Early Embryology of the Vertebrate Ear

From "self-differentiation" to organoids-the quest for the units of development

As proposed by Wilhelm Roux in 1885, the key goal of experimental embryology ("Entwicklungsmechanik") was to elucidate whether organisms or their parts develop autonomously ("self-differentiation") or require interactions with other parts or the environment. However, experimental embryologists soon realized that concepts like "self-differentiation" only make sense when applied to particular parts or units of the developing embryo as defined both in time and space. Whereas the formation of tissues or organs may initially depend on interactions with surrounding tissues, they later become independent of such interactions or "determined." Moreover, the determination of a particular tissue or organ primordium has to be distinguished from the spatially coordinated determination of its parts-what we now refer to as "patterning." While some primordia depend on extrinsic influences (e.g., signals from adjacent tissues) for proper patterning, others rely on intrinsic mechanisms. Such intrinsically patterned units may behave as "morphogenetic fields" that can compensate for lost parts and regulate their size and proper patterning. While these insights were won by experimental embryologists more than 100 years ago, they retain their relevance today. To enable the generation of more life-like organoids in vitro for studying developmental processes and diseases in a dish, questions about the spatiotemporal units of development (when and how tissues and organs are determined and patterned) need to be increasingly considered. This review briefly sketches this conceptual history and its continued relevance by focusing on the determination and patterning of the inner ear with a specific emphasis on some studies published in this journal.

Development and evolution of the vestibular apparatuses of the inner ear

The vertebrate inner ear is a labyrinthine sensory organ responsible for perceiving sound and body motion. While a great deal of research has been invested in understanding the auditory system, a growing body of work has begun to delineate the complex developmental program behind the apparatuses of the inner ear involved with vestibular function. These animal studies have helped identify genes involved in inner ear development and model syndromes known to include vestibular dysfunction, paving the way for generating treatments for people suffering from these disorders. This review will provide an overview of known inner ear anatomy and function and summarize the exciting discoveries behind inner ear development and the evolution of its vestibular apparatuses.

How do red-eyed treefrog embryos sense motion in predator attacks? Assessing the role of vestibular mechanoreception

The widespread ability to alter timing of hatching in response to environmental cues can serve as a defense against threats to eggs. Arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, can hatch up to 30% prematurely to escape predation. This escape-hatching response is cued by physical disturbance of eggs during attacks, including vibrations or motion, and thus depends critically on mechanosensory ability. Predator-induced hatching appears later in development than flooding-induced, hypoxia-cued hatching; thus, its onset is not constrained by the development of hatching ability. It may, instead, reflect the development of mechanosensor function. We hypothesize that vestibular mechanoreception mediates escape-hatching in snake attacks, and that the developmental period when hatching-competent embryos fail to flee from snakes reflects a sensory constraint. We assessed the ontogenetic congruence of escape-hatching responses and an indicator of vestibular function, the vestibulo-ocular reflex (VOR), in three ways. First, we measured VOR in two developmental series of embryos 3-7 days old to compare with the published ontogeny of escape success in attacks. Second, during the period of greatest variation in VOR and escape success, we compared hatching responses and VOR across sibships. Finally, in developmental series, we compared the response of individual embryos to a simulated attack cue with their VOR. The onset of VOR and hatching responses were largely concurrent at all three scales. Moreover, latency to hatch in simulated attacks decreased with increasing VOR. These results are consistent with a key role of the vestibular system in the escape-hatching response of A. callidryas embryos to attacks.

