Neuronotrophic effect of developing otic vesicle on cochleo-vestibular neurons: evidence for nerve growth factor involvement

[Cochlear nerve canal stenosis: a review of recent research]

Some patients with severe-profound sensorineural hearing loss （SNHL） with normal cochlear anatomical structure received cochlear implantation （CI） and the hearing and speech rehabilitation effect was not ideal. Through retrospective analysis, it was found that some of these patients had cochlear never canal （CNC） stenosis, or atresia in severe cases.This article reviews the development of the CNC, the diagnostic criteria of CNC stenosis and the results of hearing and speech rehabilitation in these patients after CI.

Cochlear nerve deficiency in SOX11-related Coffin-Siris syndrome

The phenotypic spectrum of SOX11-related Coffin-Siris syndrome (CSS) is expanding with reports of new associations. SOX11 is implicated in neurogenesis and inner ear development. Cochlear nerve deficiency, absence or hypoplasia, is commonly associated with cochlear canal stenosis or with CHARGE syndrome, a monogenic condition that affects inner ear development. SOX11 is a transcription factor essential for neuronal identity, highly correlated with the expression of CHD7, which regulates SOX11. We present two unrelated probands, each with novel de novo SOX11 likely pathogenic variants and phenotypic manifestations of CSS including global developmental delay, growth deficiency, and hypoplastic nails. They have unilateral sensorineural hearing loss due to cochlear nerve deficiency confirmed on MRI. SOX11 is implicated in sensory neuron survival and maturation. It is highly expressed in the developing inner ear. Homozygous ablation of SOX11 in a mouse model resulted in a reduction in sensory neuron survival and decreased axonal growth. A heterozygous knockout mice model had hearing impairment with grossly normal inner ear structures like the two probands reported. We propose cochlear nerve deficiency as a new phenotypic feature of SOX11-related CSS. Magnetic resonance imaging is useful in delineating the cochlear nerve deficiency and other CSS-related brain malformations.

Recommendations for Measuring the Electrically Evoked Compound Action Potential in Children With Cochlear Nerve Deficiency

OBJECTIVES

This study reports a method for measuring the electrically evoked compound action potential (eCAP) in children with cochlear nerve deficiency (CND).

DESIGN

This method was developed based on experience with 50 children with CND who were Cochlear Nucleus cochlear implant users.

RESULTS

This method includes three recommended steps conducted with recommended stimulating and recording parameters: initial screen, pulse phase duration optimization, and eCAP threshold determination (i.e., identifying the lowest stimulation level that can evoke an eCAP). Compared with the manufacturer-default parameters, the recommended parameters used in this method yielded a higher success rate for measuring the eCAP in children with CND.

CONCLUSIONS

The eCAP can be measured successfully in children with CND using recommended parameters. This specific method is suitable for measuring the eCAP in children with CND in clinical settings. However, it is not suitable for intraoperative eCAP recordings due to the extensive testing time required.

Children with unilateral cochlear nerve canal stenosis have bilateral cochleovestibular anomalies

OBJECTIVES/HYPOTHESIS

To investigate the cochleovestibular apparatus bilaterally in children with isolated unilateral bony cochlear nerve canal (bCNC) stenosis.

STUDY DESIGN

Retrospective review.

METHODS

Imaging studies of children with unilateral bCNC stenosis (<1.0 mm) on computed tomography imaging (N = 36) were compared with controls imaged due to trauma without temporal bone injury (N = 32). Twenty-six measurements were obtained in each ear, assessing the bony internal auditory canal (IAC), cochlea, and vestibular end-organs, and were analyzed using one-way analysis of variance for intersubject comparisons and paired t tests for intrasubject comparisons with a Bonferroni adjustment for multiple comparisons (P = .0006).

RESULTS

Patients with bCNC stenosis had a smaller IAC (P < .000) and cochlea (P < .000) on the stenotic side as compared with controls. Although the vestibular end-organ was also smaller in bCNC ears, this difference was not significant. The contralateral ear also had a smaller bCNC (P < .000) and cochlea (P < .000) as compared with controls, although to a lesser degree than the stenotic side.

CONCLUSIONS

Children with unilateral bCNC stenosis have abnormal biometry of both the cochlea and the vestibular end-organ in the affected and the normal contralateral ear as compared with controls.

LEVEL OF EVIDENCE

3b Laryngoscope, 129:2403-2408, 2019.

Does the Width of the Bony Cochlear Nerve Canal Predict the Outcomes of Cochlear Implantation?

