overexpression.

Competence, specification and commitment to an olfactory placode fate

The nasal placode shares a common origin with other sensory placodes within a pre-placodal domain at the cranial neural plate border. However, little is known about early events in nasal placode development as it segregates from prospective lens, neural tube and epidermis. Here, Dlx3, Dlx5, Pax6 and the pan-neuronal marker Hu serve as molecular labels to follow the maturation of olfactory precursors over time. When competence to form olfactory placode was tested by grafting ectoderm from different axial levels to the anterior neural fold, we found that competence is initially broad for head, but not trunk, ectoderm and declines rapidly with time. Isolated olfactory precursors are specified by HH10, concomitant with their complete segregation from other placodal, epidermal and neural progenitors. Heterotopic transplantation of olfactory progenitors reveals they are capable of autonomous differentiation only 12 hours later, shortly before overt placode invagination at HH14. Taken together, these results show that olfactory placode development is a step-wise process whereby signals from adjacent tissues specify competent ectoderm at or before HH10, followed by gradual commitment just prior to morphological differentiation.

Rapid identification of PAX2/5/8 direct downstream targets in the otic vesicle by combinatorial use of bioinformatics tools

A novel bioinformatics pipeline is used to discover PAX2/5/8 direct downstream targets involved in inner ear development.

Background

The pax2/5/8 genes belonging to the PAX family of transcription factors are key developmental regulators that are involved in the patterning of various embryonic tissues. More particularly, their function in inner ear specification has been widely described. However, little is known about the direct downstream targets and, so far, no global approaches have been performed to identify these target genes in this particular tissue.

Results

Here we present an original bioinformatics pipeline composed of comparative genomics, database querying and text mining tools, which is designed to rapidly and specifically discover PAX2/5/8 direct downstream targets involved in inner ear development. We provide evidence supported by experimental validation in medaka fish that brain 2 (POU domain, class 3, transcription factor 2), claudin-7, secretory pathway component sec31-like and meteorin-like precursor are novel direct downstream targets of PAX2/5/8.

Conclusions

This study illustrates the power of extensive mining of public data repositories using bioinformatics methods to provide answers for a specific biological question. It furthermore demonstrates how the usage of such a combinatorial approach is advantageous for the biologist in terms of experimentation time and costs.

Hindbrain-derived Wnt and Fgf signals cooperate to specify the otic placode in Xenopus

Induction of the otic placode, the rudiment of the inner ear, is believed to depend on signals derived from surrounding tissues, the head mesoderm and the prospective hindbrain. Here we report the first attempt to define the specific contribution of the neuroectoderm to this inductive process in Xenopus. To this end we tested the ability of segments of the neural plate (NP), isolated from different axial levels, to induce the otic marker Pax8 when recombined with blastula stage animal caps. We found that one single domain of the NP, corresponding to the prospective anterior hindbrain, had Pax8-inducing activity in this assay. Surprisingly, more than half of these recombinants formed otic vesicle-like structures. Lineage tracing experiments indicate that these vesicle-like structures are entirely derived from the animal cap and express several pan-otic markers. Pax8 activation in these recombinants requires active Fgf and canonical Wnt signaling, as interference with either pathway blocks Pax8 induction. Furthermore, we demonstrate that Fgf and canonical Wnt signaling cooperate to activate Pax8 expression in isolated animal caps. We propose that in the absence of mesoderm cues the combined activity of hindbrain-derived Wnt and Fgf signals specifies the otic placode in Xenopus, and promotes its morphogenesis into an otocyst.

Epithelial-mesenchymal transition as a possible mechanism of semicircular canal morphogenesis in chick inner ear

Semicircular canals are sensory organs for balance, consisting of fluid-filled tubules that are arranged perpendicularly to each other in inner ear. The precise mechanism of the morphogenesis of this unique organ is still under investigation. Semicircular canals arise from the flattened pouch of epithelium. The centers of two apposing wall of the pouch approach each other and form a fusion plate. The clearing of the fusion plate makes a hole and leaves the remaining tissue as semicircular canals. Three mechanisms have been proposed for this clearing: programmed cell death, epithelial-mesenchymal transition, and retraction of the cells in the fusion plate to surrounding semicircular canals. Previous studies have revealed programmed cell death in the fusion plate, although other two hypotheses were not disproved. Here we examined the contribution of epithelial-mesenchymal transition and epithelial retraction to the morphogenesis of semicircular canals. We analyzed immunohistochemically the structural change in the epithelium of the developing fusion plate using molecular markers, basal lamina component laminin, cytoskeletal F-actin, and cellular junctional marker beta-catenin. Our observation revealed that fusion plate epithelium lost its apico-basal polarity and intermingled with facing fusion plate cells, associated with the disruption of basal lamina. Moreover, there were several cells with mesenchymal appearance adjacent to the torn basal lamina. We also found the merging of apposing basal laminae at the border between forming canal and breaking fusion plate. These observations suggest that the epithelial-mesenchymal transition, rather than the epithelial retraction, may be responsible for clearing fusion plate cells.