Transplantation of Ears Provides Insights into Inner Ear Afferent Pathfinding Properties

Numerous tissue transplantations have demonstrated that otocysts can develop into normal ears in any location in all vertebrates tested thus far, though the pattern of innervation of these transplanted ears has largely been understudied. Here, expanding on previous findings that transplanted ears demonstrate capability of local brainstem innervation and can also be innervated themselves by efferents, we show that inner ear afferents grow toward the spinal cord mostly along existing afferent and efferent fibers and preferentially enter the dorsal spinal cord. Once in the dorsal funiculus of the spinal cord, they can grow toward the hindbrain and can diverge into vestibular nuclei. Inner ear afferents can also project along lateral line afferents. Likewise, lateral line afferents can navigate along inner ear afferents to reach hair cells in the ear. In addition, transplanted ears near the heart show growth of inner ear afferents along epibranchial placode-derived vagus afferents. Our data indicate that inner ear afferents can navigate in foreign locations, likely devoid of any local ear-specific guidance cues, along existing nerves, possibly using the nerve-associated Schwann cells as substrate to grow along. However, within the spinal cord and hindbrain, inner ear afferents can navigate to vestibular targets, likely using gradients of diffusible factors that define the dorso-ventral axis to guide them. Finally, afferents of transplanted ears functionally connect to native hindbrain vestibular circuitry, indicated by eliciting a startle behavior response, and providing excitatory input to specific sets of extraocular motoneurons.

Ear transplantations reveal conservation of inner ear afferent pathfinding cues

Vertebrate inner ear neurons project into the correct brainstem nuclei region before target neurons become postmitotic, or even in their absence. Moreover, afferents from transplanted ears in frogs have been shown to navigate to vestibular nuclei, suggesting that ear afferents use molecular cues to find their target. We performed heterochronic, xenoplastic, and heterotopic transplantations in chickens to investigate whether inner ear afferents are guided by conserved guidance molecules. We show that inner ear afferents can navigate to the vestibular nuclei following a delay in afferent entry and when the ear was from a different species, the mouse. These data suggest that guidance molecules are expressed for some time and are conserved across amniotes. In addition, we show that chicken ears transplanted adjacent to the spinal cord project dorsally like in the hindbrain. These results suggest that inner ear afferents navigate to the correct dorsoventral brainstem column using conserved cues.

Inner ear development: building a spiral ganglion and an organ of Corti out of unspecified ectoderm

The mammalian inner ear develops from a placodal thickening into a complex labyrinth of ducts with five sensory organs specialized to detect position and movement in space. The mammalian ear also develops a spiraled cochlear duct containing the auditory organ, the organ of Corti (OC), specialized to translate sound into hearing. Development of the OC from a uniform sheet of ectoderm requires unparalleled precision in the topological developmental engineering of four different general cell types, namely sensory neurons, hair cells, supporting cells, and general otic epithelium, into a mosaic of ten distinctly recognizable cell types in and around the OC, each with a unique distribution. Moreover, the OC receives unique innervation by ear-derived spiral ganglion afferents and brainstem-derived motor neurons as efferents and requires neural-crest-derived Schwann cells to form myelin and neural-crest-derived cells to induce the stria vascularis. This transformation of a sheet of cells into a complicated interdigitating set of cells necessitates the orchestrated expression of multiple transcription factors that enable the cellular transformation from ectoderm into neurosensory cells forming the spiral ganglion neurons (SGNs), while simultaneously transforming the flat epithelium into a tube, the cochlear duct, housing the OC. In addition to the cellular and conformational changes forming the cochlear duct with the OC, changes in the surrounding periotic mesenchyme form passageways for sound to stimulate the OC. We review molecular developmental data, generated predominantly in mice, in order to integrate the well-described expression changes of transcription factors and their actions, as revealed in mutants, in the formation of SGNs and OC in the correct position and orientation with suitable innervation. Understanding the molecular basis of these developmental changes leading to the formation of the mammalian OC and highlighting the gaps in our knowledge might guide in vivo attempts to regenerate this most complicated cellular mosaic of the mammalian body for the reconstitution of hearing in a rapidly growing population of aging people suffering from hearing loss.