A narrow bony cochlear nerve canal (BCNC) is associated with sensorineural hearing loss necessitating cochlear implantation (CI). This study evaluated the implications of BCNC width for post-CI outcomes. A total of 56 children who had received CIs were included. The patients were divided into three groups according to the width of the BCNC (Group 1: diameter < 1.4 mm, n = 17; Group 2: diameter 1.4-2.0 mm, n = 14; Group 3: diameter > 2.0 mm, n = 25). The post-CI speech performances were compared among the three groups according to BCNC width. The correlation between BCNC width and post-CI speech performance was evaluated. Logistic regression analysis was also performed to investigate factors that can impact post-CI speech performance. Cochlear nerve deficiency (CND) occurred more frequently in Group 1. Groups 1 and 2 had significantly worse post-CI outcomes. Patients with intact cochlear nerves had significantly better post-CI outcomes than those with CND. When the cochlear nerve was intact, patients with a narrower BCNC showed less favorable results. Therefore, patients with either a narrow BCNC or CND seemed to have poorer outcomes. A narrow BCNC is associated with higher CND rates and poor outcomes. Measurement of BCNC diameter may help predict CI outcomes.

Comparison of the Motor Performance and Vestibular Function in Infants with a Congenital Cytomegalovirus Infection or a Connexin 26 Mutation: A Preliminary Study

OBJECTIVES

Hearing-impaired children are at risk for vestibular damage and delayed motor development. Two major causes of congenital hearing loss are cytomegalovirus (CMV) infection and connexin (Cx) 26 mutations. Comparison of the motor performance and vestibular function between these specific groups is still underexplored. The objective of this study was to investigate the impact of congenital (c)CMV and Cx26 on the motor performance and vestibular function in 6 months old infants.

DESIGN

Forty children (mean age 6.7 months; range 4.8 to 8.9 months) participated in this cross-sectional design and were recruited from the Flemish CMV registry. They were divided into five age-matched groups: normal-hearing control, asymptomatic cCMV, normal-hearing symptomatic cCMV, hearing-impaired symptomatic cCMV, and hearing-impaired Cx26. Children were examined with the Peabody Developmental Motor Scales-2 and cervical vestibular-evoked myogenic potential (cVEMP) test.

RESULTS

Symptomatic hearing-impaired cCMV children demonstrated a significantly lower gross motor performance compared with the control group (p = 0.005), the asymptomatic cCMV group (p = 0.034), and the Cx26 group (0.016). In this symptomatic hearing-impaired cCMV group, 4 out of 8 children had absent cVEMP responses that were related to the weakest gross motor performance. The Cx26 children showed no significant delay in motor development compared with the control children and none of these children had absent cVEMP responses.

CONCLUSIONS

The weakest gross motor performance was found in symptomatic hearing-impaired cCMV-infected children with absent cVEMP responses. These results suggest that abnormal saccular responses are a major factor for this delayed motor development, although more work is needed including comprehensive vestibular function testing to verify this.

Radiological assessment of the Indian children with congenital sensorineural hearing loss

Introduction. Congenital sensorineural hearing loss is one of the most common birth defects with incidence of approximately 1 : 1000 live births. Imaging of cases of congenital sensorineural hearing loss is frequently performed in an attempt to determine the underlying pathology. There is a paucity of literature from India and for this reason we decided to conduct this study in Indian context to evaluate the various cochleovestibular bony and nerve anomalies by HRCT scan of temporal bone and MRI with 3D scan of inner ear in a tertiary care centre. Material and Methods. A total of 280 children with congenital deafness (158 males and 122 females), between January 2002 to June 2013 were included in the study and they were assessed radiologically by HRCT scan of temporal bone and MRI with 3D scan of inner ear. Results. In the present study we found various congenital anomalies of bony labyrinth and vestibulocochlear nerve. Out of 560 inner ears we found 78 anomalous inner ears. Out of these 78 inner ears 57 (73%) had cochlear anomaly, 68 (87.1%) had anomalous vestibule, 44 (56.4%) had abnormal vestibular aqueduct, 24 (30.7%) had anomalous IAC, and 23 (29.4%) had abnormal cochleovestibular nerves. Conclusion. In present study, we found lower incidences of congenital anomalies comparative to existing literature.

Inner ear anomalies seen on CT images in people with Down syndrome

BACKGROUND

Although dysplasia of inner ear structures in Down syndrome has been reported in several histopathological studies, the imaging findings have not been widely studied.

OBJECTIVE

To evaluate the prevalence and clinical significance of inner ear anomalies detected on CT images in patients with Down syndrome.

MATERIALS AND METHODS

The temporal bone CT images of patients with Down syndrome were assessed for inner ear anomalies; clinical notes and audiograms were reviewed for hearing loss. Logistic regression models were employed to identify which CT findings were associated with sensorineural hearing loss (SNHL).