The transmembrane inner ear (tmie) gene contributes to vestibular and lateral line development and function in the zebrafish (Danio rerio)

The inner ear is a complex organ containing sensory tissue, including hair cells, the development of which is not well understood. Our long-term goal is to discover genes critical for the correct formation and function of the inner ear and its sensory tissue. A novel gene, transmembrane inner ear (Tmie), was found to cause hearing-related disorders when defective in mice and humans. A homologous tmie gene in zebrafish was cloned and its expression characterized between 24 and 51 hours post-fertilization. Embryos injected with morpholinos (MO) directed against tmie exhibited circling swimming behavior (approximately 37%), phenocopying mice with Tmie mutations; semicircular canal formation was disrupted, hair cell numbers were reduced, and maturation of electrically active lateral line neuromasts was delayed. As in the mouse, tmie appears to be required for inner ear development and function in the zebrafish and for hair cell maturation in the vestibular and lateral line systems as well.

Retinoic acid-induced inner ear teratogenesis caused by defective Fgf3/Fgf10-dependent Dlx5 signaling

BACKGROUND

Retinoic acid (RA) is essential for inner ear development. However, exposure to excess RA at a critical period leads to inner ear defects. These defects are associated with disruption in epithelial-mesenchymal interactions.

METHODS

This study investigates the role of Dlx5 in the epithelial-mesenchymal interactions that guide otic capsule chondrogenesis, as well as the effect of excess in utero RA exposure on Dlx5 expression in the developing mouse inner ear. Control of Dlx5 by Fgf3 and Fgf10 under excess RA conditions is investigated by examining the developmental window during which Fgf3 and Fgf10 are altered by in utero RA exposure and by testing the ability of Fgf3 and Fgf10 to mitigate the reduction in chondrogenesis and Dlx5 expression mediated by RA in high-density cultures of periotic mesenchyme containing otic epithelium, a model of epithelial-mesenchymal interactions in which chondrogenic differentiation of periotic mesenchyme ensues in response to induction by otic epithelium.

RESULTS

Dlx5 deletion alters expression of TGFbeta(1), important for otic capsule chondrogenesis, in the developing inner ear and compromises the ability of cultured periotic mesenchyme containing otic epithelium, harvested from Dlx5 null embryos, to differentiate into cartilage when compared with control cultures. Downregulation in Dlx5 ensues as a consequence of in utero RA exposure in association with inner ear dysmorphogenesis. This change in Dlx5 is noted at embryonic day 10.5 (E10.5), but not at E9.5, suggesting that Dlx5 is not a direct RA target. Before Dlx5 downregulation, Fgf3 and Fgf10 expression is modified in the inner ear by excess RA, with the ability of exogenous Fgf3 and Fgf10 to rescue chondrogenesis and Dlx5 expression in RA-treated cultures of periotic mesenchyme containing otic epithelium supporting these fibroblast growth factors (FGFs) as intermediary genes by which RA mediates its effects.

CONCLUSIONS

Disruption in an Fgf3, -10/Dlx5 signaling cascade is operant in molecular mechanisms of inner ear teratogenesis by excess RA.

Notch signaling augments the canonical Wnt pathway to specify the size of the otic placode

The inner ear derives from a patch of ectoderm defined by expression of the transcription factor Pax2. We recently showed that this Pax2(+) ectoderm gives rise not only to the otic placode but also to the surrounding cranial epidermis, and that Wnt signaling mediates this placode-epidermis fate decision. We now present evidence for reciprocal interactions between the Wnt and Notch signaling pathways during inner ear induction. Activation of Notch1 in Pax2(+) ectoderm expands the placodal epithelium at the expense of cranial epidermis, whereas loss of Notch1 leads to a reduction in the size of the otic placode. We show that Wnt signaling positively regulates Notch pathway genes such as Jag1, Notch1 and Hes1, and we have used transgenic Wnt reporter mice to show that Notch signaling can modulate the canonical Wnt pathway. Gain- and loss-of-function mutations in the Notch and Wnt pathways reveal that some aspects of otic placode development - such as Pax8 expression and the morphological thickening of the placode - can be regulated independently by either Notch or Wnt signals. Our results suggest that Wnt signaling specifies the size of the otic placode in two ways, by directly upregulating a subset of otic genes, and by positively regulating components of the Notch signaling pathway, which then act to augment Wnt signaling.

