35

Generating the diversity of cell types in the inner ear mayrequire an interplay between regional compartmentalizationand local cellular interactions. Recent evidence has come fromgene targeting, lineage analysis, fate mapping and geneexpression studies. Notch signaling and neurogenic gene reg-ulation are involved in patterning or specification of sensoryorgans, ganglion cells and hair cell mechanoreceptors.

Addresses*Department of Biological Sciences, Purdue University, 1392 Lilly Hallof Science, West Lafayette, Indiana 47907-1392, USA; e-mail: dfekete@purdue.edu †National Institute on Deafness and Other Communication Disorders,5 Research Court, Room 2B34, Rockville, Maryland 20850, USA; e-mail: wud@nidcd.nih.gov

Current Opinion in Neurobiology 2002, 12:35–42

0959-4388/02/$ — see front matter© 2002 Elsevier Science Ltd. All rights reserved.

AbbreviationsA–P anterior–posteriorbHLH basic helix–loop–helixBMP bone morphogenetic proteinD–V dorsal–ventralFgf Fibroblast growth factorHes Hairy/Enhancer of SplitJag JaggedLufng lunatic fringeM–L medial–lateralngn neurogeninSer SerrateVIIIg eighth cranial ganglion (statoacoustic ganglion)

IntroductionThe vertebrate inner ear houses the receptor cells for twodistinct sensory pathways: the auditory and vestibular systems. Both systems use similar types of mechano-receptors, called hair cells, to transduce mechanical motioninto electrochemical energy.

Several types of hair cells are distinguishable by morphologicaland functional criteria. Hair cells are always buffered fromone another by the processes of non-excitable supportingcells. Together, these two types of cell comprise the innerear sensory organs, which are themselves structurally andfunctionally divergent. The sensory organs reside in an intricate series of fluid conduits filled with a potassium-richextracellular fluid called endolymph. The strial and otherspecialized cells that secrete endolymph are physically separated from the sensory organs, although both are equallycrucial to mechanotransduction.

During development, the otic epithelium interacts withand remodels the surrounding loose connective tissue,forming an intimate association. These tissues becomeencased in cartilage that will ossify to form the otic capsule.The otic epithelial cells, along with the primary sensory

neurons that innervate the hair cells, share a common origin during embryogenesis. They derive from the oticplacode — a small patch of ectodermal cells adjacent to thedorsal hindbrain. The otic placode folds in on itself, sepa-rates from the adjacent non-placodal ectoderm and sinksinto the head. It then undergoes elaborate morphogeneticchanges and tissue interactions to create the maturelabyrinth, complete with all its specialized cell types.

The molecular basis of cell fate specification in the ear wasreviewed in Current Opinion in Neurobiology in 1996, with afocus on how cell fate decisions might be regulated by patterning genes that are expressed regionally [1]. Thefield has advanced considerably in the intervening years,and we wish to provide an update. Although there havebeen several recent advances in our understanding of oticinduction [2,3••,4] here, we review progress in the stagesthat occur immediately after this important event.

A compartment-boundary model of cell fatespecificationThe cells of the developing inner ear probably undergo asequence of cell fate decisions to generate the myriad ofdifferent cell types (Figure 1). Moreover, the cell typesmust arise in the correct spatial position with respect toone another. This complex task would be simplified if theear were segregated from a relatively early stage into different compartments (Figure 2). In other developingsystems, such as the fly wing and the vertebrate hindbrain,developmental compartments are essential for regionalidentity and cell fate specification. They are defined bylineage restrictions (at least transiently), specified by differential expression of so-called ‘selector genes’ and, inthe case of the wing, can pattern nearby cells using signalsemanating from their boundaries [5].

Regional gene expression in the otocyst is what initially led usand our coworkers [1,6,7] to consider that compartmentsmight be present. Regional expression in the otocyst can beinduced and/or maintained by external signals emanatingfrom the surrounding tissues, including the hindbrain [8–12].Once established, the boundaries between compartmentscould then serve as sites of focal cell–cell interactions to mediate local patterning and cell fate specification in the ear.

