Number Of Hair Cells Research Articles

Cochlear damage caused by continuous and intermittent noise exposure

We compared the extent of permanent threshold shifts (PTS) and cochlear hair cell damage caused by continuous noise exposure with those caused by intermittent noise exposure. Twenty male pigmented guinea pigs that had been exposed to a one-octave band of noise at 4 kHz for 5 h were placed in four groups: exposure to 115 dB SPL continuous noise (group 1, n=5), 115 dB SPL intermittent noise (group 2, n=5), 125 dB SPL continuous noise (group 3, n=5), and 125 dB SPL intermittent noise (group 4, n=5). PTS at 2, 4, 8, and 16 kHz were assessed by means of auditory brainstem responses measured before noise exposure and 10 days after. The guinea pigs were killed 15 days after noise exposure, and the number of hair cells missing counted in surface preparations of the organs of Corti stained with rhodamine phalloidin. Groups 1 and 3 had significantly greater PTS ( P<0.05) at all frequencies than intermittent groups 2 and 4. Group 3 had the greatest PTS at all the frequencies. Intermittent 125 dB noise total energy was greater than that of continuous 115 dB noise, but the latter elicited more PTS than the former. The extent of hair cell damage was comparable to the physiological findings. This indicates that continuous noise causes greater damage to the cochlea than intermittent noise of the same intensity and that, at the intensities tested, damage to the cochlea is not proportional to the total noise energy.

Activating Influences on Supporting Cell Proliferation in the Regenerating Avian Cochlea

Aminoglycoside treatment is associated with hair cell death and hearing loss. In the avian inner ear, it has been well-documented that this hair cell death stimulates the proliferation of adjacent supporting cells, leads to a structural regeneration of hair cells, and results in a subsequent recovery of auditory function. We have been studying the mechanisms that lead to hair cell death in the avian cochlea following aminoglycoside treatment and how this process reactivates the cell cycle in a normally quiescent population of supporting cells. Hair cell death appears to be caused by the induction of apoptosis, or programmed cell death. This apoptotic pathway is initiated within the first 12 hours after a single systemic injection of gentamicin (300 mg/kg), as identified by the nuclear-to-cytoplasmic translocation of the T cell-restricted intracellular antigen-related protein (TIAR). TIAR is an RNA binding protein that is normally found in the cell nucleus but that has been shown to exhibit increased cytoplasmic localization in a variety of tissues undergoing apoptosis. Confocal microscopy demonstrated that within 12 hours of gentamicin injection, punctate cytoplasmic TIAR labeling was seen only in those hair cells in the gentamicin-damaged region of the cochlea that would later proceed to die. Even though these early signs of apoptosis could be detected by 12 hours, signs of structural damage to the hair cells and expression of activated forms of execution stage apoptotic enzymes such as caspase-3 were not seen until 24 to 36 hours after the gentamicin injection. These terminal-stage changes in hair cells were accompanied by an intensification and alteration in the immunocytochemical labeling for myosin VIIa, a hair cell-specific unconventional myosin. The final expulsion of hair cells in the gentamicin-damaged region of the cochlea occurred between 48 and 72 hours after the gentamicin injection and was accompanied by pyknotic and TUNEL-labeled hair cell nuclei. Concurrently, the supporting cells in the damaged region of the cochlea began to enter the S (DNA synthesis) phase of the cell cycle by 65 hours and reached a peak in S phase between 72 and 96 hours. The period of S phase was associated with a loss of connexin43-containing gap junctions between supporting cells in the damaged region of the cochlea. New hair cells were identified in the damaged region by their expression of myosin VIIa at 96 hours. The number of new hair cells increased dramatically by 120 hours and was accompanied by a restoration of connexin43-containing gap junctions between supporting cells in the damaged region of the cochlea. These data reinforce the role of apoptosis in the death of cochlear hair cells after gentamicin treatment and suggest that TIAR, myosin VIIa, and caspase-3 are key components in this apoptotic pathway. Moreover, our data suggest that steps in the hair cell apoptotic pathway directly stimulate supporting cell gap junction disassembly, and this leads to proliferation of a subset of these supporting cells.