Dynamic expression of Lgr6 in the developing and mature mouse cochlea

The Wnt/β-catenin signaling pathway plays important roles in mammalian inner ear development. Lgr5, one of the downstream target genes of the Wnt/β-catenin signaling pathway, has been reported to be a marker for inner ear hair cell progenitors. Lgr6 shares approximately 50% sequence homology with Lgr5 and has been identified as a stem cell marker in several organs. However, the detailed expression profiles of Lgr6 have not yet been investigated in the mouse inner ear. Here, we first used Lgr6-EGFP-Ires-CreERT2 mice to examine the spatiotemporal expression of Lgr6 protein in the cochlear duct during embryonic and postnatal development. Lgr6-EGFP was first observed in one row of prosensory cells in the middle and basal turn at embryonic day 15.5 (E15.5). From E18.5 to postnatal day 3 (P3), the expression of Lgr6-EGFP was restricted to the inner pillar cells (IPCs). From P7 to P15, the Lgr6-EGFP expression level gradually decreased in the IPCs and gradually increased in the inner border cells (IBCs). At P20, Lgr6-EGFP was only expressed in the IBCs, and by P30 Lgr6-EGFP expression had completely disappeared. Next, we demonstrated that Wnt/β-catenin signaling is required to maintain the Lgr6-EGFP expression in vitro. Finally, we demonstrated that the Lgr6-EGFP-positive cells isolated by flow cytometry could differentiate into myosin 7a-positive hair cells after 10 days in-culture, and this suggests that the Lgr6-positive cells might serve as the hair cell progenitor cells in the cochlea.

Potential treatments for genetic hearing loss in humans: current conundrums

Genetic defects are a major cause of hearing loss in newborns. Consequently, hearing loss has a profound negative impact on human daily living. Numerous causative genes for genetic hearing loss have been identified. However, presently, there are no truly curative treatments for this condition. There have been several recent reports on successful treatments in mice using embryonic gene therapy, neonatal gene therapy and neonatal antisense oligonucleotide therapy. Herein, we describe state-of-the-art research on genetic hearing loss treatment through gene therapy and discuss the obstacles to overcome in curative treatments of genetic hearing loss in humans.

Sensory afferent segregation in three-eared frogs resemble the dominance columns observed in three-eyed frogs

The formation of proper sensory afferent connections during development is essential for brain function. Activity-based competition is believed to drive ocular dominance columns (ODC) in mammals and in experimentally-generated three-eyed frogs. ODC formation is thus a compromise of activity differences between two eyes and similar molecular cues. To gauge the generality of graphical map formation in the brain, we investigated the inner ear projection, known for its well-defined and early segregation of afferents from vestibular and auditory endorgans. In analogy to three eyed-frogs, we generated three-eared frogs to assess to what extent vestibular afferents from two adjacent ears could segregate. Donor ears were transplanted either in the native orientation or rotated by 90 degrees. These manipulations should result in either similar or different induced activity between both ears, respectively. Three-eared frogs with normal orientation showed normal swimming whereas those with a rotated third ear showed aberrant behaviors. Projection studies revealed that only afferents from the rotated ears segregated from those from the native ear within the vestibular nucleus, resembling the ocular dominance columns formed in three-eyed frogs. Vestibular segregation suggests that mechanisms comparable to those operating in the ODC formation of the visual system may act on vestibular projection refinements.

Transforming the vestibular system one molecule at a time: the molecular and developmental basis of vertebrate auditory evolution

We review the molecular basis of auditory development and evolution. We propose that the auditory periphery (basilar papilla, organ of Corti) evolved by transforming a newly created and redundant vestibular (gravistatic) endorgan into a sensory epithelium that could respond to sound instead of gravity. Evolution altered this new epithelia's mechanoreceptive properties through changes of hair cells, positioned the epithelium in a unique position near perilymphatic space to extract sound moving between the round and the oval window, and transformed its otolith covering into a tympanic membrane. Another important step in the evolution of an auditory system was the evolution of a unique set of "auditory neurons" that apparently evolved from vestibular neurons. Evolution of mammalian auditory (spiral ganglion) neurons coincides with GATA3 being a transcription factor found selectively in the auditory afferents. For the auditory information to be processed, the CNS required a dedicated center for auditory processing, the auditory nuclei. It is not known whether the auditory nucleus is ontogenetically related to the vestibular or electroreceptive nuclei, two sensory systems found in aquatic but not in amniotic vertebrates, or a de-novo formation of the rhombic lip in line with other novel hindbrain structures such as pontine nuclei. Like other novel hindbrain structures, the auditory nuclei express exclusively the bHLH gene Atoh1, and loss of Atoh1 results in loss of most of this nucleus in mice. Only after the basilar papilla, organ of Corti evolved could efferent neurons begin to modulate their activity. These auditory efferents most likely evolved from vestibular efferent neurons already present. The most simplistic interpretation of available data suggest that the ear, sensory neurons, auditory nucleus, and efferent neurons have been transformed by altering the developmental genetic modules necessary for their development into a novel direction conducive for sound extraction, conduction, and processing.