In the past five years, evidence has been slowly accumu-lating in support of this ‘compartment-boundary’ model,although many important details are still missing. The earliest version of the model presumed that outgrowth of theendolymphatic duct — a structure involved in endolymphcirculation — would be specified by compartment bound-aries intersecting near the dorsal pole of the otocyst. Fate-mapping data from the chick now support this contention,placing the duct near the intersection of an anterior–posterior

Revisiting cell fate specification in the inner earDonna M Fekete* and Doris K Wu†

(A–P) boundary and a medial–lateral (M–L) boundary. Atthe dorsal pole, these two boundaries seem to serve as linesof lineage restriction [13•]. In the case of the M–L boundary,its position immediately lateral to the budding duct is further suggested by gene expression data [6]. Additionalsupport for the compartment-boundary model has comefrom an analysis of sensory organ determination (see below).

Specification of neurogenic versus non-neurogenic cellsAs the otic placode invaginates, a population of progenitorsnear the center of the closing cup and ventral otic vesicleemigrates into the adjacent mesoderm. These cells are neuroblasts; they may continue to divide for several hours ordays (depending on the species) as they colonize the eighthcranial ganglion (VIIIg). After becoming post-mitotic, eachneuron makes synaptic connections with one of the sensoryorgans, and with appropriate brainstem neurons of eitherauditory or vestibular nuclei.

Morphologically and molecularly, the specification of someotocyst cells as neuroblasts is perhaps the earliest cell fatedecision that takes place in the ear. Interestingly, the mol-ecular basis of this decision involves transcription factorsthat are related to Atonal — a neurogenic protein that con-fers neural competence to ectodermal cells. The formationof the otic ganglion is defective in mice lacking two distantmembers of this basic helix–loop–helix (bHLH) gene family, neurogenin-1 (Ngn-1) and NeuroD [14••,15••,16••].

Several other genes may be required for VIIIg specifica-tion, including GATA3, Eya1 and Fgfr2(IIIb), the putativereceptor for both Fibroblast growth factor (Fgf)3 and Fgf10[17–19]. Mutations in each of these genes leads to severehypomorphic development of not only the VIIIg, but alsothe whole inner ear.

So far, there is no evidence to indicate that neurons in theauditory and vestibular parts of the VIIIg can be clonally

36 Development

Figure 1

The possible decisions that underlie cell fatespecification in the developing inner ear ofmammals. Some of the genes shown to beinvolved in the acquisition of cell fate areindicated on the right, and a hierarchy ofpossible fate decisions, along with placeswhere the division of the otic epithelium intolineage-based developmental compartmentsmight contribute to these decisions (see text),is shown on the left. Genes that may beinvolved in compartmentalization are not listed(see Figure 2 for some examples). In somecases (such as for activated Notch, Hes1 andHes5), it is uncertain whether the moleculeshelp to define the sensory/non-sensoryborder, regulate the hair cell/supporting celldecision, or both. Not all known cell types inthe inner ear are indicated. Note that not allregions are likely to contain cells withequivalent potentials. For example, neurogeniccells only emigrate from the anteroventral oticepithelium. This region probably also givesrise to other cells types (see Figures 2 and 3).It is unknown whether neural-competentprogenitors that fail to become specified asneurons can be directed to other cell fates,such as sensory and/or non-sensory cells, asindicated by the dotted lines. aNotch:activated Notch; AG: auditory ganglion; HC: hair cell; IHC: inner hair cell;NB: neuroblast; NC: neural competence;NS: non-sensory; oCorti: organ of Corti;OHC: outer hair cell; S: sensory; SC: supporting cell; VG: vestibular ganglion.

VGAG

Type II Type I

NB

Wnt8cFgf19

Non-otic

NeuroD?Ngn1?

NS S

Crista oCorti

Otic versus ectoderm

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Type of organ/tissue

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Pillar cellsCurrent Opinion in Neurobiology

Others

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related. This leaves open the possibility that the two lineagesarise from separate compartments and thus diverge relativelyearly during gangliogenesis, as illustrated in Figure 3.Eventually, the two ganglia show differential dependence onthe neurotrophic survival factors brain derived neurotrophicfactor and Neurotrophin-3 (reviewed in [20]).

Whether the VIIIg neurons share a common precursorwith hair cells and supporting cells remains one of theforemost intriguing debates of the field (Figure 1). Thepossibility of a shared lineage was raised in the context ofevolutionary considerations, whereby the primary sensorypathway of the vertebrate inner ear was proposed to be ahomolog of the insect mechanosensory organs, includingtheir clonally related neurons [21,22]. So far, efforts todetect a common progenitor for inner ear neurons andsensory cells have failed [23•]. Notably, however, the presumptive neurogenic region of the chicken seems to bepartially overlapping with a putative sensory-competent

zone identified, in part, by expression of lunatic fringe(Lufng) [24•].

