Engraftment and differentiation of embryonic stem cell–derived neural progenitor cells in the cochlear nerve trunk: Growth of processes into the organ of corti

Hearing loss in mammals is irreversible because cochlear neurons and hair cells do not regenerate. To determine whether we could replace neurons lost to primary neuronal degeneration, we injected EYFP‐expressing embryonic stem cell–derived mouse neural progenitor cells into the cochlear nerve trunk in immunosuppressed animals 1 week after destroying the cochlear nerve (spiral ganglion) cells while leaving hair cells intact by ouabain application to the round window at the base of the cochlea in gerbils. At 3 days post transplantation, small grafts were seen that expressed endogenous EYFP and could be immunolabeled for neuron‐specific markers. Twelve days after transplantation, the grafts had neurons that extended processes from the nerve core toward the denervated organ of Corti. By 64–98 days, the grafts had sent out abundant processes that occupied a significant portion of the space formerly occupied by the cochlear nerve. The neurites grew in fasciculating bundles projecting through Rosenthal's canal, the former site of spiral ganglion cells, into the osseous spiral lamina and ultimately into the organ of Corti, where they contacted hair cells. Neuronal counts showed a significant increase in neuronal processes near the sensory epithelium, compared to animals that were denervated without subsequent stem cell transplantation. The regeneration of these neurons shows that neurons differentiated from stem cells have the capacity to grow to a specific target in an animal model of neuronal degeneration. © 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Reinnervation of hair cells by auditory neurons after selective removal of spiral ganglion neurons

Hearing loss can be caused by primary degeneration of spiral ganglion neurons or by secondary degeneration of these neurons after hair cell loss. The replacement of auditory neurons would be an important step in any attempt to restore auditory function in patients with damaged inner ear neurons or hair cells. Application of beta-bungarotoxin, a toxin derived from snake venom, to an explant of the cochlea eradicates spiral ganglion neurons while sparing the other cochlear cell types. The toxin was found to bind to the neurons and to cause apoptotic cell death without affecting hair cells or other inner ear cell types as indicated by TUNEL staining, and, thus, the toxin provides a highly specific means of deafferentation of hair cells. We therefore used the denervated organ of Corti for the study of neuronal regeneration and synaptogenesis with hair cells and found that spiral ganglion neurons obtained from the cochlea of an untreated newborn mouse reinnervated hair cells in the toxin-treated organ of Corti and expressed synaptic vesicle markers at points of contact with hair cells. These findings suggest that it may be possible to replace degenerated neurons by grafting new cells into the organ of Corti.     

Expression of TrkB and BDNF in human cochlea-an immunohistochemical study

Surgical human cochlear specimens were obtained during the removal of large posterior cranial fossa meningioma by a transcochlear approach in which the cochlea was removed for maximal exposure of the tumor and protection of important structures, such as the brainstem, cranial nerves, and pivotal blood vessels. The cochlear tissue was fixed and cryo-sectioned for tyrosine kinase receptor B (TrkB) and brain-derived neurotrophic factor (BDNF) immunohistochemistry. TrkB receptor protein was expressed in both neuronal somata and the processes of human spiral ganglion neurons (SGNs). In the human organ of Corti, TrkB immunoreactivity was mainly present in nerve fibers underneath outer hair cells. BDNF expression was found neither in the organ of Corti nor in the spiral ganglion of human cochlea. For antibody specificity and for control and comparative purposes, TrkB immunocytochemistry was performed in primary cultures of cochlear neuron/glia from adult guinea pig. Confocal laser scanning microscopy showed that TrkB was homogeneously distributed in the cytoplasm of both neuronal somata and axons. Knowledge of the expression of TrkB receptor in human cochlea should help to determine the target structures for neuron preservation in hearing-impaired patients. Our results indicate that the regeneration of SGNs under pathological conditions can be enhanced with BDNF/TrkB-based pharmaceutical or genetic strategies.     