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

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

Thyroid hormone receptor alpha1 is a critical regulator for the expression of ion channels during final differentiation of outer hair cells

Cochlear outer hair cells (OHCs) terminally differentiate prior to the onset of hearing. During this time period, thyroid hormone (TH) dramatically influences inner ear development. It has been shown recently that TH enhances the expression of the motor protein prestin via liganded TH receptor beta (TRbeta) while in contrast the expression of the potassium channel KCNQ4 is repressed by unliganded TRalpha1. These different mechanisms of TH regulation by TRalpha1 or TRbeta prompted us to analyse other ion channels that are required for the final differentiation of OHCs. We analysed the onset of expression of the Ca(2+) channel Ca(V)1.3, and the K(+) channels SK2 and BK and correlated the results with the regulation via TRalpha1 or TRbeta. The data support the hypothesis that proteins expressed in rodents prior to or briefly after birth like Ca(V)1.3 and prestin are either independent of TH (e.g. Ca(V)1.3) or enhanced through TRbeta (e.g. prestin). In contrast, proteins expressed in rodents later than P6 like KCNQ4 ( approximately P6), SK2 ( approximately P9) and BK ( approximately P11) are repressed through TRalpha1. We hypothesise that the precise regulation of expression of the latter genes requires a critical local TH level to overcome the TRalpha1 repression.

Retinoid signaling in inner ear development

The inner ear originates from an embryonic ectodermal placode and rapidly develops into a three-dimensional structure (the otocyst) through complex molecular and cellular interactions. Many genes and their products are involved in inner ear induction, organogenesis, and cell differentiation. Retinoic acid (RA) is an endogenous signaling molecule that may play a role during different phases of inner ear development, as shown from pathological observations. To gain insight into the function of RA during inner ear development, we have investigated the spatio-temporal expression patterns of major components of RA signaling pathway, including cellular retinoic acid binding proteins (CRABPs), cellular retinoid binding proteins (CRBPs), retinaldehyde dehydrogenases (RALDHs), catabolic enzymes (CYP26s), and nuclear receptors (RARs). Although the CrbpI, CrabpI, and -II genes are specifically expressed in the inner ear throughout development, loss-of-function studies have revealed that these proteins are dispensable for inner development and function. Several Raldh and Cyp26 gene transcripts are expressed at embryological day (E) 9.0-9.5 in the otocyst and show mainly complementary distributions in the otic epithelium and mesenchyme during following stages. From Western blot, RT-PCR, and in situ hybridization analysis, there is a low expression of Raldhs in the early otocyst at E9, while Cyp26s are strongly expressed. During the following days, there is an up-regulation of Raldhs and a down-regulation for Cyp26s. Specific RA receptor (Rar and Rxr) genes are expressed in the otocyst and during further development of the inner ear. At the otocyst stage, most of the components of the retinoid pathway are present, suggesting that the embryonic inner ear might act as an autocrine system, which is able to synthesize and metabolize RA necessary for its development. We propose a model in which two RA-dependent pathways may control inner ear ontogenesis: one indirect with RA from somitic mesoderm acting to regulate gene expression within the hindbrain neuroepithelium, and another with RA acting directly on the otocyst. Current evidence suggests that RA may regulate several genes involved in mesenchyme-epithelial interactions, thereby controlling inner ear morphogenesis. Our investigations suggest that RA signaling is a critical component not only of embryonic development, but also of postnatal maintenance of the inner ear.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

Differential expression of Eph receptors and ephrins in the cochlear ganglion and eighth cranial nerve of the chick embryo