The molecular biology of ear development - "Twenty years are nothing"

Views of classical biological problems changed dramatically with the rise of molecular biology as a common framework. It was indeed the new language of life sciences. Molecular biology increasingly moved us towards a unified view of developmental genetics as ideas and techniques were imported to vertebrates from other biological systems where genetics was in a more advanced state. The ultimate advance has been the ability to actually perform genetic manipulations in vertebrate organisms that were almost unthinkable before. During the last two decades these technical advances entered into and affected the research on ear development. These events are still very recent and have been with us for no longer than two decades, which is the reason for the title of this article. This new scenario forms the basis of the current and productive work of many laboratories, and this is what this Special Issue of The International Journal of Developmental Biology wants to show, presenting a snapshot of insights at the beginning of the 21st Century. In this article, we give an overview of the topics that are addressed in this Ear Development Special Issue, and also we take the opportunity to informally dig into the genealogy of some of those topics, trying to link the current work with some classical work of the past.

Molecular evolution of the vertebrate mechanosensory cell and ear

The molecular basis of mechanosensation, mechanosensory cell development and mechanosensory organ development is reviewed with an emphasis on its evolution. In contrast to eye evolution and development, which apparently modified a genetic program through intercalation of genes between the master control genes on the top (Pax6, Eya1, Six1) of the hierarchy and the structural genes (rhodopsin) at the bottom, the as yet molecularly unknown mechanosensory channel precludes such a firm conclusion for mechanosensors. However, recent years have seen the identification of several structural genes which are involved in mechanosensory tethering and several transcription factors controlling mechanosensory cell and organ development; these warrant the interpretation of available data in very much the same fashion as for eye evolution: molecular homology combined with potential morphological parallelism. This assertion of molecular homology is strongly supported by recent findings of a highly conserved set of microRNAs that appear to be associated with mechanosensory cell development across phyla. The conservation of transcription factors and their regulators fits very well to the known or presumed mechanosensory specializations which can be mostly grouped as variations of a common cellular theme. Given the widespread distribution of the molecular ability to form mechanosensory cells, it comes as no surprise that structurally different mechanosensory organs evolved in different phyla, presenting a variation of a common theme specified by a conserved set of transcription factors in their cellular development. Within vertebrates and arthropods, some mechanosensory organs evolved into auditory organs, greatly increasing sensitivity to sound through modifications of accessory structures to direct sound to the specific sensory epithelia. However, while great attention has been paid to the evolution of these accessory structures in vertebrate fossils, comparatively less attention has been spent on the evolution of the inner ear and the central auditory system. Recent advances in our molecular understanding of ear and brain development provide novel avenues to this neglected aspect of auditory neurosensory evolution.

Development and regeneration of hair cells

The vertebrate inner ear is derived from the otic placode and undergoes a complicated series of morphogenetic processes to differentiate into an elaborate structure harboring mechanosensory epithelia featuring hair cells, the mechanoreceptors of hearing and balance. Recently, the principal mechanisms producing hair cells and the key molecules involved in their fate determination and differentiation have been gradually unveiled. The in-depth understanding of hair cell development is consequently providing clues to strategies for mammalian hair cell regeneration. Among them, the identification and characterization of progenitor cells for the hair cell lineage, which is just emerging, is of particular interest. Herein, we review the molecular mechanisms of inner ear development with particular focus on perspectives for hair cell regeneration.

Cadherin expression in the developing chicken cochlea

In this study, we demonstrate that eight classic cadherins are differentially expressed in distinct anatomical regions of the cochlea during late stages of chicken embryonic development. Cadherin-6B is expressed in hair cells and spindle-shaped cells, while cadherin-8 mRNA is found only in supporting cells. Cadherin-11 is widely expressed not only in mesenchymal cell around the cochlea, but also in supporting cells and homogene cells. N-cadherin is found in the sensory epithelium, the neurons of the acoustic ganglion and on their neurites that target the hair cells. Three closely related cadherins (cadherin-7, cadherin-19, and cadherin-20) are expressed in a partially complementary manner in spindle-shaped cells and acoustic ganglion cells. R-cadherin is observed in homogene cells, acoustic ganglion cells, and their projections to hair cells. The expression of classic cadherins in the developing cochlea suggests a role for cadherins in the development of the cochlea.

Developing a sense of scents: plasticity in olfactory placode formation

The sense organs of the vertebrate head arise predominantly from sensory placodes. The sensory placodes have traditionally been grouped as structures that share common developmental and evolutionary characteristics. In attempts to build a coherent model for development of all placodes, the fascinating differences that make placodes unique are often overlooked. Here I review olfactory placode development with special attention to the origin and cell movements that generate the olfactory placode, the derivatives of this sensory placode, and the degree to which it shows plasticity during development. Next, through comparison with adenohypophyseal, and lens placodes I suggest we revise our thinking and terminology for these anterior placodes, specifically by: (1) referring to the peripheral olfactory sensory system as neural ectoderm because it expresses the same series of genes involved in neural differentiation and differentiates in tandem with the olfactory bulb, and (2) grouping the anterior placodes with their corresponding central nervous system structures and emphasizing patterning mechanisms shared between placodes and these targets. Sensory systems did not arise independent of the central nervous system; they are part of a functional unit composed of peripheral sensory structures and their targets. By expanding our analyses of sensory system development to also include cell movements, gene expression and morphological changes observed in this functional unit, we will better understand the evolution of sensory structures.