内耳有毛細胞再生の誘導

Since the discovery of the spontaneous regeneration of inner ear hair cells in birds and mammals from the late 1980's to the early 1990's, its mechanisms have been studied. Three mechanisms of regeneration of the inner ear hair cells have been reported so far: regeneration by mitosis of supporting cells, transdifferentiation of supporting cells into hair cells, and self-repair of the sublethal hair cells. However, which mechanism contributes most to the regeneration of the inner ear hair cells has remained to be elucidated.Extrinsic and diffusible factors like various growth factors have been reported to proliferate hair cell numbers in the inner ear. Regeneration of organs could be achieved not only by induction of proliferation with extrinsic factors but also by repeating their developmental pathways in adult organs. Molecular biology and gene targeting reveal that various intrinsic factors and signaling pathways such as cell cycle regulators, WNT signaling, and Notch signaling have important roles in the development of inner ears, and that progenitors of sensory epithelium would exist at the greater epithelial ridge in the inner ear. Further analysis and application of these factors will contribute to regeneration of the inner ear hair cells.

M40403, a superoxide dismutase mimetic, protects cochlear hair cells from gentamicin, but not cisplatin toxicity

Gentamicin, an aminoglycoside antibiotic, and cisplatin, a platinum-based anticancer drug, are two commonly used clinical drugs with ototoxic side effects. The ototoxicity of gentamicin and cisplatin has been linked to the production of reactive oxygen species (ROS), although the specific ROS pathways have not been identified. One ROS that might play a role in ototoxicity is the superoxide radical, which is enzymatically dismutated to molecular oxygen and hydrogen peroxide by endogenous superoxide dismutase (SOD) enzymes. M40403, a manganese-based nonpeptidyl molecule that mimics the activity of SOD, was tested for its ability to protect against gentamicin and cisplatin toxicity in cochlear organotypic cultures from neonatal C57BL/10J mice. Cultures were treated with gentamicin or cisplatin alone or in combination with M40403. M40403 alone had no effect on outer hair cell (OHC) or inner hair cell (IHC) survival at doses of 1, 5, and 10 μM, but a high dose of 30 μM reduced hair cell numbers by approximately 30%. Gentamicin alone and cisplatin alone killed OHCs and IHCs in a dose-dependent manner. The addition of M40403 to gentamicin-treated cultures significantly increased OHC and IHC survival in a dose-dependent manner, whereas M40403 failed to protect hair cells in cisplatin-treated cultures at any dose. The results suggest that the toxicity of gentamicin and cisplatin to cochlear hair cells are mediated by different pathways. Clinically, increased levels of SOD or SOD mimetics might provide significant protection against aminoglycoside ototoxicity.

An expanded domain of fgf3 expression in the hindbrain of zebrafish valentino mutants results in mis-patterning of the otic vesicle.

The valentino (val) mutation in zebrafish perturbs hindbrain patterning and, as a secondary consequence, also alters development of the inner ear. We have examined the relationship between these defects and expression of fgf3 and fgf8 in the hindbrain. The otic vesicle in val/val mutants is smaller than normal, yet produces nearly twice the normal number of hair cells, and some hair cells are produced ectopically between the anterior and posterior maculae. Anterior markers pax5 and nkx5.1 are expressed in expanded domains that include the entire otic epithelium juxtaposed to the hindbrain, and the posterior marker zp23 is not expressed. In the mutant hindbrain, expression of fgf8 is normal, whereas the domain of fgf3 expression expands to include rhombomere 4 through rhombomere X (an aberrant segment that forms in lieu of rhombomeres 5 and 6). Depletion of fgf3 by injection of antisense morpholino (fgf3-MO) suppresses the ear patterning defects in val/val embryos: Excess and ectopic hair cells are eliminated, expression of anterior otic markers is reduced or ablated, and zp23 is expressed throughout the medial wall of the otic vesicle. By contrast, disruption of fgf8 does not suppress the val/val phenotype but instead interacts additively, indicating that these genes affect distinct developmental pathways. Thus, the inner ear defects observed in val/val mutants appear to result from ectopic expression of fgf3 in the hindbrain. These data also indicate that val normally represses fgf3 expression in r5 and r6, an interpretation further supported by the effects of misexpressing val in wild-type embryos. This is in sharp contrast to the mouse, in which fgf3 is normally expressed in r5 and r6 because of positive regulation by kreisler, the mouse ortholog of val. Implications for co-evolution of the hindbrain and inner ear are discussed.