The cochleovestibular ganglion of the chick differentiates at early embryonic stages as VIIIth nerve axons enter the brainstem. The tonotopic organization of the auditory portion of the VIIIth nerve can be discerned at the time axons initially reach their brainstem targets. The mechanisms underlying this early organization are not known. Eph receptor tyrosine kinases and their ligands, the ephrins, have a demonstrated role in guiding axons to topographically appropriate locations in other areas of the nervous system. In order to begin to test whether Eph proteins have a similar role in the auditory system, we investigated the tonotopic expression of several Eph receptors and ephrins in the VIIIth nerve during embryonic ages corresponding to the initial innervation of the auditory brainstem. Expression patterns of EphA4, EphB2, EphB5, ephrin-A2, and ephrin-B1 were evaluated immunohistochemically at embryonic days 4 through 10. Protein expression was observed in the cochlear ganglion and VIIIth nerve axons at these ages. EphB5, ephrin-A2, and ephrin-B1 were expressed throughout the nerve. EphA4 and EphB2 had complementary expression patterns within the nerve, with EphA4 expression higher in the dorsolateral part of the nerve and EphB2 expression higher in the ventromedial part of the nerve. These regions may correspond to auditory and vestibular components, respectively. Moreover, EphA4 expression was higher toward the low-frequency region of both the centrally and peripherally projecting branches of cochlear ganglion cells. Regional variation of Eph protein expression may influence the target selection and topography of developing VIIIth nerve projections.

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.

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.

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.

Comparative expression patterns of T-, N-, E-cadherins, beta-catenin, and polysialic acid neural cell adhesion molecule in rat cochlea during development: implications for the nature of Kölliker's organ

We investigated the expression patterns of several cell adhesion molecules (CAMs) during rat cochlea ontogeny, from embryo day 16 to adulthood, with the use of immunohistochemistry: neural cadherin (N-cad) and polysialic acid neural CAM (PSA-NCAM) as two different neural CAM paradigms; epithelial cadherin (E-cad), which was restricted to the epitheloid phenotype; and the cytoplasmic domain-free truncated-cadherin (T-cad). We made the following observations. (1) T-cad was present in all types of fibrocyte and in subdomains within the pillar cells. (2) E- and N-cad were expressed with mutually exclusive patterns and did not overlap with T-cad. All cochlear epithelial cells, including the sensory outer hair cells (OHCs), were E-cad-positive, except for the negative inner hair cells (IHCs) and the nonsensory Kölliker's organ domain close to the IHCs. N-cad expression appeared first in the developing IHCs and then in the neighboring Kölliker's organ in an increasingly mediolateral gradient in opposition to the E-cad gradient. The OHCs, which are never N-cad positive, intensively expressed E-cad, as did the Hensen cells at the beginning of their differentiation. (3) The cadherin-linked molecule beta-catenin, absent in fibrocytes, was detected in all epithelial cell membranes and was prominent in the E-cad-rich modiolar extremity of Kölliker's organ. (4) Gradual PSA-NCAM expression was observed in the lateral portion of Kölliker's organ, and the intense PSA-NCAM expression was seen surrounding the IHCs. As development proceeded, PSA-NCAM immunoreactivity progressively became restricted to the basal poles of the IHCs, where it remained in the adult rat cochlea, suggesting a synaptic plasticity. Synaptic plasticity in rat cochlea and hypotheses about T-cad functions and neosensory features of the Kölliker's organ are discussed.

Retinoic acid repression of bone morphogenetic protein 4 in inner ear development

Bone morphogenetic protein 4 (BMP4) and retinoic acid are important for normal development of the inner ear, but whether they are linked mechanistically is not known. BMP4 antagonists disrupt semicircular canal formation, as does exposure to retinoic acid. We demonstrate that retinoic acid directly down-regulates BMP4 transcription in a mouse inner ear-derived cell line, and we identify a novel promoter in the second intron of the BMP4 gene that is a target of this regulation both in the cell line and in the mouse embryonic inner ear in vivo. The importance of this down-regulation is demonstrated in chicken embryos by showing that the retinoic acid effect on semicircular canal development can be overcome by exogenous BMP4.