Targeted connexin26 ablation arrests postnatal development of the organ of Corti

Mutations in the gene coding for connexin26 (Cx26) is the most common cause of human nonsyndromic hereditary deafness. To investigate deafness mechanisms underlying Cx26 null mutations, we generated three independent lines of conditional Cx26 null mice. Cell differentiation and gross cochlear morphology at birth seemed normal. However, postnatal development of the organ of Corti was stalled as the tunnel of Corti and the Nuel’s space were never opened. Cell degeneration was first observed in the Claudius cells around P8. Outer hair cell loss was initially observed around P13 at middle turn when inner hair cells were still intact. Massive cell death occurred in the middle turn thereafter and gradually spread to the basal turn, resulting in secondary degeneration of spiral ganglion neurons in the corresponding cochlear locations. These results demonstrated that Cx26 plays essential roles in postnatal maturation and homoeostasis of the organ of Corti before the onset of hearing.     

腺病毒介导的NT3基因转染对噪音损伤的耳蜗螺旋神经节细胞的保护作用

神经营养素 3(neurotrophin 3,NT3)作为螺旋神经节细胞特异的营养因子 ,可有效地支持内耳传入神经元的存活 ,因此有望成为治疗因其退变而引起的感音性神经性耳聋的有效因子。实验采用腺病毒介导lacZ基因 ,检测了外源基因在豚鼠内耳中的长期表达。用噪音制备了豚鼠耳聋模型 ,在噪音损伤后第 7天 ,通过圆窗膜注入 1× 10 8重组腺病毒。注入神经营养素 3重组腺 (Ad NT3)的组为实验组 ,注入Ad lacZ的为对照组。 4周后 ,经NT3抗体免疫细胞化学染色可见 ,在注入Ad NT3病毒的实验组中 ,在内耳多种细胞中有明显的NT3蛋白的表达。HE染色显示 ,注射Ad lacZ组的豚鼠耳蜗螺旋神经节细胞明显退变 ,螺旋神经节内细胞间隙拉大 ,细胞密度明显低于注射Ad NT3实验组动物 (P <0 .0 1)。这一结果说明 ,腺病毒介导的NT3基因可长期表达于内耳中 ,并且可在噪音引起毛细胞死亡后有效地抑制螺旋神经节细胞的退变。     

Fibroblast growth factor-2 immunoreactivity is present in the central and peripheral auditory pathways of adult rats

Recent findings have pointed out the role of neurotrophic factors in the survival and maintenance of neurons of the auditory system. Basic fibroblast growth factor (bFGF, FGF-2) is a potent neurotrophic molecule whose actions can be seen in the central and peripheral nervous systems. In the present study, FGF-2 immunoreactivity was analyzed in the auditory pathways of the adult rat, employing a well-characterized polyclonal antibody against FGF-2. In the cochlea, FGF-2 immunoreactivity was observed in the inner and outer hair cells of the organ of Corti, spiral ganglion neurons, spiral limbus, and stria vascularis. Stereological methods employing optical fractionator revealed the presence of 84.5, 15, and 0.5% of spiral ganglion neurons possessing FGF-2 immunoreactivity of strong, moderate, and weak intensity, respectively. In the central auditory pathways, FGF-2 immunoreactivity was found in the cytoplasm of the neurons of the cochlear nuclei, trapezoid body nuclei, medial geniculate nucleus, and inferior colliculus. The two-color immunoperoxidase method showed FGF-2 immunoreactivity in the nuclei of astrocytes throughout the central auditory pathway. Computer-assisted microdensitometric image analysis revealed higher levels of specific mean gray values of FGF-2 immunoreactivity in the trapezoid body and ventral cochlear nucleus and also in the spiral ganglion and inner hair cells. Sections incubated with FGF-2 antibody preabsorbed with human recombinant FGF-2 showed no immunoreaction in the majority of the studied regions, exhibiting only a slight immunoreactive product in the hair cells of the organ of Corti. Furthermore, no changes in immunoreactivity were observed in sections incubated with FGF-2 antiserum preincubated with human recombinant acidic FGF (FGF-1). The findings suggest that FGF-2 may exert paracrine and autocrine actions on neurons of the central and peripheral auditory systems and may be of importance in the mechanism of hearing diseases.     