Specifically, NeuroD-positive cells, presumed to be neuroblasts, are sprinkled throughout the anterior part ofthe Lufng domain in the mouse otocyst (Figure 4a,b).Later, this same domain seems to give rise to sensory cellsof the utricle, and possibly part of the saccule and cochlea(Figures 2 and 3). But the posterior part of the Lufngdomain does not express NeuroD (Figure 4c,d), suggestingthat some of the Lufng-positive sensory patches originatebeyond the neurogenic region. The possibility that neuronsand at least some of the sense organs share a common origin is supported by Ngn1 null mutant mice, which, inaddition to the loss of the VIIIg, have sensory defects in the utricle, saccule and cochlea [14••]. Likewise, NeuroDknockout mice have both a shortened cochlea and a severereduction in the size of the VIIIg [15•• ,16•• ]. By itself, thisevidence is insufficient to confirm a lineage relationship

Revisiting cell fate specification in the inner ear Fekete and Wu 37

Figure 2

A compartment-boundary model of earmorphogenesis. (a) Model of thecompartmentalized otocyst viewed from ananteromedial perspective, shown bisected bythree boundaries (A–P, M–L and D–V) intoeight developmental compartments(posterodorsolateral and posteroventrolateralnot visible). The budding endolymphatic ductarises near the dorsal pole. (b) Several genesexpressed in different parts of the otocyst areindicated, along with the compartments thatthey are most likely to encompass. In very fewcases have the genes been shown definitively tomeet one another at boundaries. Data arecompiled from studies of both mouse andchicken. (c) Predicted fate map for the earlylabyrinth, showing where cells from eachcompartment of the early otocyst are likely toreside after morphogenesis. The ear is viewedfrom an anteromedial perspective. The possiblelocation of sensory patches relative tocompartment boundaries is indicated. The fatemap is especially hypothetical in the cochlearduct, as very few data are available to define thelocation of compartments and boundaries in thispart of the ear. It is assumed that the posteriorpart of the cochlear duct arises from theposteroventrolateral and posteroventromedialcompartments. Because the A–P boundary hasno molecular markers to define it, the preciselocation of the developing saccule and organ ofCorti (oC) in mammals (or basilar papilla inbirds) is uncertain and so these are shown asgray stripes. If an A–P boundary does exist,these two organs could straddle the boundaryor each might be confined to a singlecompartment. (d) Possible arrangement ofsensory organs and regions of the ear arisingfrom the different developmental compartments.AC: anterior crista; BP: basilar papilla; LC: lateral crista; oC: organ of Corti;PC: posterior crista; SM: saccular macula; UM: utricular macula.

Anterior

Dorsal

Medial

Otocyst gene expression Compartment Sensorypatch

Region

ADM

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AVM

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SM and oC?

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AC

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Current Opinion in Neurobiology

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1 (

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ory

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)M

sx1

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boundary

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(b)

(c)

(d)

between sensory cells and neurons, because NeuroD is alsoexpressed later in sensory hair cells. A direct labelingmethod is required to unequivocally demonstrate that neurons and sensory organs have a shared lineage.

Specification of sensory versus non-sensorytissueThere seems to be a broad, sensory-competent region fromwhich individual sensory patches arise in both mice andchicken. Knowlton [25] recognized this as a thickenedregion in the ventral and medial otocyst of the chicken.The combined expression domains of two genes, Lufngand Serrate (Ser)1/Jagged (Jag)1, can now be used to definethis region molecularly. Expression of Lufng is highest inthe anterior part of this domain, whereas that of Ser1 ishighest in the posterior part in the chicken [24•].

The expression of bone morphogenetic protein 4 (BMP4)marks some (in the mouse) or all (in the chicken) of thepunctate sensory anlagen, confirming that each of theseorgans arises within the Lufng- and/or Ser1-expressingdomains. Exactly how the proteins encoded by Lfng,Ser1/Jag1 and BMP4 are involved in specifying the sensoryorgans is not clear. Efforts to block BMP4 signaling havenot provided meaningful insight into this issue [26,27].