Fibroblast Growth Factor Signaling Regulates Pillar Cell Development in the Organ of Corti

One of the most striking aspects of the cellular pattern within the sensory epithelium of the mammalian cochlea is the presence of two rows of pillar cells in the region between the single row of inner hair cells and the first row of outer hair cells. The factors that regulate pillar cell development have not been determined; however, previous results suggested a key role for fibroblast growth factor receptor 3 (FGFR3). To examine the specific effects of FGFR3 on pillar cell development, we inhibited receptor activation in embryonic cochlear explant cultures. Results indicated that differentiation of pillar cells is dependent on continuous activation of FGFR3. Moreover, transient inhibition of FGFR3 did not inhibit the pillar cell fate permanently, because reactivation of FGFR3 resulted in the resumption of pillar cell differentiation. The effects of increased FGFR3 activation were determined by exposing cochlear explants to FGF2, a strong ligand for several FGF receptors. Treatment with FGF2 led to a significant increase in the number of pillar cells and to a small increase in the number of inner hair cells. These effects were not dependent on cellular proliferation, suggesting that additional pillar cells and inner hair cells were a result of increased recruitment into the prosensory domain. These results indicate that FGF signaling plays a critical role in the commitment and differentiation of pillar cells. Moreover, the position of the pillar cells appears to be determined by the activation of FGFR3 in a subset of the progenitor cells that initially express this receptor.

Notch/ Notch ligands and Math1 expression patterns in the organ of Corti of wild-type and Hes1 and Hes5 mutant mice

The sensory epithelium of the mammalian cochlea (the organ of Corti) represents an excellent developmental system. The organ of Corti contains two main cell types: the sensory hair cells and the supporting cells which are organized in a defined mosaic pattern. Previous results have demonstrated the participation of Notch signaling in the regulation of the pattern of hair cell differentiation within this sensory mosaic. It has also been shown that the basic helix-loop-helix (bHLH) transcription factor Math1 is a positive regulator of hair cell differentiation. We demonstrated that Hes1 and Hes5, two members of the inhibitory bHLH transcription factors, act as negative regulators of hair cell differentiation. Loss-of-function studies implicating the neurogenic genes Notch1, Jag2, Hes1 and Hes5 generated a significant increase in the number of hair cells. However, their functional interplay within the organ of Corti has not been determined. To clarify the mechanisms that regulate hair cell differentiation, we examined the expression of Notch/Notch ligand system and Math1 in the developing organ of Corti of Hes1- and Hes5-deficient mice. Our study suggests complex specific relationships between Notch signaling, Math1 and Hes1/ Hes5 in the control of hair cell differentiation in the developing organ of Corti.

FGFR1 Is Required for the Development of the Auditory Sensory Epithelium

The mammalian auditory sensory epithelium, the organ of Corti, comprises the hair cells and supporting cells that are pivotal for hearing function. The origin and development of their precursors are poorly understood. Here we show that loss-of-function mutations in mouse fibroblast growth factor receptor 1 ( Fgfr1) cause a dose-dependent disruption of the organ of Corti. Full inactivation of Fgfr1 in the inner ear epithelium by Foxg1-Cre-mediated deletion leads to an 85% reduction in the number of auditory hair cells. The primary cause appears to be reduced precursor cell proliferation in the early cochlear duct. Thus, during development, FGFR1 is required for the generation of the precursor pool, which gives rise to the auditory sensory epithelium. Our data also suggest that FGFR1 might have a distinct later role in intercellular signaling within the differentiating auditory sensory epithelium.

The middle ear of gekkonoid lizards: interspecific variation of structure in relation to body size and to auditory sensitivity

Wishing to assess the effects of the dimensions of the middle ear on the auditory sensitivity of gekkonoid lizards, we measured middle ear components in preserved geckos, which in life had yielded ‘cochlear microphonics’ audiograms. We examined two to seven specimens of 14 species. The measures of middle ear elements varied relative to head or body length similarly within species and among species. The areas of the external ear opening, tympanic membrane, and columellar footplate, and the ratio between the last two (‘hydraulic lever’), were correlated with animal length. The hydraulic and mechanical (extracolumellar) lever ratios appeared to complement each other, the former being emphasized in large animals, the latter in small animals. The apparent auditory sensitivity correlated with the sizes of the animal, head and external ear opening, and negatively (insignificantly) correlated with the mechanical lever ratio. The correlation of sensitivity with the hydraulic lever was insignificant, perhaps due to a ‘tympanic membrane lever’ (catenary effect). The most sensitive frequency negatively correlated with the area of the external ear opening, the area of the tympanic membrane, and with the level of greatest sensitivity. It was positively correlated with the relative length of the cartilaginous portion of the ossicular chain. However, the number of hair cells in the basilar papilla, too, is known to correlate with animal size. Moreover, the least sensitive species were not only the smallest species, they were also the species known to lack a zone of unidirectional hair cells in the basilar papilla. Hence the apparent sensitivity hypothetically depends on both middle ear dimensions and summation of inner ear output. This hypothesis requires verification by other methods.