Apoptosis of inner hair cells caused by laser ablation of their spiral ganglion neurons in cultures of the mouse organ of Corti

Laser beam ablation of spiral ganglion neurons was performed in seven organotypic cultures of the newborn mouse cochlea between 5 and 8 days in vitro, with a recovery period of from 18 hours to 3 days. Direct somatic injury (laser or mechanical) inflicted on hair cells does not necessarily cause their death; many of them survive, repair damage and re-establish their neurosensory connections. By contrast, laser irradiation and ablation of their afferent spiral ganglion neurons causes a most spectacular degeneration of sensory cells within 18–48 hours after the insult. Ultrastructurally, the degenerated hair cells—characteristically the inner hair cells—display “dark-cell vacuolar degeneration” that combines the signs of apoptotic death (the peripheral condensation of nuclear chromatin and nuclear pyknosis) with signs of cell edema, vacuolization and necrosis. The ultimate condensation of the cytoplasm gives the dead cells a jet black appearance. The irradiated spiral ganglion neurons die displaying similar pathological characteristics. The extent and locus of inner hair cell degeneration correspond to that of ablated spiral ganglion neurons: ultimately the ablation of one neuron causes degeneration of a single inner hair cell within the closest radial segment of the afferent innervation. The elimination of spiral ganglion neurons by mechanical means does not affect hair cell survival. It is inferred that the laser pulse acts as a stimulus depolarizing the neuronal membrane of the spiral ganglion neurons and their radial fibers and causing the excitotoxic death of their synaptic sensory cells through excessive stimulation of the glutamatergic receptors. Reciprocal pre-and postsynaptic synapses between the afferent dendrites and inner hair cells in culture could possibly serve as entryways of the stimulus. The pathogenesis of this apparent transsynaptically-induced apoptotic death of inner hair cells will be further examined in culture.     

Primary Culture and Plasmid Electroporation of the Murine Organ of Corti.

In all mammals, the sensory epithelium for audition is located along the spiraling organ of Corti that resides within the conch shaped cochlea of the inner ear (fig 1). Hair cells in the developing cochlea, which are the mechanosensory cells of the auditory system, are aligned in one row of inner hair cells and three (in the base and mid-turns) to four (in the apical turn) rows of outer hair cells that span the length of the organ of Corti. Hair cells transduce sound-induced mechanical vibrations of the basilar membrane into neural impulses that the brain can interpret. Most cases of sensorineural hearing loss are caused by death or dysfunction of cochlear hair cells.An increasingly essential tool in auditory research is the isolation and in vitro culture of the organ explant ^1,2,9. Once isolated, the explants may be utilized in several ways to provide information regarding normative, anomalous, or therapeutic physiology. Gene expression, stereocilia motility, cell and molecular biology, as well as biological approaches for hair cell regeneration are examples of experimental applications of organ of Corti explants.This protocol describes a method for the isolation and culture of the organ of Corti from neonatal mice. The accompanying video includes stepwise directions for the isolation of the temporal bone from mouse pups, and subsequent isolation of the cochlea, spiral ligament, and organ of Corti. Once isolated, the sensory epithelium can be plated and cultured in vitro in its entirety, or as a further dissected micro-isolate that lacks the spiral limbus and spiral ganglion neurons. Using this method, primary explants can be maintained for 7-10 days. As an example of the utility of this procedure, organ of Corti explants will be electroporated with an exogenous DsRed reporter gene. This method provides an improvement over other published methods because it provides reproducible, unambiguous, and stepwise directions for the isolation, microdissection, and primary culture of the organ of Corti.     

Neurotrophin Gene Therapy for Sustained Neural Preservation after Deafness

The cochlear implant provides auditory cues to profoundly deaf patients by electrically stimulating the residual spiral ganglion neurons. These neurons, however, undergo progressive degeneration after hearing loss, marked initially by peripheral fibre retraction and ultimately culminating in cell death. This research aims to use gene therapy techniques to both hold and reverse this degeneration by providing a sustained and localised source of neurotrophins to the deafened cochlea. Adenoviral vectors containing green fluorescent protein, with or without neurotrophin-3 and brain derived neurotrophic factor, were injected into the lower basal turn of scala media of guinea pigs ototoxically deafened one week prior to intervention. This single injection resulted in localised and sustained gene expression, principally in the supporting cells within the organ of Corti. Guinea pigs treated with adenoviral neurotrophin-gene therapy had greater neuronal survival compared to contralateral non-treated cochleae when examined at 7 and 11 weeks post injection. Moreover; there was evidence of directed peripheral fibre regrowth towards cells expressing neurotrophin genes after both treatment periods. These data suggest that neurotrophin-gene therapy can provide sustained protection of spiral ganglion neurons and peripheral fibres after hearing loss.     