RESULTS

Inner ear anomalies were observed in 74.5% (38/51) of patients. Malformed bone islands of lateral semicircular canal (LSCC), narrow internal auditory canals (IACs), cochlear nerve canal stenoses, semicircular canal dehiscence (SCCD), and enlarged vestibular aqueducts were detected in 52.5% (53/101), 24.5% (25/102), 21.4% (21/98), 8.8% (9/102) and 2% (2/101) of patients' ears, respectively. IAC stenosis had the highest odds ratio (OR = 5.37, 95% CI: 1.0-28.9, P = 0.05) for SNHL.

CONCLUSION

Inner ear anomalies occurred in 74.5% of our population, with malformed (<3 mm) bone island of LSCC being the most common (52.5%) anomaly. Narrow IAC was seen in 24.5% of patients with Down syndrome and in 57.1% of ears with SNHL. High-resolution CT is a valuable for assessing the cause of hearing loss in people with Down syndrome.

The cytokine macrophage migration inhibitory factor (MIF) acts as a neurotrophin in the developing inner ear of the zebrafish, Danio rerio

Macrophage migration inhibitory factor (MIF) plays versatile roles in the immune system. MIF is also widely expressed during embryonic development, particularly in the nervous system, although its roles in neural development are only beginning to be understood. Evidence from frogs, mice and zebrafish suggests that MIF has a major role as a neurotrophin in the early development of sensory systems, including the auditory system. Here we show that the zebrafish mif pathway is required for both sensory hair cell (HC) and sensory neuronal cell survival in the ear, for HC differentiation, semicircular canal formation, statoacoustic ganglion (SAG) development, and lateral line HC differentiation. This is consistent with our findings that MIF is expressed in the developing mammalian and avian auditory systems and promotes mouse and chick SAG neurite outgrowth and neuronal survival, demonstrating key instructional roles for MIF in vertebrate otic development.

Brain stem and inner ear abnormalities in children with auditory neuropathy spectrum disorder and cochlear nerve deficiency

BACKGROUND AND PURPOSE

Cranial abnormalities, including CND, are common in children with ANSD. The purpose of this study was to assess whether CND is associated with brain or inner ear abnormalities in a cohort of children with ANSD.

MATERIALS AND METHODS

Two neuroradiologists retrospectively reviewed cranial MR imaging examinations in 103 children with ANSD. Brain, cochlear nerve, and temporal bone abnormalities were described and tabulated. Findings were stratified on the basis of the presence and laterality of CND, and differences in the presence of associated inner ear or intracranial abnormalities were assessed by using 2-tailed Fisher exact tests.

RESULTS

CND was identified in 33.0% of children and 26.9% of ears with ANSD. Significantly more patients with bilateral CND had intracranial abnormalities than those with unilateral CND (60.0% versus 15.8%; P = .012). Forty percent of patients with bilateral CND, 0% of patients with unilateral CND, and 10.1% of those without CND demonstrated hindbrain malformations. Patients with bilateral CND were more likely to demonstrate hindbrain malformations than patients with normal nerves (P = .01) or unilateral CND (P = .004). Labyrinthine abnormalities were significantly more common in patients with bilateral CND than in those without CND (P ≤ .001). Cochlear anomalies were more common in patients with bilateral versus unilateral CND (P = .01). IAC and cochlear aperture stenosis were more common in those with unilateral and bilateral CND than those without CND (both P < .001).

CONCLUSIONS

Cochlear and hindbrain abnormalities are significantly more common among patients with ANSD with bilateral CND compared with those with at least 1 intact cochlear nerve.

Complete labyrinthine aplasia: clinical and radiologic findings with review of the literature

BACKGROUND AND PURPOSE

Complete labyrinthine aplasia (CLA), also referred to as Michel aplasia, is a severe congenital anomaly of the inner ear, defined by the complete absence of inner ear structures. The purpose of this study was to document the imaging findings in a series of patients with CLA, with review of the literature, to better understand this anomaly.

MATERIALS AND METHODS

The CT and MR imaging findings of 9 patients (14 ears with CLA) were retrospectively evaluated. The audiologic tests and patient charts were also retrospectively reviewed.

RESULTS

CLA was bilateral in 5 and unilateral in 4 patients. The petrous bone was hypoplastic in all 14 ears, but the otic capsule was aplastic in only 5. The middle ear and mastoid volumes were decreased in most of the ears. The stapes was aplastic in 1 ear and was dysplastic in 5 ears. The internal acoustic canal was aplastic in 4 ears and markedly narrowed in 10 ears. The facial nerve canal showed a variety of anomalies and aberrant courses in 11/14 ears. The bony covering of the jugular bulb was defective in 9 ears. Tegmen tympani defects were seen in 3 patients, and there were several accompanying skull base and posterior fossa anomalies.