Expression of marker genes during otolith development in medaka

Little is known about the genes and processes involved in the development of otoliths. In this study, we isolated the biomineralization-related genes otolin and chondromodulin-1 (chm1) from medaka, and examined their spatiotemporal expression pattern as well as that of two other genes also related to biomineralization, i.e., sparc/osteonectin and type II collagen (col2a), during otic development in medaka. Our results demonstrated that all the tested genes were expressed in the otic vesicle, and that chm1 was exclusively expressed in the semicircular canal of the otic vesicle.

Insulin-like growth factor binding protein (IGFBP)-mediated hair cell survival on the mouse utricle exposed to neomycin: the roles of IGFBP-4 and IGFBP-5

CONCLUSION

This study suggests for the first time that 1) IGF-I, IGFBP-4, and -5 alone and IGF-I+IGFBP-5 mixture stimulated hair cell survival and prevented neomycin-induced hair cell loss in the sensory epithelial culture of mouse utricles, 2) When administered together, IGFBP-4 diminished the effect of IGF-I, 3) In P3-5 mice utricle, IGF-I, IGFBP-4, and IGFBP-5 are expressed in the cytoplasm of hair cells. And Insulin/IGF-I Receptor is expressed in the nucleus of hair cells.

OBJECTIVES

Several growth factors have been demonstrated to protect auditory sensory cells in vitro and in vivo from aminoglycoside toxicity. IGF-I is one of the most well-known mitogenic and protective substance working in the inner ear. However, there are no reports available regarding the function of IGFBPs in the inner ear. In the present study, the effects of IGFBP-4 and -5 on hair cell survival were investigated in mouse utriclular organ cultures.

MATERIALS AND METHODS

The amount of cellular damage and cell viability in vestibular organs were assessed by counting hair cells stained with a rhodamine-phalloidin probe. The expressions of IGFBP-4, IGFBP-5, IGF-IR, and IGF-I were localized by immunohistochemistry.

RESULTS

When treated with IGF-I, IGFBP-4, or IGFBP-5 for 24 h, explant culture showed hair cell survival rates of 136+/-18%, 140+/-15%, and 133+/-6%, respectively, compared to controls. Neomycin (1 mM) induced hair cell loss resulted in 45+/-17% of hair cell survival. However, pre-treatment of IGF-I, IGFBP-4, or -5 before neomycin insult showed survival rates of 113+/-14%, 98+/-8%, and 73+/-24%, respectively. Similar to IGF-I, IGFBP-4 and IGFBP-5 were significantly protective. IGFBP-4 and -5 immunoreactivities were observed in the cytoplasm of normal explanted vestibular hair cells as well as in the P3 mouse utricular hair cells in vivo.

Differential expression of Sox2 and Sox3 in neuronal and sensory progenitors of the developing inner ear of the chick

The generation of the mechanosensory elements of the inner ear during development proceeds in a precise temporal and spatial pattern. First, neurosensory precursors form sensory neurons. Then, prosensory patches emerge and give rise to hair and supporting cells. Hair cells are innervated by cochleovestibular neurons that convey sound and balance information to the brain. SOX2 is an HMG transcription factor characteristic of the stem-cell genetic network responsible for progenitor self-renewal and commitment, and its loss of function generates defects in ear sensory epithelia. The present study shows that SOX2 protein is expressed in a spatially and temporally restricted manner throughout development of the chick inner ear. SOX2 is first expressed in the neurogenic region that gives rise to sensory neurons. SOX2 is then restricted to the prosensory patches in E4 and E5 embryos, as revealed by double and parallel labelling with SOX2 and Tuj1, MyoVIIa, or Islet1. Proliferating cell nuclear antigen labelling showed that SOX2 is expressed in proliferating cells during those stages. By E5, SOX2 is also expressed in the Schwann cells of the cochleovestibular ganglion, but not in the otic neurons. At E8 and E17, beyond stages of sensory cell specification, SOX2 is transiently expressed in hair cells, but its level remains high in supporting cells. SOX3 is concomitantly expressed with SOX2 in the neurogenic domain of the otic cup, but not in prosensory patches. Our data are consistent with a role for SOX2 in specifying a population of otic progenitors committed to a neural fate, giving rise to neurons and hair cells.