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.

Development and evolution of inner ear sensory epithelia and their innervation

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

Extracellular matrices associated with the apical surfaces of sensory epithelia in the inner ear: molecular and structural diversity

The ultrastructure and molecular composition of the extracellular matrices that are associated with the apical surfaces of the mechanosensory epithelia in the mouse inner ear are compared. A progressive increase in molecular and structural organization is observed, with the cupula being the simplest, the otoconial membrane exhibiting an intermediate degree of complexity, and the tectorial membrane being the most elaborate of the three matrices. These differences may reflect changes that occurred in the acellular membranes of the inner ear as a mammalian hearing organ arose during evolution from a simple equilibrium receptor. A comparison of the molecular composition of the acellular membranes in the chick inner ear suggests the auditory epithelium and the striolar region of the maculae are homologous, indicating the basilar papilla may have evolved from the striolar region of an otolithic organ. A comparison of the tectorial membranes in the chick cochlear duct and the mouse cochlea reveals differences in the structure of the noncollagenous matrix in the two species that may result from differences in the stochiometry of alpha- and beta-tectorin and/or differences in the post-translational modification of alpha-tectorin. This comparison also indicates that the appearance of collagen in the mammalian tectorial membrane may have been a major step in the evolution of an electromechanically tuned vertebrate hearing organ that operates over an extended frequency range.

Auditory system development: primary auditory neurons and their targets

The neurons of the cochlear ganglion transmit acoustic information between the inner ear and the brain. These placodally derived neurons must produce a topographically precise pattern of connections in both the inner ear and the brain. In this review, we consider the current state of knowledge concerning the development of these neurons, their peripheral and central connections, and their influences on peripheral and central target cells. Relatively little is known about the cellular and molecular regulation of migration or the establishment of precise topographic connection to the hair cells or cochlear nucleus (CN) neurons. Studies of mice with neurotrophin deletions are beginning to yield increasing understanding of variations in ganglion cell survival and resulting innervation patterns, however. Finally, existing evidence suggests that while ganglion cells have little influence on the differentiation of their hair cell targets, quite the opposite is true in the brain. Ganglion cell innervation and synaptic activity are essential for normal development of neurons in the cochlear nucleus.

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.

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.

Evolution and development of the vertebrate ear

This review outlines major aspects of development and evolution of the ear, specifically addressing issues of cell fate commitment and the emerging molecular governance of these decisions. Available data support the notion of homology of subsets of mechanosensors across phyla (proprioreceptive mechanosensory neurons in insects, hair cells in vertebrates). It is argued that this conservation is primarily related to the specific transducing environment needed to achieve mechanosensation. Achieving this requires highly conserved transcription factors that regulate the expression of the relevant structural genes for mechanosensory transduction. While conserved at the level of some cell fate assignment genes (atonal and its mammalian homologue), the ear has also radically reorganized its development by implementing genes used for cell fate assignment in other parts of the developing nervous systems (e.g., neurogenin 1) and by evolving novel sets of genes specifically associated with the novel formation of sensory neurons that contact hair cells (neurotrophins and their receptors). Numerous genes have been identified that regulate morphogenesis, but there is only one common feature that emerges at the moment: the ear appears to have co-opted genes from a large variety of other parts of the developing body (forebrain, limbs, kidneys) and establishes, in combination with existing transcription factors, an environment in which those genes govern novel, ear-related morphogenetic aspects. The ear thus represents a unique mix of highly conserved developmental elements combined with co-opted and newly evolved developmental elements.

Effects of microgravity on vestibular development and function in rats: genetics and environment

Our anatomical and behavioral studies of embryonic rats that developed in microgravity suggest that the vestibular sensory system, like the visual system, has genetically mediated processes of development that establish crude connections between the periphery and the brain. Environmental stimuli also regulate connection formation including terminal branch formation and fine-tuning of synaptic contacts. Axons of vestibular sensory neurons from gravistatic as well as linear acceleration receptors reach their targets in both microgravity and normal gravity, suggesting that this is a genetically regulated component of development. However, microgravity exposure delays the development of terminal branches and synapses in gravistatic but not linear acceleration-sensitive neurons and also produces behavioral changes. These latter changes reflect environmentally controlled processes of development.