Loss And Survival of Spiral Ganglion Neurons in the Guinea Pig After Intracochlear Perfusion with Aminoglycosides

Loss of cochlear hair cells results in a loss of ganglion cells and further neurodegenerative changes throughout the auditory pathway. Understanding more about the early stages of ganglion cell loss in vivo may lead to ways of ameliorating or preventing the loss of these neurons. To examine these stages, the effects of intracochlear perfusion with aminoglycoside antibiotics on the organ of Corti and spiral ganglion cells were evaluated in young adult guinea pigs at survival periods ranging from 1 hour to 12 weeks, using immunocytochemical and ultrastructural techniques. At 1 hour survival a base-to-apex gradient of damage was indicated in the cochlea by the appearance of severely damaged hair cells and injured ganglion cells in the basal coil while in the apical coil, hair cells were damaged but intact and ganglion cells appeared normal. By 4 hours the appearance of severely disrupted hair cells and damaged ganglion cells had extended throughout the cochlea. The ultrastructural appearance of many injured ganglion cells demonstrated features characteristic of cell death including condensed cytoplasm, non-marginal clumping of nuclear chromatin, and wrinkled nuclear membrane. Despite the loss of many ganglion cells, a population of these cells remained at 12 weeks survival. These contained large amounts of rough endoplasmic reticulum, were unmyelinated apart from the central process and were surrounded by satellite cells. These features are typical of ganglion cells during development, before the onset of hearing. Immunolabelling of cochlear whole mounts after hair cell destruction with protein gene product 9.5 (PGP 9.5) revealed the presence of neural elements in the organ of Corti at up to 12 weeks survival. These may associated with the remaining ganglion cells. In these surviving ganglion cells, the intense labelling with PGP 9.5 together with the increase in rough endoplasmic reticulum, indicates the presence of active protein synthesis which may be connected with their survival.     

Stem cells for the replacement of inner ear neurons and hair cells

Stem cells in the nervous system have some capacity to restore damaged tissue. Proliferation of stem cells endows them with self-renewal ability and accounts for in vitro formation of neurospheres, clonally derived colonies of floating cells. However, damage to the nervous system is not readily repaired, suggesting that the stem cells do not provide an easily recruited source of cells for regeneration. The vestibular and auditory organs, despite their limited ability to replace damaged cells, appear to contain cells with stem cell properties. These inner ear stem cells, identified by neurosphere formation and by their expression of markers of inner ear progenitors, can differentiate to hair cells and neurons. Differentiated cells obtained from inner ear stem cells expressed sensory neuron markers and, after co-culture with the organ of Corti, grew processes that extended to hair cells. The neurons expressed synaptic vesicle markers at points of contact with hair cells. Exogenous stem cells have also been used for hair cell and neuron replacement. Embryonic stem cells are one potential source of both hair cells and sensory neurons. Neural progenitors made from embryonic stem cells, transplanted into the inner ear of gerbils that had been de-afferented by treatment with a toxin, differentiated into cells that expressed neuronal markers and grew processes both peripherally into the organ of Corti and centrally. The regrowth of these neurons suggests that it may be possible to replace auditory neurons that have degenerated with neurons that restore auditory function by regenerating connections to hair cells.     

顺铂耳毒性大鼠耳聋模型的研究

摘要 目的：探讨顺铂对大鼠造成的听力损伤及耳蜗细胞形态学变化。方法：体内实验,运用顺铂腹腔注射的方法,连续七天注射,通过听性脑干反应检测,观察顺铂对不同日龄的大鼠听力损伤情况;测听后取耳蜗,通过基底膜铺片和冰冻切片的免疫荧光染色,观察听力损伤后对耳蜗毛细胞和螺旋神经元的影响。体外实验,耳蜗器官培养免疫荧光染色,观察顺铂对耳蜗毛细胞和螺旋神经元的影响。结果：顺铂具有耳毒性,会对大鼠听力造成损伤,高频听力损伤更加严重,而且对不同日龄的大鼠造成的听力损失不同,小日龄的大鼠对顺铂耳毒性更加敏感。体内实验,顺铂耳毒性造成听力损失,会引起大鼠耳蜗毛细胞的缺失,但未观察到明显的螺旋神经元缺失,也没有观察到明显的Cleaved caspase-3阳性螺旋神经元细胞。体外实验,可以观察到顺铂同时引起毛细胞和螺旋神经元产生明显的损伤。结论：体、内外实验,都可以建立稳定的顺铂耳毒性大鼠耳聋模型,对研究顺铂损伤耳蜗毛细胞的发生机制和保护奠定了实验基础。     