Genetic and epigenetic mechanisms of gene regulation during lens development

Recent studies demonstrated a number of links between chromatin structure, gene expression, extracellular signaling and cellular differentiation during lens development. Lens progenitor cells originate from a pool of common progenitor cells, the pre-placodal region (PPR) which is formed from a combination of extracellular signaling between the neural plate, naïve ectoderm and mesendoderm. A specific commitment to the lens program over alternate choices such as the formation of olfactory epithelium or the anterior pituitary is manifested by the formation of a thickened surface ectoderm, the lens placode. Mouse lens progenitor cells are characterized by the expression of a complement of lens lineage-specific transcription factors including Pax6, Six3 and Sox2, controlled by FGF and BMP signaling, followed later by c-Maf, Mab21like1, Prox1 and FoxE3. Proliferation of lens progenitors together with their morphogenetic movements results in the formation of the lens vesicle. This transient structure, comprised of lens precursor cells, is polarized with its anterior cells retaining their epithelial morphology and proliferative capacity, whereas the posterior lens precursor cells initiate terminal differentiation forming the primary lens fibers. Lens differentiation is marked by expression and accumulation of crystallins and other structural proteins. The transcriptional control of crystallin genes is characterized by the reiterative use of transcription factors required for the establishment of lens precursors in combination with more ubiquitously expressed factors (e.g. AP-1, AP-2alpha, CREB and USF) and recruitment of histone acetyltransferases (HATs) CBP and p300, and chromatin remodeling complexes SWI/SNF and ISWI. These studies have poised the study of lens development at the forefront of efforts to understand the connections between development, cell signaling, gene transcription and chromatin remodeling.

Regulation of otic vesicle and hair cell stereocilia morphogenesis by Ena/VASP-like (Evl) in Xenopus

The inner ear is derived from a thickening in the embryonic ectoderm, called the otic placode. This structure undergoes extensive morphogenetic movements throughout its development and gives rise to all components of the inner ear. Ena/VASP-like (Evl) is an actin binding protein involved in the regulation of cytoskeletal dynamics and organization. We have examined the role of Evl during the morphogenesis of the Xenopus inner ear. Evl (hereafter referred to as Xevl) is expressed throughout otic vesicle formation and is enriched in the neuroblasts that delaminate to form the vestibulocochlear ganglion and in hair cells that possess mechanosensory stereocilia. Knockdown of Xevl perturbs epithelial morphology and intercellular adhesion in the otic vesicle and disrupts formation of the vestibulocochlear ganglion, evidenced by reduction of ganglion size, disorganization of the ganglion, and defects in neurite outgrowth. Later in embryogenesis, Xevl is required for development of mechanosensory hair cells. In Xevl knockdown embryos, hair cells of the ventromedial sensory epithelium display multiple abnormalities including disruption of the cuticular plate at the base of stereocilia and disorganization of the normal staircase appearance of stereocilia. Based on these data, we propose that Xevl plays an integral role in regulating morphogenesis of the inner ear epithelium and the subsequent development of the vestibulocochlear ganglion and mechanosensory hair cells.

Changes in retinoic acid signaling alter otic patterning

Retinoic acid (RA) has pleiotropic functions during embryogenesis. In zebrafish, increasing or blocking RA signaling results in enlarged or reduced otic vesicles, respectively. Here we elucidate the mechanisms that underlie these changes and show that they have origins in different tissues. Excess RA leads to ectopic foxi1 expression throughout the entire preplacodal domain. Foxi1 provides competence to adopt an otic fate. Subsequently, pax8, the expression of which depends upon Foxi1 and Fgf, is also expressed throughout the preplacodal domain. By contrast, loss of RA signaling does not affect foxi1 expression or otic competence, but instead results in delayed onset of fgf3 expression and impaired otic induction. fgf8 mutants depleted of RA signaling produce few otic cells, and these cells fail to form a vesicle, indicating that Fgf8 is the primary factor responsible for otic induction in RA-depleted embryos. Otic induction is rescued by fgf8 overexpression in RA-depleted embryos, although otic vesicles never achieve a normal size, suggesting that an additional factor is required to maintain otic fate. fgf3;tcf2 double mutants form otic vesicles similar to RA-signaling-depleted embryos, suggesting a signal from rhombomere 5-6 may also be required for otic fate maintenance. We show that rhombomere 5 wnt8b expression is absent in both RA-signaling-depleted embryos and in fgf3;tcf2 double mutants, and inactivation of wnt8b in fgf3 mutants by morpholino injection results in small otic vesicles, similar to RA depletion in wild type. Thus, excess RA expands otic competence, whereas the loss of RA impairs the expression of fgf3 and wnt8b in the hindbrain, compromising the induction and maintenance of otic fate.