Ectopic noggin blocks sensory and nonsensory organ morphogenesis in the chicken inner ear

Bone morphogenetic protein 4 (Bmp4) is expressed during multiple stages of development of the chicken inner ear. At the otocyst stage, Bmp4 is expressed in each presumptive sensory organ, as well as in the mesenchymal cells surrounding the region of the otocyst that is destined to form the semicircular canals. After the formation of the gross anatomy of the inner ear, Bmp4 expression persists in some sensory organs and restricted domains of the semicircular canals. To address the role of this gene in inner ear development, we blocked BMP4 function(s) by delivering one of its antagonists, Noggin, to the developing inner ear in ovo. Exogenous Noggin was delivered to the developing otocyst by using a replication-competent avian retrovirus encoding the Noggin cDNA (RCAS-N) or implanting beads coated with Noggin protein. Noggin treatment resulted in a variety of phenotypes involving both sensory and nonsensory components of the inner ear. Among the nonsensory structures, the semicircular canals were the most sensitive and the endolymphatic duct and sac most resistant to exogenous Noggin. Noggin affected the proliferation of the primordial canal outpouch, as well as the continual outgrowth of the canal after its formation. In addition, Noggin affected the structural patterning of the cristae, possibly via a decrease of Msx1 and p75NGFR expression. These results suggest that BMP4 and possibly other BMPs are required for multiple phases of inner ear development.

Nerve dependency of developing and mature sensory receptor cells

Old and recent data concerning development of sensory cells and trophic interdependency of sensory neurons and sensory cells is reviewed for the ear, the lateral line system, the electroreceptive system, and the taste system. All sensory neurons originate from placodes. However, only most ear, lateral line and electrosensory cells derive from placodes, while the taste sensory cell originate locally. All sensory cells apparently are nerve independent for their formation, and at least sensory cells in the ear and the taste system share the neurotrophic support for their specific sensory neurons. Later, most of these sensory cells appear to depend, to a variable degree, on some innervation for maintenance. While the molecular nature of the signal cascade from sensory cells to sensory neurons is known in at least two systems, nothing is known about the molecular nature of the signal cascade from the sensory neurons back to the sensory cells.

Recent insights into regeneration of auditory and vestibular hair cells

Advances in hair cell regeneration are progressing at a rapid rate. This review will highlight and critique recent attempts to understand some of the cellular and molecular mechanisms underlying hair cell regeneration in non-mammalian vertebrates and efforts to induce regeneration in the mammalian inner ear sensory epithelium.

The expression domain of two related homeobox genes defines a compartment in the chicken inner ear that may be involved in semicircular canal formation

Homeobox-containing genes encode a class of proteins that control patterning in developing systems, in some cases by acting as selector genes that define compartment identity. In an effort to demonstrate a similar role for such genes during ear development in the chicken, we present a detailed expression study of two related homeobox-containing genes, SOHo-1 and GH6, using in situ hybridization. At otocyst stages the two genes define a broad lateral domain of expression, which may represent a developmental compartment. Three-dimensional computer reconstructions of SOHo-1 expression at these and later stages revealed that the lateral domain becomes progressively restricted to the three semicircular canals. Thus, SOHo-1 and GH6 are among a small group of markers for a specific structural component of the inner ear. The gene expression domain initially includes the sensory regions of the semicircular canals, known as the cristae ampullaris, but none of the other four sensory organs which were recognizable by BMP4 expression during early morphogenesis (stages 19-24). Significantly, two of the sensory organs (the superior and posterior cristae) were found at the limits, or boundaries, of the SOHo-1/GH6 expression domain, suggesting that compartment boundaries may be involved in specifying sensory organ location as well as identity. Maintained expression at the boundaries may aid in specifying the location of canal outgrowth. These concepts are presented as a formal model which emphasizes that patterning information could be provided at the boundaries of gene expression domains in the inner ear.