An easy introduction to organismal and cellular mechanisms for graviperception

In this review paper, organismal and also cellular mechanisms for perception of gravity are explained. A statolith and a number of hair cells which surround the statolith is a basic structure of statocysts for detecting the direction of gravity or tilting of the body in various animals. The vestibular system of vertebrate was explained, especially on the process from the body-tilting to impulse frequencies which travel to brain. For the cellular responses to gravity, contribution of various organella (??) and cytoskeleton are introduced. Such cellular responses may change when the gravity values become less or null. Gravity perception mechanisms of plants are also explained.

Determination and commitment of mechanosensory hair cells.

Sound and movement are perceived through the vibration of modified ciliary bundles located on the apical surfaces of specialized mechanosensory hair cells. These hair cells derive from specific regions of the otocyst that become determined to develop initially as sensory epithelia and ultimately as either hair cells or supporting cells. The number of hair cells in an individual vertebrate is surprisingly small and the ability to replace these cells varies among different classes. The molecular and cellular factors that specify hair cell identity are not known, but the results of recent experiments have begun to identify some of the signaling pathways that play important roles in hair cell development. This review will describe recent findings related to the factors that influence the final choice of a progenitor cell to develop as a hair cell and discuss their implications for the overall development of the auditory and vestibular systems.

Synthetic speech stimuli spectrally normalized for nonhuman cochlear dimensions

Behavioral or neural measures of speech encoding are often taken from animals with auditory systems that differ substantially from those of humans. Absolute distance between spectral peaks of speech sounds along the basilar membrane is typically much greater in humans than in smaller animals. To address this problem, a synthesizer was developed for creating speech scaled for nonhuman cochleae in which spectra are warped to account for differences in cochlear physiology. Absolute cochlear distance between spectral peaks (formants) is held constant across species while formant bandwidths are also scaled to span equal numbers of hair cells as for humans. This was accomplished via a flexible reimplementation of the KLATT80 speech synthesizer in MATLAB.

Efferent axons in the avian auditory nerve.

The sensory hair cells of the inner ear receive both afferent and efferent innervation. The efferent supply to the auditory organ has evolved in birds and mammals into a separate complex system, with several types of neurons of largely unknown function. In this study, the efferent axons in four different species of birds (chicken, starling, barn owl and emu) were examined anatomically. Total numbers of efferents supplying the cochlear duct (auditory basilar papilla and the vestibular lagenar macula) were determined; separate estimates of the efferents to the lagenar macula only were also derived and subtracted. The numbers for auditory efferents thus varied between 120 (chicken) and 1068 (barn owl). Considering the much larger numbers of hair cells in the basilar papilla, each efferent is predicted to branch extensively. However, pronounced species-specific differences as well as regional differences along the tonotopic gradient of the basilar papilla were documented. Myelinated and unmyelinated axons were found, with mean diameters of about 1 microm and about 0.5 microm, respectively. This suggests two basic populations of efferents, however, they did not appear to be distinguished sharply. Evidence is presented that some efferents lose their myelination at the transition from central oligodendrocyte to peripheral Schwann cell myelin. Finally, a comparison of the four bird species evaluated suggests that the efferent population with smaller, unmyelinated axons is the phylogenetically more primitive one. A new population probably arose in parallel with the evolution and differentiation of the specialized hair-cell type it innervates, the short hair cell.

Expression of proneural and neurogenic genes in the zebrafish lateral line primordium correlates with selection of hair cell fate in neuromasts.