CONCLUSIONS

Although CLA is a rare developmental anomaly, its accurate diagnosis and its differential diagnosis from labyrinthine ossificans is crucial. Proper guidance of these patients for brain stem implantation in the critical period of brain development depends on the recognition of the characteristic imaging findings of CLA.

Survival and stimulation of neurite outgrowth in a serum-free culture of spiral ganglion neurons from adult mice

We have developed a reliable protocol for the serum-free dissociation and culture of spiral ganglion neurons from adult mice, an important animal model for patients with post-lingual hearing loss. Pilot experiments indicated that the viability of spiral ganglion cells in vitro depended critically on the use of Hibernate medium with B27 supplement. With an optimized protocol, we obtained 2 x 10(3) neurons immediately after dissociation, or about one-fifth of those present in the intact spiral ganglion. After four days in culture, 4% of the seeded neurons survived without any exogenous growth factors other than insulin. This yield was highly reproducible in five independent experiments and enabled us to measure systematically the numbers and lengths of the regenerating neurites. Furthermore, the survival rate compared well to the few published protocols for culturing adult spiral ganglion neurons from other species. Enhanced survival and neurite outgrowth upon the addition of brain-derived neurotrophic factor and leukemia inhibitory factor demonstrated that both are potent stimulants for damaged spiral ganglion neurons in adults. This responsiveness to exogenous growth factors suggested that our culture protocol will facilitate the screening of molecular compounds as potential treatments for sensorineural hearing loss.

Immortalized mouse inner ear cell lines demonstrate a role for chemokines in promoting the growth of developing statoacoustic ganglion neurons

The target-derived factors necessary for promoting initial outgrowth from the statoacoustic ganglion (SAG) to the inner ear have not been fully characterized. In the present study, conditioned medium from embryonic Immortomouse inner ear cell lines that maintain many characteristics of developing inner ear sensory epithelia were screened for neurite-promoting activity. Conditioned medium found to be positive for promoting SAG neurite outgrowth and neuronal survival was then tested for the presence of chemokines, molecules that have not previously been investigated for promoting SAG outgrowth. One candidate molecule, monocyte chemotactic protein 1 (MCP-1), was detected in the conditioned medium and subsequently localized to mouse hair cells by immunocytochemistry. In vitro studies demonstrated that function-blocking MCP-1 antibodies decreased the amount of SAG neurite outgrowth induced by the conditioned medium and that subsequent addition of MCP-1 protein was able to promote outgrowth when added to the antibody-treated conditioned medium. The use of the Immortomouse cell lines proved valuable in identifying this candidate cofactor that promotes outgrowth of early-stage SAG nerve fibers and is expressed in embryonic hair cells.

Temporal bone findings on computed tomography imaging in branchio-oto-renal syndrome

OBJECTIVES/HYPOTHESIS

To describe temporal bone findings using visual inspection and direct measurement on computerized tomography (CT) in individuals with branchio-oto-renal syndrome (BOR). We ask if it is possible for the untrained observer to use a battery of CT observations as a tool in the overall evaluation of the BOR phenotype.

STUDY DESIGN

Retrospective evaluation of CT findings in individuals with a clinical diagnosis of BOR based on criteria derived from genotype-phenotype analyses.

METHODS

Prospective measurement of temporal bone CT imaging in 21 individuals (42 ears) with BOR and 21 normally hearing controls (21 ears) was performed. Thirty-nine aspects of each temporal bone were evaluated: 17 by direct measurement, 5 computed from direct measurement, and 17 by visual inspection. Thirty-eight recordings from each ear were made on axial section and 1 was made on coronal section.

RESULTS

Statistically significant differences were found between BOR and control groups in 30 of 39 categories (76.9%). The most common and easily identifiable characteristics of BOR by visual inspection were 1) hypoplastic apical turn of the cochlea, 2) facial nerve deviated to the medial side of the cochlea, 3) funnel-shaped internal auditory canal, and 4) patulous eustachian tube. The embryological origin of temporal bone anomalies in BOR are described.

CONCLUSIONS

CT evaluation of the temporal bone, when properly investigated, should be used as an important tool in the overall evaluation of the BOR phenotype.

Imaging of the temporal bone

Classical descriptions of the temporal bone are based on its five embryologically distinct osseous components: the petrous, tympanic, mastoid, squamous, and styloid portions.