Lufng is one of the three vertebrate orthologs of the fringegene [28]. Members of this gene family are important forestablishing boundaries during development in both fliesand vertebrates [29–32]. Lufng is important for somite segmentation in mice, whereas radical fringe is involved inpositioning the apical ectodermal ridge at the dorsal–ventral(D–V) boundary in the developing chick limb [33,34]. It isintriguing that several sensory organs arise immediatelyadjacent to the dorsal boundary of Lufng expression in thechicken: the anterior and posterior cristae above it, and the

saccular and utricular maculae straddling or below it(Figure 3). The possibility that a Fringe protein may havea similar role in localizing sensory organs at a putative D–Vboundary in the ear raises an interesting parallel. Eventhough Lfng knockout mice do not show an inner ear phenotype [28], possible redundant functions with the twoother vertebrate fringe genes, manic and radical, have notbeen explored.

The sensory versus non-sensory decision may also be regulated by activated Notch signaling through the Jag1ligand (Figure 1). In the dominant mouse mutantsHeadturner and Slalom, missense mutations in Jag1 lead toabsence or severe reduction of the anterior and/or posteriorcristae, their ampullae and the proximal attachments of thesemicircular canals [35•,36•]. Because Jag1 is expressedearly in the sensory patches but not in the non-sensoryampullae [37], its role in the development of non-sensorystructures may be indirect.

Specification of sensory organ typeFurther division of the sensory competent region into distinct sensory organs may be dictated by other genes thatcompartmentalize the otocyst (Figure 2). Figure 3 illustrateshow the Lufng-positive area is bisected at the anterior pole(along the M–L boundary) by the expression of two genes,Pax2 and Fgf3 [6,10,11,18,38,39]. In fact the three genes,Lufng, Pax2 and Fgf3, can be used to differentiate betweenfour different domains that intersect near the anterior pole.Three of these domains are expected to develop sensorypatches near this pole: the anterior crista, the utricular macula and the saccular macula (Figures 2 and 3). The sensory patches arising near this intersection might be acquir-ing their unique identities by the specific proteins expressedin each region, as well as by different signals that may be emanating from the M–L and D–V boundaries.

38 Development

Figure 3

Expression domains of three genes that maycontribute to the specification of sensorypatches and neuroblasts near the anteriorpole of the otocyst. Orientation is the same asin Figure 2. Lufng expression is highest in theanterior pole and fades off in the posteriorotocyst (not indicated here). It has beensuggested that Lufng may confer sensorycompetence to otic epithelial cells. Theexpression domains of Pax2 and Fgf3 bothbisect the Lufng domain near the anteriorpole. Pax2 is primarily expressed medially andencompasses the developing endolymphaticduct, which is shown as a bulge near thedorsal pole of the otocyst. Fgf3 is primarilyexpressed anteroventrolaterally, and thuswould only partially overlap with the Lufngdomain. On the right is a possible model forwhere different sensory organ primordia andneuroblasts might arise with respect to thehypothetical boundaries defined by these

three genes. It is tempting to speculate thatthe neurogenic region (crosses) may be splitby the M–L boundary such that neuroblastsgiving rise to auditory ganglia (AG) and thosegiving rise to vestibular ganglia (VG) may

come from different compartments, asindicated by question marks. AC: anteriorcrista; LC: lateral crista, oC: organ of Corti;PC: posterior crista; SM: saccular macula;UM: utricular macula.

Lufng Pax2 Fgf3Sensory patches

and neurogenic region

oC

UM

VG?AG?

ACLC PC

D–Vboundary A–P

boundary

M–Lboundary

Anterior

Dorsal

Current Opinion in Neurobiology

Medial

SM

In addition to the gene expression data, there is evidencefrom both transplantation studies and gene knockoutexperiments to support the hypothesis that sensory organidentity depends upon compartment identity. When theaxes of the chicken otocyst are systematically reversed bytransplantation, the resulting respecification of the sensoryorgans is paralleled by a global respecification of compart-ments. For example, in a normal chicken inner ear, thecristae are positioned dorsal to the maculae; when the D–Vaxis of a donor otocyst was inverted relative to the D–Vaxis of the host, the sensory organs were respecified, suchthat the cristae again formed dorsal to the maculae [12]. Inconcert with the resulting structural pattern, the expres-sion domains of several regionally expressed genes, such asSensory Organ Homebox (SOHo)-1 and Otx1, were alsorespecified to coincide with the surrounding host tissue.These data are consistent with predictions of the compart-ment-boundary model, whereby respecification of regionalgenes would be a necessary prerequisite to respecifyingsensory organ types.