Expression of a mouse atonal homologue, math1, defines cells with the potential to become sensory hair cells in the mouse inner ear (Science 284 (1999) 1837) and Notch signaling limits the number of cells that are permitted to adopt this fate (Nat. Genet. 21 (1999) 289; J. Neurocytol. 28 (1999) 809). Failure of lateral inhibition mediated by Notch signaling is associated with an overproduction of ear hair cells in the zebrafish mind bomb (mib) and deltaA mutants (Development 125 (1998a) 4637; Development 126 (1999) 5669), suggesting a similar role for these genes in limiting the number of hair cells in the zebrafish ear. This study extends the analysis of proneural and neurogenic gene expression to the lateral line system, which detects movement via clusters of related sensory hair cells in specialized structures called neuromasts. We have compared the expression of a zebrafish atonal homologue, zath1, and neurogenic genes, deltaA, deltaB and notch3, in neuromasts and the posterior lateral line primordium (PLLP) of wild-type and mib mutant embryos. We describe progressive restriction of proneural and neurogenic gene expression in the migrating PLLP that appears to correlate with selection of hair cell fate in maturing neuromasts. In mib mutants there is a failure to restrict expression of zath1 and Delta homologues in the neuromasts revealing similarities with the phenotype previously described in the ear.

Supernumerary Outer Hair Cells Arise External to the Last Row of Sensory Cells in the Organ of Corti

During the development of the mammalian inner ear, the number of hair cells produced is highly regulated and remains constant throughout life. The mechanism underlying this regulation is beginning to be understood although many aspects still remain obscure. When late embryonic or early postnatal rat organs of Corti were cultured, the production of supernumerary hair cells was observed. This overproduction of sensory cells could be modulated by the addition of several growth factors. In this study, we examined explants of rat organs of Corti that produced supernumerary hair cells. In the supernumerary hair cell region, up to two rows of inner hair cells and five rows of outer hair cells were observed. Morphological evaluation of these specimens revealed that less mature hair cells were located in the most external rows of these sensory cells. When a supernumerary hair cell was produced, a supporting cell (i.e. Deiters' cell) was also produced, strongly suggesting that the conversion of a Deiters' cell into a hair cell was not the mechanism that produced these extra hair cells. Based on these results, we propose that prosensory cells located at the external edge of the organ of Corti retain a capacity to form hair cells and that it is these prosensory cells that differentiate into supernumerary hair cells and Deiters' cells.

A quantitative analysis of the nerve fibers in the VIIIth nerve of Belgian Waterslager canaries with a hereditary sensorineural hearing loss

The number of auditory nerve fibers was determined for non-Belgian Waterslager canaries (non-BWS) and Belgian Waterslager canaries (BWS) that are affected by a sensorineural high frequency hearing loss and a 30% reduction in the number of auditory hair cells. Counts were obtained from semithin cross sections of the Durcupan-embedded auditory nerve at the level of the internal auditory meatus. In addition, the number of lagenar fibers was determined from cross sections near the apical end of the cochlear duct in order to separate them from the total number of auditory nerve fibers. The mean number of auditory nerve fibers was 6076 in non-BWS and 5363 in BWS canaries, representing a 12% reduction in BWS. This small reduction in the number of auditory nerve fibers, as compared to the larger reduction in hair cell number, might be explained by a predominant loss of abneural hair cells in BWS, since it has been shown for other species that a large proportion of abneural hair cells are devoid of afferent innervation. In addition, we observed that despite the prominent hair cell pathologies documented for BWS canaries, the mean diameter of auditory nerve fibers from non-BWS canaries (2.22±0.81 μm) did not differ from those of BWS canaries (2.21±0.96 μm).

Hes1 is a negative regulator of inner ear hair cell differentiation.

Hair cell fate determination in the inner ear has been shown to be controlled by specific genes. Recent loss-of-function and gain-of-function experiments have demonstrated that Math1, a mouse homolog of the Drosophila gene atonal, is essential for the production of hair cells. To identify genes that may interact with Math1 and inhibit hair cell differentiation, we have focused on Hes1, a mammalian hairy and enhancer of split homolog, which is a negative regulator of neurogenesis. We report here that targeted deletion of Hes1 leads to formation of supernumerary hair cells in the cochlea and utricle of the inner ear. RT-PCR analysis shows that Hes1 is expressed in inner ear during hair cell differentiation and its expression is maintained in adulthood. In situ hybridization with late embryonic inner ear tissue reveals that Hes1 is expressed in supporting cells, but not hair cells, of the vestibular sensory epithelium. In the cochlea, Hes1 is selectively expressed in the greater epithelial ridge and lesser epithelial ridge regions which are adjacent to inner and outer hair cells. Co-transfection experiments in postnatal rat explant cultures show that overexpression of Hes1 prevents hair cell differentiation induced by Math1. Therefore Hes1 can negatively regulate hair cell differentiation by antagonizing Math1. These results suggest that a balance between Math1 and negative regulators such as Hes1 is crucial for the production of an appropriate number of inner ear hair cells.