Opposing gradients of Gli repressor and activators mediate Shh signaling along the dorsoventral axis of the inner ear

Organization of the vertebrate inner ear is mainly dependent on localized signals from surrounding tissues. Previous studies demonstrated that sonic hedgehog (Shh) secreted from the floor plate and notochord is required for specification of ventral (auditory) and dorsal (vestibular) inner ear structures, yet it was not clear how this signaling activity is propagated. To elucidate the molecular mechanisms by which Shh regulates inner ear development, we examined embryos with various combinations of mutant alleles for Shh, Gli2 and Gli3. Our study shows that Gli3 repressor(R) is required for patterning dorsal inner ear structures, whereas Gli activator (A) proteins are essential for ventral inner ear structures. A proper balance of Gli3R and Gli2/3A is required along the length of the dorsoventral axis of the inner ear to mediate graded levels of Shh signaling,emanating from ventral midline tissues. Formation of the ventral-most otic region, the distal cochlear duct, requires robust Gli2/3A function. By contrast, the formation of the proximal cochlear duct and saccule, which requires less Shh signaling, is achieved by antagonizing Gli3R. The dorsal vestibular region requires the least amount of Shh signaling in order to generate the correct dose of Gli3R required for the development of this otic region. Taken together, our data suggest that reciprocal gradients of GliA and GliR mediate the responses to Shh signaling along the dorsoventral axis of the inner ear.

Distinct roles of Fgf8, Foxi1, Dlx3b and Pax8/2 during otic vesicle induction and maintenance in medaka

The development of the vertebrate inner ear is a complex process that has been investigated in several model organisms. In this work, we examined genetic interactions regulating early development of otic structures in medaka. We demonstrate that misexpression of Fgf8, Dlx3b and Foxi1 during early gastrulation is sufficient to produce ectopic otic vesicles. Combined misexpression strongly increases the appearance of this phenotype. By using a heat-inducible promoter we were furthermore able to separate the regulatory interactions among Fgf8, Foxi1, Dlx3b, Pax8 and Pax2 genes, which are active during different stages of early otic development. In the preplacodal stage we suggest a central position of Foxi1 within a regulatory network of early patterning genes including Dlx3b and Pax8. Different pathways are active after the placodal stage with Dlx3b playing a central role. There Dlx3b regulates members of the Pax-Six-Eya-Dach network and also strongly affects the early dorsoventral marker genes Otx1 and Gbx2.

Early regionalization of the otic placode and its regulation by the Notch signaling pathway

Otic neuronal precursors are the first cells to be specified and do so in the anterior domain of the otic placode, the proneural domain. In the present study, we have explored the early events of otic proneural regionalization in relation to the activity of the Notch signaling pathway. The proneural domain was characterized by the expression of Sox3, Fgf10 and members of the Notch pathway such as Delta1, Hes5 and Lunatic Fringe. The complementary non-neural domain expressed two patterning genes, Lmx1b and Iroquois1, and the members of the Notch pathway, Serrate1 and Hairy1. Fate map studies and double injections with DiI/DiO showed that labeled cells remained confined to anterior or posterior territories with limited cell intermingling. To explore whether Notch signaling pathway plays a role in the initial regionalization of the otic placode, Notch activity was blocked by a gamma-secretase inhibitor (DAPT). Notch blockade induced the expansion of non-neural genes, Lmx1 and Iroquois1, into the proneural domain. Combined gene expression and DiI experiments showed that these effects were not due to migration of non-neural cells into the proneural domain, suggesting that Notch activity regulates the expression of non-neural genes. This was further confirmed by the electroporation of a dominant-negative form of the Mastermind-like1 gene that caused the up-regulation of Lmx1 within the proneural domain. In addition, Notch pathway was involved in neuronal precursor selection, probably by a classical mechanism of lateral inhibition. We propose that the regionalization of the otic domain into a proneural and a non-neural territory is a very early event in otic development, and that Notch signaling activity is required to exclude the expression of non-neural genes from the proneural territory.

Foxe view of lens development and disease

The recent identification of a mutation in Foxe3 that causes congenital primary aphakia in humans marks an important milestone. Congenital primary aphakia is a rare developmental disease in which the lens does not form. Previously, Foxe3 had been shown to play a crucial role in vertebrate lens formation and this gene is one of the earliest integrators of several signaling pathways that cooperate to form a lens. In this review, we highlight recent advances that have led to a better understanding of the developmental processes and gene regulatory networks involved in lens development and disease.

Fgf-dependent otic induction requires competence provided by Foxi1 and Dlx3b

Background

The inner ear arises from a specialized set of cells, the otic placode, that forms at the lateral edge of the neural plate adjacent to the hindbrain. Previous studies indicated that fibroblast growth factors (Fgfs) are required for otic induction; in zebrafish, loss of both Fgf3 and Fgf8 results in total ablation of otic tissue. Furthermore, gain-of-function studies suggested that Fgf signaling is not only necessary but also sufficient for otic induction, although the amount of induced ectopic otic tissue reported after misexpression of fgf3 or fgf8 varies among different studies. We previously suggested that Foxi1 and Dlx3b may provide competence to form the ear because loss of both foxi1 and dlx3b results in ablation of all otic tissue even in the presence of a fully functional Fgf signaling pathway.