Another prediction is that the loss of a compartment ‘selector gene’ should alter regional identity and affect thesensory organs within or abutting its expression domain. Inthis context, Pax2 may contribute to cochlear identity,given that a Pax2 knockout model results in agenesis of the mouse cochlea [40]. Likewise, Otx1 is expressed in a posteroventrolateral domain that includes the presumptivelateral crista, and the Otx1 null mutant mouse lacks boththe lateral canal and a normal lateral crista [41,42]. But thetiming of gene expression confounds the issue: Otx1 is notexpressed in the presumptive lateral crista until afterBMP4, leading to the suggestion that Otx1 is needed fordevelopment, rather than specification of the lateral crista[42]. Alternatively, because expression of BMP4 is sharedby all three cristae, the BMP4-positive focus may reflectacquisition of a general crista fate, with the specification ofthe organ as the ‘lateral’ crista requiring Otx1.

The compartment-boundary model assumes that sensoryorgan diversity is specified, at least in part, because theorgans arise within lineage-restricted compartments. Thisleads to the prediction that most of the organs will not berelated by lineage once the compartment boundaries areestablished. So far, lineage mapping in the chick ear supports this prediction, although the small clone sizesobserved would make such relationships hard to detect[23•]. By contrast, fate mapping of slightly larger cell clusters in the frog has uncovered an unexpectedly largedispersion of sensory precursors, with labeled cells oftencolonizing several and far-flung sensory organs [43]. Thislatter observation is inconsistent with a rigorous interpreta-tion of the compartment-boundary model, stressing theneed for similar fate-mapping data in chick.

Specification of hair cells and supporting cellsOur understanding of cell specification in the sensoryorgans has advanced considerably in recent years. Lineage

studies have confirmed that sensory hair cells and supportingcells can arise from common progenitors in the inner ear[23•,44]. It has been suggested that the alternatingarrangement of hair cells and supporting cells might arisethrough lateral inhibition, and might be mediated by theNotch/Delta signaling pathway (Figure 1) [45]. This hasnot proved to be as simplistic as was first imagined [21].

Initially, the idea was that the population of sensory precursors is uniform with respect to their potential to formeither hair cells or supporting cells. As a cell begins to differentiate into a hair cell, it suppresses its immediateneighbors from assuming the same fate through Notch-mediated signaling (Figure 1). The inhibited cells wouldthus adopt the supporting cell fate. As predicted, theNotch1 receptor and several of its ligands (Ser1/Jag1,Ser2/Jag2, Delta1) are expressed in the sensory patches atthe appropriate stages [37,45–47]. Furthermore, interfer-ence of Notch signaling alters the mosaic arrangement ofsensory cells, usually by generating additional rows of hair

Revisiting cell fate specification in the inner ear Fekete and Wu 39

Figure 4

Comparison of Lufng and NeuroD expression domains in mouse otocyst at 10.5 days post coitum. (a,b) Adjacent anterior sectionsprobed for Lufng (a) and NeuroD (b) transcripts. The Lufng-positivedomain overlaps with that of NeuroD in this region. NeuroD is also expressed in the delaminating neuroblasts (arrows in [b]).(c,d) Adjacent posterior sections through the same ear. In more poste-rior sections, the Lufng-positive domain (c) is negative for NeuroD (d).D: dorsal; M: medial. Scale bar, 100 µm.

(a) (b)

(c) (d)

Lufng NeuroD

D

Current Opinion in NeurobiologyM

cells in the well-organized cochlea [35•,36•,48–53]. Theexpected result of converting all sensory cells to hair cellsby inhibiting Notch signaling is observed rarely, however,perhaps because there are several Notch ligands.

Only in the zebrafish mind bomb mutant, which altersNotch signaling by an unknown mechanism, is the predicted phenotype obtained [54]. In the mammaliancochlea, the results are more consistent with activatedNotch limiting the size, or outer border, of the sensorypatch. In the chicken ear, the situation is complicated byevidence that Notch1 signaling may have an additionalrole in mediating lateral induction between supportingcells through a different Notch ligand (Ser1) to those(Delta1, Ser2) proposed to mediate lateral inhibition fromhair cells [21].