Imaging of the temporal bone

Classical descriptions of the temporal bone are based on its five embryologically distinct osseous components: the petrous, tympanic, mastoid, squamous, and styloid portions.

Congenital malformations of the inner ear and the vestibulocochlear nerve in children with sensorineural hearing loss: evaluation with CT and MRI

PURPOSE

The purpose of this work was to study the diagnostic value of CT and MRI in children with sensorineural hearing loss and to analyze anatomic abnormalities of the inner ear and the vestibulocochlear nerve in this patient group.

METHOD

We evaluated 42 inner ears in 21 children with congenital deafness who had congenital inner ear malformations and who were candidates for cochlear implants. All patients were studied with high resolution MR and helical CT examinations. The MR study included a T2-weighted 3D fast SE sequence. We describe and tabulate the anatomic abnormalities. Special attention was given to abnormalities of the vestibulocochlear nerve. The field of view in the plane according to the length axis of the internal auditory canal (IAC) was 4 cm. Additional continuous parasagittal reformations perpendicular to the length axis of the IAC were studied with a field of view of 3 cm.

RESULTS

CT and MRI allowed accurate identification of malformations of the inner ear in children with congenital deafness. We identified 99 malformations, with a majority of patients demonstrating multiple abnormalities. Common imaging findings were Mondini abnormality and Mondini variants (12/42) and fusion of the lateral or superior semicircular canal with the vestibule (12/42). MRI demonstrated in 9 of 21 patients a rudimentary or absent vestibulocochlear nerve in the auditory canal.

CONCLUSION

CT and MRI are important modalities to analyze the inner ear in children who are candidates for cochlear implants. MRI with an extremely small field of view should be used to study possible abnormalities of the vestibulocochlear nerves. This may alter clinical care and allow cochlear implant placement in patients whose electrodiagnostic studies suggest that the implant should not be performed. The detailed analysis of abnormalities of the inner ear might establish prognostic factors.

Eighth nerve aplasia and hypoplasia in cochlear implant candidates: the clinical perspective

OBJECTIVE

To identify the clinical and radiologic characteristics of aplasia and hypoplasia of the eighth nerve.

STUDY DESIGN

Retrospective case-note review.

SETTING

Cochlear implant program.

PATIENTS

All children at the authors' institution in whom the cochlear implant assessment failed because of absence or hypoplasia of the eighth nerve.

INTERVENTION

Computed tomography of petrous bones and magnetic resonance imaging of the brain.

MAIN OUTCOME MEASURES

Presence or absence of eighth nerve and other radiologic factors contraindicating implantation.

RESULTS

Of 143 cochlear implant candidates, 237 were judged ineligible for cochlear implantation. The preimplant assessment failed in 10 candidates of 143 because of bilateral aplasia or hypoplasia of the eighth nerve (7 cases) or unilateral aplasia or hypoplasia of the eighth nerve and a contraindication to operation on the other side (3 cases). The aplasia or hypoplasia of the eighth nerve was confirmed by magnetic resonance imaging in seven cases (5%): six were syndromic (3 CHARGE, 1 VATER-RAPADILLINO, 1 Möbius, 1 Okihiro), and one was nonsyndromic autosomal-recessive. All seven children had delayed motor milestones and absence of auditory brainstem responses.

CONCLUSION

Aplasia and hypoplasia of the eighth nerve are not uncommon in pediatric cochlear implant candidates, particularly in the presence of a syndrome such as CHARGE. Magnetic resonance imaging of the brain is mandatory before implantation because it can identify the presence or absence of the eighth nerve. Parents of children with profound hearing loss, delayed motor milestones, absence of auditory brainstem responses, and a syndromic diagnosis, should be made aware of this possible abnormality.

Imaging evaluation of sensorineural hearing loss

The imaging evaluation of patients with sensorineural hearing loss (SNHL) focuses on the acoustic pathways from the cochlea to the auditory cortex. Magnetic resonance imaging (MRI) is the modality of choice for most patients with SNHL, though computed tomography (CT) also plays an important role in the evaluation of bony changes and in patients for whom MRI is contraindicated. Conventional enhanced MRI is the most commonly used technique in this clinical setting. High-resolution fast spin-echo T2 MRI is an adjunctive technique that provides exquisite evaluation of the cerebellopontine angle (CPA), internal auditory canal (IAC), cranial nerves, and membranous labyrinth, and plays a significant role in the diagnosis and surgical evaluation of SNHL. Categories of lesions that cause SNHL include brain lesions involving central auditory pathways; neoplasms of the CPA and IAC, the most common being schwannoma; other neoplastic, congenital, and cystic masses of the CPA and IAC; congenital anomalies of the inner ear; intrinsic cochlear nerve defects, inflammatory processes of the inner ear; and temporal bone trauma.

