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

Fgf10 expression patterns in the developing chick inner ear

The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. It originates from the otic placode, which invaginates, forming the otic vesicle; the latter gives rise to neurosensory and nonsensory elements of the adult membranous labyrinth. A hypothesis based on descriptive and experimental evidence suggests that the acquisition of discrete sensory patches during evolution of this primordium may be related to subdivision of an early pansensory domain. In order to gain insight into this developmental mechanism, we carried out a detailed analysis of the spatial and temporal expression pattern of the gene Fgf10, by comparing different markers of otic patterning and hair cell differentiation. Fgf10 expression labels a sensory-competent domain included in a Serrate-positive territory from which most of the sensory epithelia arise. Our data show that Fgf10 transcripts are present initially in a narrow ventromedial band of the rudimentary otocyst, extending between its rostral and caudal poles. During development, this Fgf10-expressing area splits repetitively into several separate subareas, creating six of the eight sensory organs present in birds. Only the lateral crista and the macula neglecta were initially Fgf10 negative, although they activated Fgf10 expression after their specification as sensory elements. These results allowed us to determine a timetable of sensory specification in the developing chick inner ear. The comparison of the expression pattern of Fgf10 with those of other markers of sensory differentiation contributes to our understanding of the mechanism by which vertebrate inner ear prosensory domains have arisen during evolution.

Patterning and cell fate in the inner ear: a case for Notch in the chicken embryo

The development of the inner ear provides a beautiful example of one basic problem in development, that is, to understand how different cell types are generated at specific times and domains throughout embryonic life. The functional unit of the inner ear consists of hair cells, supporting cells and neurons, all deriving from progenitor cells located in the neurosensory competent domain of the otic placode. Throughout development, the otic placode resolves into the complex inner ear labyrinth, which holds the auditory and vestibular sensory organs that are innervated in a highly specific manner. How does the early competent domain of the otic placode give rise to the diverse specialized cell types of the different sensory organs of the inner ear? We review here our current understanding on the role of Notch signaling in coupling patterning and cell fate determination during inner ear development, with a particular emphasis on contributions from the chicken embryo as a model organism. We discuss further the question of how these two processes rely on two modes of operation of the Notch signaling pathway named lateral induction and lateral inhibition.

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.

BMP4 signaling is involved in the generation of inner ear sensory epithelia

Background

The robust expression of BMP4 in the incipient sensory organs of the inner ear suggests possible roles for this signaling protein during induction and development of auditory and vestibular sensory epithelia. Homozygous BMP4-/- animals die before the inner ear's sensory organs develop, which precludes determining the role of BMP4 in these organs with simple gene knockout experiments.

Results

Here we use a chicken otocyst culture system to perform quantitative studies on the development of inner ear cell types and show that hair cell and supporting cell generation is remarkably reduced when BMP signaling is blocked, either with its antagonist noggin or by using soluble BMP receptors. Conversely, we observed an increase in the number of hair cells when cultured otocysts were treated with exogenous BMP4. BMP4 treatment additionally prompted down-regulation of Pax-2 protein in proliferating sensory epithelial progenitors, leading to reduced progenitor cell proliferation.

Conclusion

Our results implicate BMP4 in two events during chicken inner ear sensory epithelium formation: first, in inducing the switch from proliferative sensory epithelium progenitors to differentiating epithelial cells and secondly, in promoting the differentiation of hair cells within the developing sensory epithelia.

Clonal analysis of the relationships between mechanosensory cells and the neurons that innervate them in the chicken ear

In vertebrates, hair-cell-bearing mechanosensory organs and the neurons that innervate them share a common placodal origin. In the inner ear, the peripheral neurons for both auditory and vestibular systems emigrate from the otic placode as neuroblasts, and divide, differentiate and innervate only one of six to eight distinct sensory organs. How these neurons find their correct target is unknown, although one suggestion is that they synapse with clonally related cells. To test this idea for both the middle and inner ears of chicken embryos, lineage analysis was initiated at the time of neuroblast delamination by labeling progenitors with replication-defective retroviruses. The vast majority (89%) of clones were restricted to a single anatomical subdivision of the sensory periphery or its associated ganglia, indicating limited clonal dispersion. Among the remaining clones, we found evidence of a shared neurosensory lineage in the middle ear. Likewise, in the inner ear, neurons could be related to cells of the otic epithelium, although the latter cells were not widely distributed. Rather, they were restricted to a region in or near the utricular macula. None of the other seven sensory organs was related to the ganglion neurons, suggesting that a common lineage between neurons and their targets is not a general mechanism of establishing synaptic connections in the inner ear. This conclusion is further strengthened by finding a shared lineage between the vestibular and acoustic ganglia, revealing the presence of a common progenitor for the two functional classes of neurons.

Planar polarity of hair cells in the chick inner ear is correlated with polarized distribution of c-flamingo-1 protein

Hair cells of the vertebrate inner ear are directional mechanosensors: they have a polarity, defined by a vector in the plane of the sensory epithelium. It has been suggested that this polarity might be controlled by genes homologous to those that control planar cell polarity (PCP) in Drosophila, and vertebrate homologues of the Drosophila PCP genes Van Gogh/strabismus and flamingo/starry night are indeed essential for normal hair cell PCP. The underlying molecular mechanism is unclear, however. Although the PCP protein Flamingo shows a polarized intracellular distribution in the fly, it is unknown whether this is necessary for its function. Here, we describe the expression pattern of a flamingo homologue, c-flamingo-1 (c-fmi-1), in the developing chick ear and show that its protein product, like that of flamingo in the fly, has a polarized distribution in each hair cell, defining an axis that corresponds to the structural PCP axis. This conservation between fly and vertebrate suggests that the polarized protein localization is functionally important. In the basilar papilla, the same localization is seen in supporting cells also, suggesting that supporting cells are cryptically polarized, despite having no overt structural polarity; they may thus participate in PCP signal transmission across the sensory patch.