In the fly and vertebrate nervous systems, the Notch/Deltasignaling pathway negatively regulates the expression ofthe bHLH neurogenic genes. It does so through an intracellular signaling pathway that activates transcriptionof another bHLH family — the Hairy/Enhancer of Split(Hes) transcriptional suppressors [55,56]. (Note, however,that in mammals the Notch3 receptor has the oppositeeffect, acting to suppress rather than activate Hes transcrip-tion [57].) Genes related to Atonal and Hes are expressed in the developing sense organs in addition to Notch and its ligands [46,49,50,58], suggesting that the complete regulatory cascade is active in the ear.

Math1, a murine bHLH Atonal homolog, is absolutelyrequired for hair cell specification in mice [59••]. Ectopicexpression of Math1 can induce certain non-sensory cells inthe postnatal rat cochlea to adopt a hair cell fate [60••].The ectopic induction of hair cells can be inhibited byHes1 [50]. Moreover, knockout of Hes1 leads to super-numerary inner hair cells, whereas knockout of Hes5 leadsto supernumerary outer hair cells [49,50]. The broadexpression pattern of Hes1 in the cochlea is more consistentwith a possible role in cell fate decisions of sensory versusnon-sensory, rather than hair cell versus supporting cell.For Hes5, its predominant expression only in supportingcells in and around the outer hair cell region makes it amore likely candidate for supporting cell fate determina-tion [49,60••]. Normal expression of Hes5 is downregulatedin supporting cells of the Jag2 knockout mouse, presumablythrough lack of Notch activation [58]. It remains to beshown whether Hes-mediated repression of Math1 is a normal part of supporting cell fate specification (Figure 1),and whether it is controlled by Notch receptor activation aspredicted by both a lateral inhibitory model and homologywith fly mechanoreceptor specification.

Once hair cells have been specified, their continued differentiation requires the transcription factor Pou4f3(Brn3c/Brn3.1). In the absence of this gene, the hair cellphenotype aborts at an early stage, eventually leading tothe death of hair cells, supporting cells and the primary

ganglionic neurons [61–63]. However, the latter cells areknown to require neurotrophins, provided by the sensoryepithelium, for their long-term survival, and therefore theirloss may be indirect.

ConclusionsMany issues remain unresolved in regard to cell fate spec-ification in the inner ear and the possible role of otocystcompartmentalization. Relatively little is known about thesequence in which the boundaries are established. As yet,we have no direct evidence that signaling occurs acrossboundaries in the ear. Furthermore, there are no genemarkers for the putative A–P boundary, which makes itimpossible to place the developing organ of Corti or saccule accurately on the compartment-boundary modelshown in Figure 2. Finally, there might be subdivisionswithin some of these compartments — for example, onethat separates the utricle and saccule from the cochlea inthe ventral compartment, and another that separates theprospective lateral canal from the anterior and posteriorcanals in the lateral compartment.

Virtually nothing is known about the specification of different subtypes of cells — such as vestibular versus auditoryganglion neurons, type I versus type II spiral ganglion neurons, inner versus outer hair cells, type I versus type IIvestibular hair cells and different classes of supporting cell— or the specification of various non-sensory cell types,such as the vestibular dark cells or those in the stria vascularis or the endolymphatic sac. Further understandingof these decisions may be aided by more extensive fate-mapping and lineage analysis, especially in the mammal.

Also unresolved is how sensory cell polarity, specificallystereociliary bundle orientation, is determined. Answers tothis may be found by searching for molecular homologieswith other systems, such as that mediating bristle cellpolarity in the fly cuticle. There is evidence that theWnt/Ca2+ signaling pathway is involved in the stereociliabundle polarity of outer hair cells in mouse cochlear cultures (M Kelley, personal communication).

Another area ripe for investigation concerns the establish-ment and maintenance of longitudinal gradients along theorgan of Corti that underlie the frequency selectivity socrucial to hearing acuity. It is already known that differen-tial mRNA splicing generates an array of calcium-activatedpotassium channels that regulate electrical tuning in auditory hair cells [64,65]. It is tempting to speculate thatthe systematic control of potassium channel mRNA splicing might ultimately share upstream genetic controlmechanisms with those regulating other longitudinal gradients such as organ width, organ stiffness, hair cell sizeand even stereociliary bundle length. Meanwhile, thenext few years promise even more revelations about the control of cell fate specification in the exquisitelycomplex receptive organs and associated tissues of thevertebrate inner ear.