Molecular signaling and pulpal nerve development

The purpose of this review is to discuss molecular factors influencing nerve growth to teeth. The establishment of a sensory pulpal innervation occurs concurrently with tooth development. Epithelial/mesenchymal interactions initiate the tooth primordium and change it into a complex organ. The initial events seem to be controlled by the epithelium, and subsequently, the mesenchyme acquires odontogenic properties. As yet, no single initiating epithelial or mesenchymal factor has been identified. Axons reach the jaws before tooth formation and form terminals near odontogenic sites. In some species, local axons have an initiating function in odontogenesis, but it is not known if this is also the case with mammals. In diphyodont mammals, the primary dentition is replaced by a permanent dentition, which involves a profound remodeling of terminal pulpal axons. The molecular signals underlying this remodeling remain unknown. Due to the senescent deterioration of the dentition, the target area of tooth nerves shrinks with age, and these nerves show marked pathological-like changes. Nerve growth factor and possibly also brain-derived neurotrophic factor seem to be important in the formation of a sensory pulpal innervation. Neurotrophin-3 and -4/5 are probably not involved. In addition, glial cell line-derived neurotrophic factor, but not neurturin, seems to be involved in the control of pulpal axon growth. A variety of other growth factors may also influence developing tooth nerves. Many major extracellular matrix molecules, which can influence growing axons, are present in developing teeth. It is likely that these molecules influence the growing pulpal axons.

Programmed cell death in the mouse cochleovestibular ganglion during development

Programmed cell death (PCD) is as essential to development as is cell proliferation. Our objective was to elucidate the spatiotemporal occurrence of PCD during the development of the cochleovestibular ganglion. We performed a time-sequence study on the distribution of in situ PCD, apoptosis, during the development of the mouse cochleovestibular ganglion by using the TUNEL method to detect apoptosis histochemically. Apoptosis in the cochleovestibular ganglion was observed from the 11.5th gestational day (GD) to the 18. 5th GD. Apoptosis was seen most extensively in the vestibular ganglion cells at the 13.5th GD, while in the spiral ganglion cells apoptosis was maximal between the 15.5th and 16.5th GD. Because these times of peak apoptotic activity almost exactly corresponded to the events of innervation and terminal mitosis of the cochleovestibular ganglion cells, we infer that PCD is involved in the innervation and modulates the number of cochleovestibular ganglion cells overproduced by terminal mitosis.

Developmental changes in growth factors released by the embryonic inner ear

Recent studies have demonstrated a role for neurotrophins in regulating the survival of developing auditory and vestibular neurons. However, the developmental time-course for neurotrophin production and release by inner ear tissues has not been defined. In the present study, neurotrophin-like activity was evaluated from culture medium conditioned by early- or midembryonic stage inner ears. Examination of the proteinaceous properties of conditioned medium revealed a developmental change in growth factor release by the inner ear. Neurotrophin-like molecules were not detected in medium conditioned by early stage otocysts. In contrast, neurotrophin-like bioactivity was detected in medium conditioned by middevelopmental stage inner ears. Western blot analysis revealed that NT-3 was released by the rat inner ear at midstages of inner ear development. ELISA measurements revealed that both NT-3 and BDNF are produced by the middevelopmental stage inner ear, and that NT-3 protein levels are higher than BDNF levels. These results suggest that there are developmental changes in the release of growth factors by the inner ear.

Properties of cochlear nucleus neurons in primary culture

Dissociated primary cell cultures were derived from the cochlear nuclei (CN) of postnatal rats using standard techniques. Cultured cells differentiated morphologically, but their dendritic profiles were generally less specialized than those of CN cells in vivo. Physiologically, cultured cells could be divided into three classes: tonic, phasic and non-spiking cells, which differed in many of their fundamental biophysical properties. The percentage of cultured cells that spiked repetitively increased over time to a maximum of 85% at 6 days. However, the percentage of cells that produced action potentials decreased with time in culture, from 91% during the first 8 days to less than 40% after 9 days. CN cells were successfully cultured in both serum-supplemented and serum-free (Neurobasal) media. More neurons survived at low plating densities in Neurobasal than in medium containing serum, although neuronal survival was similar at higher densities. Few neurons raised in the serum-free medium were spontaneously active; other response properties were similar to those of cells grown in the presence of serum. Although differentiation of CN cells in culture did not completely mirror the in vivo developmental pattern, these experiments demonstrate that primary culture represents a viable method for the in vitro study of CN neurons.