Results

Using a transgenic line that allows us to misexpress fgf8 under the control of the zebrafish temperature-inducible hsp70 promoter, we readdressed the role of Fgf signaling and otic competence during placode induction. We find that misexpression of fgf8 fails to induce formation of ectopic otic vesicles outside of the endogenous ear field and has different consequences depending upon the developmental stage. Overexpression of fgf8 from 1-cell to midgastrula stages leads to formation of no or small otic vesicles, respectively. Overexpression of fgf8 at these stages never leads to ectopic expression of foxi1 or dlx3b, contrary to previous studies that indicated that foxi1 is activated by Fgf signaling. Consistent with our results we find that pharmacological inhibition of Fgf signaling has no effect on foxi1 or dlx3b expression, but instead, Bmp signaling activates foxi1, directly and dlx3b, indirectly. In contrast to early activation of fgf8, fgf8 overexpression at the end of gastrulation, when otic induction begins, leads to much larger otic vesicles. We further show that application of a low dose of retinoic acid that does not perturb patterning of the anterior neural plate leads to expansion of foxi1 and to a massive Fgf-dependent otic induction.

Conclusion

These results provide further support for the hypothesis that Foxi1 and Dlx3b provide competence for cells to respond to Fgf and form an otic placode.

Dan is required for normal morphogenesis and patterning in the developing chick inner ear

During vertebrate inner ear development, compartmentalization of the auditory and vestibular apparatuses along two axes depends on the patterning of transcription factors expressed in a region-specific manner. Although most of the patterning is regulated by extrinsic signals, it is not known how Nkx5.1 and Msx1 are patterned. We focus on Dan, the founding member of the Cerberus/Dan gene family that encodes BMP antagonists, and describe its function in morphogenesis and patterning. First, we confirmed that Dan is expressed in the dorso-medial region of the otic vesicle that corresponds to the presumptive endolymphatic duct and sac (ed/es). Second, we used siRNA knockdown to demonstrate that depletion of Dan induced both a severe reduction in the size of the ed/es and moderate deformities of the semicircular canals and cochlear duct. Depletion of Dan also caused suppression of Nkx5.1 in the dorso-lateral region, suppression of Msx1 in the dorso-medial region, and ectopic induction of Nkx5.1 and Msx1 in the ventro-medial region. Most of these phenotypes also appeared following misexpression of the constitutively active form of BMP receptor type Ib. Thus, Dan is required for the normal morphogenesis of the inner ear and, by inhibiting BMP signaling, for the patterning of the transcription factors Nkx5.1 and Msx1.

Role of Gbx2 and Otx2 in the formation of cochlear ganglion and endolymphatic duct

The boundary of gene expression of transcription factors often plays a role in making a signaling center in development. In the otic vesicle, Gbx2 is expressed in the dorso-medial region including the endolymphatic duct, and Otx2 in the ventral region. Fgf10 is expressed between their expression boundaries, and the cochleovestibular ganglion develops close to the medial side of the Fgf10 expressing domain. Similar expression patterns are observed in the central nervous system, where Otx2 and Gbx2 expression abut at the mid-hindbrain boundary, and the repressive interaction between Otx2 and Gbx2 defines the mid-hindbrain boundary. These analogous expression patterns raise a question about the role of the interaction between Gbx2 and Otx2 in the otic vesicle. To address this, we misexpressed Gbx2 and Otx2 to the otic epithelium. Ectopic Gbx2 expression could repress Otx2 expression and vice versa. In addition, Fgf10 expression was repressed and cochlear ganglion formation was interfered with. Moreover, endolymphatic duct was severely hypomorphic in the Otx2 misexpressing embryos. These results suggest that the interaction between Gbx2 and Otx2 in developing inner ear defines Fgf10 expression domain to induce the cochlear ganglion. It is also suggested that Gbx2 expression is important for the formation of the endolymphatic duct.

Role of the hindbrain in patterning the otic vesicle: a study of the zebrafish vhnf1 mutant

The vertebrate inner ear develops from an ectodermal placode adjacent to rhombomeres 4 to 6 of the segmented hindbrain. The placode then transforms into a vesicle and becomes regionalised along its anteroposterior, dorsoventral and mediolateral axes. To investigate the role of hindbrain signals in instructing otic vesicle regionalisation, we analysed ear development in zebrafish mutants for vhnf1, a gene expressed in the caudal hindbrain during otic induction and regionalisation. We show that, in vhnf1 homozygous embryos, the patterning of the otic vesicle is affected along both the anteroposterior and dorsoventral axes. First, anterior gene expression domains are either expanded along the whole anteroposterior axis of the vesicle or duplicated in the posterior region. Second, the dorsal domain is severely reduced, and cell groups normally located ventrally are shifted dorsally, sometimes forming a single dorsal patch along the whole AP extent of the otic vesicle. Third, and probably as a consequence, the size and organization of the sensory and neurogenic epithelia are disturbed. These results demonstrate that, in zebrafish, signals from the hindbrain control the patterning of the otic vesicle, not only along the anteroposterior axis, but also, as in amniotes, along the dorsoventral axis. They suggest that, despite the evolution of inner ear structure and function, some of the mechanisms underlying the regionalisation of the otic vesicle in fish and amniotes have been conserved.