40 Development

AcknowledgementsWe thank Michael Mulheisen for performing in situ hybridization, RodneyMcPhail for preparing the figures, and Matt Kelley and our lab members forcritically reading the manuscript. Grant support to DM Fekete comes fromthe NIH and the March of Dimes Birth Defects Foundation.

References and recommended readingPapers of particular interest, published within the annual period of review,have been highlighted as:

• of special interest••of outstanding interest

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3. Ladher RK, Anakwe KU, Gurney AL, Schoenwolf GC,•• Francis-West PH: Identification of synergistic signals initiating

inner ear development. Science 2000, 290:1965-1967.Strong evidence is provided for the involvement of FGF-19 and Wnt-8C inthe molecular control of otic induction in the chicken. Together, these twoproteins induced several otic markers from competent ectoderm in culture.Evidence supports a model in which FGF-19 emanating from the mesoderminduces and/or maintains expression of Wnt-8C in the neuroectoderm to initiate otic induction.

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2000, 227:256-270.The morphogenetic movements during early otic vesicle formation in chickenare studied by fate-mapping cells at the rim of the otic cup using fluorescent,lipophilic dyes. Results show that the dorsal rim of the otic cup gives rise tothe endolymphatic duct, whereas the posteroventral rim forms the lateral wall of the otocyst. Two lineage restriction boundaries are apparent: an A–Pboundary that bisects the endolymphatic duct, and a M–L boundary that separates the endolymphatic duct medially from the lateral region of the otocyst.

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sensory epithelia devoid of innervation. JARO 2000, 1:129-143.The inner ear of Ngn1 knockout mice lack the vestibulocochlear ganglion inaddition to several other cranial ganglia, providing the first evidence for themolecular basis of ganglion cell fate specification in the ear. The sensory

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Along with Kim et al. [16•• ], these authors report the phenotype of NeuroDknockout mice. The mice have vestibular and auditory behavioral defects and a severe reduction of sensory neurons in vestibulo-cochlear ganglion.The paucity of sensory neurons is due to late and defective delamination of neuroblasts from the otic epithelium, and enhanced cell death of the neuroblasts. The latter observation may be related to the failure of neuroblasts to express the neurotrophin receptors TrkB and TrkC.

16. Kim WY, Fritzsch B, Serls A, Bakel LA, Huang EJ, Reichardt LF, •• Barth DS, Lee JE: NeuroD-null mice are deaf due to a severe loss

of the inner ear sensory neurons during development.Development 2001, 128:417-426.

See annotation to [15•• ].

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23. Lang H, Fekete DM: Lineage analysis in the chicken inner ear• shows differences in clonal dispersion for epithelial, neuronal,

and mesenchymal cells. Dev Biol 2001, 234:120-137.Retroviral-mediated lineage analysis is conducted to investigate clonal relationships and dispersion for epithelial, neuronal and mesenchymal cellsduring chicken inner ear development. Whereas mesenchymal and neuronalcell clones can be dispersed, the epithelial clones are not. There is no evidence for a common lineage between sensory cells and their associatedneurons; however, sensory supporting cells near the inferior edge of the basilar papilla are shown to share a common lineage with adjacent non-sensory cells (border cells and hyaline cells).

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morphogenetic protein 4, serrate 1, and lunatic fringe. J CompNeurol 2000, 424:509-520.

The expression of three putative sensory organ markers, BMP4, Lufng andSer1, are compared systematically in the developing chicken inner ear.Results show that all BMP4-positive, presumptive sensory patches arise in the broad domains of Lufng and/or Ser1, suggesting that a sensory-competent region may exist in the rudimentary inner ear.

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Revisiting cell fate specification in the inner ear Fekete and Wu 41

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Together with Tsai et al. [36•], this paper describes the inner ear phenotypeof two mouse mutants, Headturner and Slalom, that have missense muta-tions in the Notch ligand, Jag1. The anterior and/or posterior ampullae aresmaller or missing, accompanied by truncation of the corresponding semi-circular canal. The number of hair cells in the organ of Corti is often aberrant.These results show that the function of the Notch signaling pathway is notrestricted to hair cell/supporting cell determination but is also important forregional specification in the inner ear.

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A seminal paper showing that Math1 is essential for the generation of sen-sory hair cells in the inner ear. Math1 knockout mice have relatively intactsensory organs but they lack sensory hair cells.

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42 Development