Trapping genes expressed in the developing mouse inner ear

Identification of the genes involved in the development of the mouse inner ear and developmental studies of mice that bear mutations in these genes is an important approach to understanding genetically determined human auditory dysfunction. Towards this end, we initiated a gene trap screen designed to simultaneously mark and mutate genes in mouse embryonic stem cells by the insertion of a lacZ reporter gene. Expression of beta-galactosidase in gene trap cell lines was monitored both before and after the addition of factors that are known to affect inner ear development. Gene trap cell lines that expressed beta-galactosidase under one or more culture conditions were used to create chimeric mouse embryos for studies of reporter gene expression in vivo. A high proportion of these gene trap insertions were expressed in the developing inner ear, suggesting that this strategy provides an effective means of identifying genes that may be involved in inner ear development or function.

Critical periods of basic fibroblast growth factor and brain-derived neurotrophic factor in the development of the chicken cochleovestibular ganglion in vitro

The temporal roles of brain-derived neurotrophic factor (BDNF) and fibroblast growth factor-2 (FGF-2) in the development of sensory neurons have been studied in a cell culture preparation which models normal embryonic inner ear development (normocytic). Previous studies showed that FGF-2 stimulated migration and differentiation of ganglion cells for the first 2 days in vitro, but after 5 days led to degeneration, implicating other factors in their later development. To see if BDNF could be such a factor, otocysts were explanted from white leghorn embryos at the time when ganglion cell precursors normally start migrating from the otic epithelium. Cultures were grown in a defined medium, either with or without human recombinant FGF-2 for 2 days or with BDNF. On Day 3, FGF-2 was replaced either with BDNF in defined medium or with defined medium only. Measurements of neuroblast migration and neurite outgrowth were made by time-lapse imaging in living cultures. In cultures receiving BDNF on Day 3, cell migration and neurite outgrowth from the explant increased for more than 3 weeks but not in cultures receiving only defined medium from Day 3. Cultures did not survive more than 3-4 days when receiving either BDNF in defined medium or defined medium alone from the first day. A neutralizing antibody to BDNF inhibited neuronal migration and neurite outgrowth, and it also blocked the effects of exogenous BDNF. BDNF did not enhance the effects of FGF-2 by interacting with it. These experiments defined a temporal sequence in which FGF-2 acts early in development, while BDNF affects a later stage.

Relationship between the development of outer hair cell electromotility and efferent innervation: a study in cultured organ of corti of neonatal gerbils

Outer hair cell (OHC) electromotility, which powers the cochlear amplifier, develops at a later stage of hearing ontogeny. There has been speculation whether efferents play a necessary role in directing or achieving OHC maturation in mammals. In this study, we examine whether the development of OHC motility depends on the establishment of efferent innervation of the cells' synaptic pole by measuring electromotility of OHCs grown in cultures, deprived of efferent innervation. Tissue cultures of the organ of Corti were prepared from the cochleas of newborn gerbils. Solitary OHCs were obtained from 4- to 15-d-old cultures by enzymatic digestion and mechanical trituration. Length changes evoked by transcellular electrical stimulation were detected and measured with a photodiode sensor. Results show that OHCs develop electromotility between 6 and 13 d in culture without the presence of efferent innervation. The timetable for the onset of OHC electromotility is comparable with that in vivo. This demonstrates that the ontogeny of OHC electromotility is an intrinsic process that does not require the influence of efferent innervation.

Survival of inner ear sensory neurons in trk mutant mice

Analysis of trkB-/-; trkC-/- double mutant mice revealed that peripheral and central inner ear sensory neurons are affected in these mice. However, a substantial amount of cochlear and vestibular neurons survive, possibly due to maintenance or upregulation of TrkA expression. To clarify the function of the TrkA receptor during development of the cochlear and vestibular ganglion we analysed trkA-/- mice and the expression of this receptor in inner ear sensory neurons of trkB-/-; trkC-/- animals. TrkA homozygous mutant mice showed normal numbers of neurons and no TrkA expression was detected in neurons of trkB-/-; trkC-/- double mutant mice. We conclude that TrkA is not essential for inner ear development and that in the absence of any of the known catalytic Trk receptors peripheral inner ear sensory neurons are prone to undergo cell death or must use a different signaling mechanism to survive.

A simple technique to efficiently dissociate primary auditory neurons from 5 day-old rat cochleas

The aim of this work was to develop a simple and reproducible method of dissociation of cochlear spiral ganglion neurons in the rat. This technique, wich was developed in 5 day-old rat pups, was based on the use of a single enzyme, thermolysin. It is easy to set up and allows the collection of a large amount of neurons. These isolated neurons were kept in a definite, serum free culture medium up to 7 days. Neurons were characterized both by standard morphological criteria and by using a specific neuronal marker (anti-neurofilament 200 kD) after 2 h and 7 days in culture. Cell viability, assessed by fluorescent dyes indicated that all isolated cells were healthy even after 7 days in vitro. The dissociation and culture methods were found very satisfactory and can be easily adapted to any kind of experiment requiring isolated spiral ganglion neurons.