Role of Somatostatin Receptor-2 in Gentamicin-Induced Auditory Hair Cell Loss in the Mammalian Inner Ear

Hair cells and spiral ganglion neurons of the mammalian auditory system do not regenerate, and their loss leads to irreversible hearing loss. Aminoglycosides induce auditory hair cell death in vitro, and evidence suggests that phosphatidylinositol-3-kinase/Akt signaling opposes gentamicin toxicity via its downstream target, the protein kinase Akt. We previously demonstrated that somatostatin—a peptide with hormone/neurotransmitter properties—can protect hair cells from gentamicin-induced hair cell death in vitro, and that somatostatin receptors are expressed in the mammalian inner ear. However, it remains unknown how this protective effect is mediated. In the present study, we show a highly significant protective effect of octreotide (a drug that mimics and is more potent than somatostatin) on gentamicin-induced hair cell death, and increased Akt phosphorylation in octreotide-treated organ of Corti explants in vitro. Moreover, we demonstrate that somatostatin receptor-1 knockout mice overexpress somatostatin receptor-2 in the organ of Corti, and are less susceptible to gentamicin-induced hair cell loss than wild-type or somatostatin-1/somatostatin-2 double-knockout mice. Finally, we show that octreotide affects auditory hair cells, enhances spiral ganglion neurite number, and decreases spiral ganglion neurite length.     

Building the mammalian cochlea — an overview

The mammalian cochlea is a highly intricate organ responsible for hearing. Numerous specialized cell types residing in the cochlear participate in processing and relaying sound information to the brain. In general, cells in the cochlea are divided into three major types: sensory, neural, and non-sensory. Sensory cells are a group of cells in the organ of Corti consisting of hair cells and supporting cells. Sensory hair cells play a primary role in detecting and processing sound in the form of vibrations. Neural cells are the neurons and glia in the spiral (cochlear) ganglion that relay the processed sound signals in the form of a neurotransmitter to the brain. Other non-sensory cells include all other cell types providing architectural and functional support. Building a functional cochlea requires tightly orchestrated, spatial and temporal regulation of gene expressions. Disruption of the normal gene expression patterns can cause developmental failure of the organ, which can lead to permanent hearing loss. Thus, comprehensive understanding of genes contributing to cochlear development is crucial for elucidating the pathological mechanisms of hearing loss. This article is intended to provide an overview of mammalian cochlear development, focusing on genes involved in its early patterning.     

Abortive synaptogenesis as a factor in the inner hair cell degeneration in the bronx waltzer (bv) mutant mouse

The bronx waltzer (vb) mutation in the mouse results in the degeneration of most but not all of the primary auditory receptors, the inner hair cells, and their afferent neurons. We analyzed the ultrastructure of 94 inner hair cells in the intact postnatal mutant mouse and in neonatal cochleas in culture to understand the pathogenesis of hair cell death and to detect factors that may prevent it. The vb spiral neurons of the bronx waltzer display two distinctive features: some of them continue to divide mitotically for at least seven postnatal days, and the type I radial fibers that innervate inner hair cells display a deficiency in immunoexpression of GAD. The growing endings of spiral neurons converge around the inner hair cells or, in their absence, invade the outer hair cell region. Their profuse sprouting among inner spiral sulcus cells contributes to the characteristic ultrastructural picture of the bv cochlea. During the first three days after birth, 40% of the inner hair cells appear normal and innervated, 40% are mostly denervated and degenerating, and 20% are immature, with minimal or no neuronal appositions. However, in mutants 6 days and older only a few inner hair cells survive, and these show either normal or superfluous afferent innervation and axosomatic GABAergic efferent innervation. Degeneration of inner hair cells begins with a distention of the nuclear envelope and the ribosomal endoplasmic reticulum. The outer nuclear membrane eventually breaks, and exudate fills the cell interior. The cellular edema leads to cell death. We propose that success or failure in synaptic acquisition is a decisive factor in the survival or decline of the mutant inner hair cells. We also suggest that the developmental delay in maturation of the spiral ganglion neurons (type I) and the failure in their synaptogenesis may be caused by an impairment in neurotrophin (NT3/BDNF) synthesis by their mutant receptor cells.     