Changes in the avian cochlea after single high-dose gentamicin

Purpose: Define the time course of functional and anatomical damage and subsequent recovery (by regeneration) of hair cells in the chicken inner ear after a single high-dose of gentamicin. Materials and Methods: Broiler chicks were given a single intraperitoneal dose (200 mg/kg) of gentamicin (n = 39) or saline (n = 39). Functional status was evaluated with auditory brainstem response (ABR) thresholds before injection and before sacrifice at 2, 5, 9, 16, 21, 28, and 70 days postinjection. The cochleae were then examined with scanning electron microscopy (SEM) to assess the extent of damage along the cochlea and absolute hair cell numbers in the basal 15% of the cochlea (high-frequency region). Results: Considerable variability between animals was seen for both ABR and SEM changes. Damage was maximal at 5 days postinjection with an average ABR threshold shift of 12 dB (range −10 to 50 dB) and basal cochlear damage of 28% (range 12%-57%). Hair cell counts were significantly decreased in the basal 15% of the cochlea at 5 days. Hair cell regeneration resulted in rapid anatomical and functional recovery, but evidence of hair cell disorganization persisted at 70 days despite improved thresholds. Conclusion: A single high dose of gentamicin produces a significant but variable anatomical and functional insult in the chick cochlea. Hair cell regeneration results in rapid but incomplete recovery. (Am J Otolaryngol 2000;21:379-388. Copyright © 2000 by W.B. Saunders Company)

Notch signaling regulates the pattern of auditory hair cell differentiation in mammals.

The development of the mammalian cochlea is an example of patterning in the peripheral nervous system. Sensory hair cells and supporting cells in the cochlea differentiate via regional and cell fate specification. The Notch signaling components shows both distinct and overlapping expression patterns of Notch1 receptor and its ligands Jagged1 (Jag1) and Jagged2 (Jag2) in the developing auditory epithelium of the rat. On embryonic day 16 (E16), many precursor cells within the Kölliker's organ immunostained for the presence of both Notch1 and Jag1, while the area of hair cell precursors did not express either Notch1 and Jag1. During initial events of hair cell differentiation between E18 and birth, Notch1 and Jag1 expression predominated in supporting cells and Jag2 in nascent hair cells. Early after birth, Jag2 expression decreased in hair cells while the pattern of Notch1 expression now included both supporting cells and hair cells. We show that the normal pattern of hair cell differentiation is disrupted by alteration of Notch signaling. A decrease of either Notch1 or Jag1 expression by antisense oligonucleotides in cultures of the developing sensory epithelium resulted in an increase in the number of hair cells. Our data suggest that the Notch1 signaling pathway is involved in a complex interplay between the consequences of different ligand-Notch1 combinations during cochlear morphogenesis and the phases of hair cell differentiation.

Expression of Math1 and HES5 in the cochleae of wildtype and Jag2 mutant mice.

The sensory epithelium within the mammalian cochlea (the organ of Corti) is a strictly ordered cellular array consisting of sensory hair cells and nonsensory supporting cells. Previous research has demonstrated that Notch-mediated lateral inhibition plays a key role in the determination of cell types within this array. Specificallly, genetic deletion of the Notch ligand, Jagged2, results in a significant increase in the number of hair cells that develop within the sensory epithelium, presumably as a result of a decrease in Notch activation. In contrast, the downstream mediators and targets of the Notch pathway in the inner ear have not been determined but they may include genes encoding the proneural gene Math1 as well as the HES family of inhibitory bHLH proteins. To determine the potential roles of these genes in cochlear development, in situ hybridization for Math1 and HES5 was performed on the cochleae of wild-type vs. Jagged2 mutants (Jag2deltaDSL). Results in wild-type cochleae show that expression of Math1 transcripts in the duct begins on E13 and ultimately becomes restricted to hair cells in the sensory epithelium. In contrast, expression of HES5 begins on E15 and becomes restricted to supporting cells in the epithelium. Results in Jag2 mutant cochleae suggest that Math1 transcripts are ultimately maintained in a larger number of cells as compared with wild-type, while transcripts for HES5 are dramatically reduced throughout the epithelium. These results are consistent with the hypothesis that activation of Notch via Jagged2 acts to inhibit expression of Math1 in cochlear progenitor cells, possibly through the activity of HES5.