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

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

Lens Specification Is the Ground State of All Sensory Placodes, from which FGF Promotes Olfactory Identity

The sense organs of the vertebrate head comprise structures as varied as the eye, inner ear, and olfactory epithelium. In the early embryo, these assorted structures share a common developmental origin within the preplacodal region and acquire specific characteristics only later. Here we demonstrate a fundamental similarity in placodal precursors: in the chick all are specified as lens prior to acquiring features of specific sensory or neurogenic placodes. Lens specification becomes progressively restricted in the head ectoderm, initially by FGF and subsequently by signals derived from migrating neural crest cells. We show that FGF8 from the anterior neural ridge is both necessary and sufficient to promote olfactory fate in adjacent ectoderm. Our results reveal that placode precursors share a common ground state as lens and progressive restriction allows the full range of placodal derivatives to form.

Cells, molecules and morphogenesis: the making of the vertebrate ear

The development and evolution of mechanosensory cells and the vertebrate ear is reviewed with an emphasis on delineating the cellular, molecular and developmental basis of these changes. Outgroup comparisons suggests that mechanosensory cells are ancient features of multicellular organisms. Molecular evidence suggests that key genes involved in mechanosensory cell function and development are also conserved among metazoans. The divergent morphology of mechanosensory cells across phyla is interpreted here as 'deep molecular homology' that was in parallel shaped into different forms in each lineage. The vertebrate mechanosensory hair cell and its associated neuron are interpreted as uniquely derived features of vertebrates. It is proposed that the vertebrate otic placode presents a unique embryonic adaptation in which the diffusely distributed ancestral mechanosensory cells became concentrated to generate a large neurosensory precursor population. Morphogenesis of the inner ear is reviewed and shown to depend on genes expressed in and around the hindbrain that interact with the otic placode to define boundaries and polarities. These patterning genes affect downstream genes needed to maintain proliferation and to execute ear morphogenesis. We propose that fibroblast growth factors (FGFs) and their receptors (FGFRs) are a crucial central node to translate patterning into the complex morphology of the vertebrate ear. Unfortunately, the FGF and FGFR genes have not been fully analyzed in the many mutants with morphogenetic ear defects described thus far. Likewise, little information exists on the ear histogenesis and neurogenesis in many mutants. Nevertheless, a molecular mechanism is now emerging for the formation of the horizontal canal, an evolutionary novelty of the gnathostome ear. The existing general module mediating vertical canal growth and morphogenesis was modified by two sets of new genes: one set responsible for horizontal canal morphogenesis and another set for neurosensory formation of the horizontal crista and associated sensory neurons. The dramatic progress in deciphering the molecular basis of ear morphogenesis offers grounds for optimism for translational research toward intervention in human morphogenetic defects of the ear.

Fgf19 expression patterns in the developing chick inner ear

The inner ear is a complex sensorial structure with hearing and balance functions. A key aim of developmental biology is to understand the molecular and cellular mechanisms involved in the induction, patterning and innervation of the vertebrate inner ear. These developmental events could be mediated by the expression of regulating genes, such as the members of the family of Fibroblast Growth Factors (Fgfs). This work reports the detailed spatial and temporal patterns of Fgf19 expression in the developing inner ear from otic cup (stage 14) to 8 embryonic days (stage 34). In the earliest stages, Fgf19 and Fgf8 expressions determine two subdomains within the Fgf10-positive proneural-sensory territory. We show that, from the earliest stages, the Fgf19 expression was detected in the acoustic-vestibular ganglion and the macula utriculi. The Fgf19 gene was also strongly, but transiently, expressed in the macula lagena, whereas the macula neglecta never expressed this gene in the period analysed. The Fgf19 expression was also clearly observed in some borders of various sensory elements. These results could be useful from further investigations into the role of FGF19 in otic patterning.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

The role of Paraxial Protocadherin in Xenopus otic placode development

Vertebrate inner ear develops from its rudiment, otic placode, which later forms otic vesicle and gives rise to tissues comprising the entire inner ear. Although several signaling molecules have been identified as candidates responsible for inner ear specification and patterning, many details remain elusive. Here, we report that Paraxial Protocadherin (PAPC) is required for otic vesicle formation in Xenopus embryos. PAPC is expressed strictly in presumptive otic placode and later in otic vesicle during inner ear morphogenesis. Knockdown of PAPC by dominant-negative PAPC results in the failure of otic vesicle formation and the loss of early inner ear markers Sox9 and Tbx2, suggesting the requirement of PAPC in the early stage of otic vesicle development. However, PAPC alone is not sufficient to induce otic placode formation.