NTPDase1 and NTPDase2 immunolocalization in mouse cochlea: implications for regulation of p2 receptor signaling.

Cellular, molecular, and physiological studies have demonstrated an important signaling role for ATP and related nucleotides acting via P2 receptors in the cochlea of the inner ear. Signal modulation is facilitated by ectonucleotidases, a heterologous family of surface-located enzymes involved in extracellular nucleotide hydrolysis. Our previous studies have implicated CD39/NTPDase1 and CD39L1/NTPDase2, members of the ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) family, as major ATP-hydrolyzing enzymes in the tissues lining the cochlear endolymphatic and perilymphatic compartments. NTPDase1 hydrolyzes both nucleoside triphosphates and diphosphates. In contrast, NTPDase2 is a preferential nucleoside triphosphatase. This study characterizes expression of these E-NTPDases in the mouse cochlea by immunohistochemistry. NTPDase1 can be immunolocalized to the cochlear vasculature and neural tissues (primary auditory neurons in the spiral ganglion). In contrast, NTPDase2 immunolabeling was principally localized to synaptic regions of the sensory inner and outer hair cells, stereocilia and cuticular plates of the outer hair cells, supporting cells of the organ of Corti (Deiters' cells and inner border cells), efferent nerve fibers located in the intraganglionic spiral bundle, and in the outer sulcus and root region of the spiral ligament. This differential expression of NTPDase1 and 2 in the cochlea suggests spatial regulation of P2 receptor signaling, potentially involving different nucleotide species and hydrolysis kinetics.     

Survival and neural differentiation of adult neural stem cells transplanted into the mature inner ear

The cochlear sensory epithelium and spiral ganglion neurons (SGNs) in the adult mammalian inner ear do not regenerate following severe injury. To replace the degenerated SGNs, neural stem cell (NSC) is an attractive alternative for substitution cell therapy. In this study, adult mouse NSCs were transplanted into normal and deafened inner ears of guinea pigs. To more efficiently drive the implanted cells into a neuronal fate, NSCs were also transduced with neurogenin 2 (ngn2) before transplantation. In deafened inner ears and in animals transplanted with ngn2-transduced NSCs, surviving cells expressed the neuronal marker neural class III beta-tubulin. Transplanted cells were found close to the sensory epithelium and adjacent to the SGNs and their peripheral processes. The results illustrate that adult NSCs can survive and differentiate in the injured inner ear. It also demonstrates the feasibility of gene transfer to generate specific progeny for cell replacement therapy in the inner ear.     

Survival, synaptogenesis, and regeneration of adult mouse spiral ganglion neurons in vitro

The inner ear spiral ganglion is populated by bipolar neurons connecting the peripheral sensory receptors, the hair cells, with central neurons in auditory brain stem nuclei. Hearing impairment is often a consequence of hair cell death, e.g., from acoustic trauma. When deprived of their peripheral targets, the spiral ganglion neurons (SGNs) progressively degenerate. For effective clinical treatment using cochlear prostheses, it is essential to maintain the SGN population. To investigate their survival dependence, synaptogenesis, and regenerative capacity, adult mouse SGNs were separated from hair cells and studied in vitro in the presence of various neurotrophins and growth factors. Coadministration of fibroblast growth factor 2 (FGF-2) and glial cell line-derived neurotrophic factor (GDNF) provided support for long-term survival, while FGF-2 alone could strongly promote neurite regeneration. Fibroblast growth factor receptor FGFR-3-IIIc was found to upregulate and translocate to the nucleus in surviving SGNs. Surviving SGNs formed contacts with other SGNs after they were deprived of the signals from the hair cells. In coculture experiments, neurites extending from SGNs projected toward hair cells. Interestingly, adult mouse spiral ganglion cells could carry out both symmetric and asymmetric cell division and give rise to new neurons. The authors propose that a combination of FGF-2 and GDNF could be an efficient route for clinical intervention of secondary degeneration of SGNs. The authors also demonstrate that the adult mammalian inner ear retains progenitor cells, which could commit neurogenesis.