Islet-1 expression in the developing chicken inner ear

The cell types of the inner ear originate from the otic placode, a thickened layer of ectoderm adjacent to the developing hindbrain. The placode invaginates and forms the otic pit, which pinches off as a small vesicle called the otocyst. Presumptive cochleovestibular neurons delaminate from the anterior ventral part of the otocyst and form the cochleovestibular ganglion of the inner ear. Here we show that the LIM/homeodomain protein islet-1 is expressed in cells of the ventral part of the otic placode and that this ventral expression is maintained at the otic pit and the otocyst stages. Auditory and vestibular neurons originate from this islet-1-positive zone of the otocyst, and these neurons maintain islet-1 expression until adulthood. We also demonstrate that islet-1 becomes up-regulated in the presumptive sensory epithelia of the inner ear in regions that are defined by the expression domains of BMP4. The up-regulation of islet-1 in developing inner ear hair and supporting cells is accompanied by down-regulation of Pax-2 in these cell types. Islet-1 expression in hair and supporting cells persists until early postnatal stages, when the transcriptional regulator is down-regulated in hair cells. Our data is consistent with a role for islet-1 in differentiating inner ear neurons and sensory epithelia cells, perhaps in the specification of cellular subtypes in conjunction with other LIM/homeodomain proteins.

FGF signaling is required for determination of otic neuroblasts in the chick embryo

The interplay between intrinsic and extrinsic factors is essential for the transit into different cell states during development. We have analyzed the expression and function of FGF10 and FGF-signaling during the early stages of the development of otic neurons. FGF10 is expressed in a highly restricted domain overlapping the presumptive neurogenic region of the chick otic placode. A detailed study of the expression pattern of FGF10, proneural, and neurogenic genes revealed the following temporal sequence for the onset of gene expression: FGF10>Ngn1/Delta1/Hes5>NeuroD/NeuroM. FGF10 and FGF receptor inhibition cause opposed effects on cell determination and cell proliferation. Ectopic expression of FGF10 in vivo promotes an increase in NeuroD and NeuroM expression. BrdU incorporation experiments showed that the increase in NeuroD-expressing cells is not due to an increase in cell proliferation. Inhibition of FGF receptor signaling in otic explants causes a severe reduction in Neurogenin1, NeuroD, Delta1, and Hes5 expression with no change in non-neural genes like Lmx1. However, it does not interfere with NeuroD expression within the CVG or with neuroblast delamination. The loss of proneural gene expression caused by FGF inhibition is not caused by decreased cell proliferation or by increased cell death. We suggest that FGF signaling in the otic epithelium is required for neuronal precursors to withdraw from cell division and irreversibly commit to neuronal fate.

Expression of Prox1 defines regions of the avian otocyst that give rise to sensory or neural cells

The simple primordium of the inner ear (otocyst) differentiates into many cell types, including sensory neurons and hair cells. We examined expression of the divergent homeobox transcription factor, cProx1, during otocyst development in chickens. Nuclear cProx1 protein is not evident in the otic placode but emerges in the otic cup by stage 12. At stage 16, cProx1-positive nuclei are scattered continuously throughout the neuroepithelium, from anteroventral to posteromedial. These labeled cells are neural precursors; they express betaIII-tubulin and migrate to the cochleovestibular ganglion between stages 13 and 21. By stage 18, two areas develop a dense pattern of cProx1 expression in which every nucleus is labeled. These areas emerge at the anterior and posterior extremes of the band of scattered cProx1 expression and express the sensory markers cSerrate1 and Cath1 by stage 23. Four discrete patches of dense cProx1 expression appear by stage 23 that correspond to the future superior crista, lateral crista, saccular macula, and posterior crista, as confirmed by immunolabeling for hair cell antigen (HCA) by stage 29. The remaining sensory epithelia display a dense pattern of cProx1 expression and label for HCA by stage 29. In the basilar papilla, nuclear cProx1 expression is down-regulated in most hair cells by stage 37 and in many supporting cells by stage 40. Our findings show that regions of the otocyst that give rise to neurons or hair cells are distinguished by their relative density of cProx1-positive nuclei, and suggest a role for cProx1 in the genesis of these cell types.

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

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

Sensory organ development in the inner ear: molecular and cellular mechanisms

The molecular mechanisms underlying the specification of sensory organs in the inner ear and the development of hair and supporting cells within these organs are described. The different organs are all derived from a common pro-sensory region, and may be specified by their proximity to the boundaries between compartments - broad domains within the otocyst defined by the asymmetric expression patterns of transcription factors. Activation of Notch may specify the pro-sensory region, and lateral inhibition mediated by Notch signalling influences whether cells of common lineage in a sensory patch differentiate as either hair cells or supporting cells. The transcription factors Math1 and Brn3.1 are required for hair cell differentiation, and supporting cells express negative regulators of neurogenesis, Hes1 and Hes5. Retinoic acid and thyroid hormone influence early aspects and timing of hair cell differentiation, respectively. Development of the hair cell's mechanosensory hair bundle involves interactions between the cytoskeleton, cell-surface adhesion molecules, receptors and associated extracellular matrix.