NGF, BDNF and NT-3 play unique roles in the in vitro development and patterning of innervation of the mammalian inner ear

Developing cochleovestibular ganglion (CVG) neurons depend upon interaction with the otocyst, their peripheral target tissue, for both trophic support and tropic guidance. RT-PCR of E11 through E14 otocyst-CVG RNA extracts have shown that NGF as well as BDNF and NT-3 are expressed in the developing inner ear (in situ RT-PCR on tissue sections of E12 otocysts localized all three neurotrophins to the otocyst). To evaluate the functional significance of NGF, BDNF and NT-3 expression, E10.5 otocyst-CVG explants were treated with antisense oligonucleotides and compared to sense treated and control cultures. Confocal microscopic analysis revealed that treatment with BDNF antisense resulted in extensive neuronal cell death, downregulation of NGF caused an inhibition of neuritogenesis and a decrease in the neuronal population of the CVG, whereas treatment with NT-3 antisense resulted in a loss of target directed CVG neuritic ingrowth in this in vitro model. The effect of NGF or BDNF antisense treatment could be prevented by the simultaneous addition of the respective growth factor. These findings demonstrate that each of the three neurotrophins have important roles during the onset of neuritic ingrowth of the CVG neurons to the otocyst.

Regenerated nerve fibers in the noise-damaged chinchilla cochlea are not efferent

Nerve-fiber regeneration in the chinchilla cochlea following a traumatic noise exposure was systematically described by Bohne and Harding (1992). However, their study did not determine the origin of the regenerated nerve fibers (RNFs). In the present study, 23 chinchillas were exposed for 12 h to a 0.5 kHz octave band of noise at 120 dB SPL. After a 3-month or 1-year recovery period, their right cochleas were incubated to demonstrate acetylcholinesterase (AChE) activity and then briefly counterstained with Neutral Red or OsO4. Their left cochleas were fixed with OsO4 and dissected using a combined organ of Corti (OC)/modiolus technique that preserved both structures for high-resolution microscopy. All cochleas were prepared as plastic-embedded flat preparations. Damage was located in the basal two-thirds of the cochlea and generally consisted of multiple lesions in the OC, often involving total degeneration of one or more OC segments (i.e., OC wipeouts). The OC wipeouts were separated from one another by areas which contained some identifiable cells of the OC (i.e., OC remnants). Most RNFs were found in OC wipeouts adjacent to OC remnants. In those animals (83%) with significant OC damage, 13 (100%) 3-month-recovery chinchillas had 1-96 RNFs while 6 (86%) 1-year-recovery chinchillas had 7-62 RNFs. In the AChE-stained cochleas, none of the RNFs were AChE-positive, but normal AChE-positive fibers were found in the undamaged apical turn. A variable number of surviving spiral ganglion cells was present in those regions of Rosenthal's canal that had originally innervated the missing hair cells in the OC wipeouts and remnants. It is concluded that RNFs are not part of the efferent cochlear system and therefore, most likely belong to the afferent system.

Developing inner ear sensory neurons require TrkB and TrkC receptors for innervation of their peripheral targets

The trkB and trkC genes are expressed during the formation of the vestibular and auditory system. To elucidate the function of trkB and trkC during this process, we have analysed mice carrying a germline mutation in the tyrosine kinase catalytic domain of these genes. Neuroanatomical analysis of homozygous mutant mice revealed neuronal deficiencies in the vestibular and cochlear ganglia. In trkB (−/−) animals vestibular neurons and a subset of cochlear neurons responsible for the innervation of outer hair cells were drastically reduced. The peripheral targets of the respective neurons showed severe innervation defects. A comparative analysis of ganglia from trkC (−/−) mutants revealed a moderate reduction of vestibular neurons and a specific loss of cochlear neurons innervating inner hair cells. No nerve fibres were detected in the sensory epithelium containing inner hair cells. A developmental study of trkB (−/−) and trkC (−/−) mice showed that some vestibular and cochlear fibres initially reached their peripheral targets but failed to maintain innervation and degenerated. TrkB and TrkC receptors are therefore required for the survival of specific neuronal populations and the maintenance of target innervation in the peripheral sensory system of the inner ear.

Neurotrophins and ciliary neurotrophic factor: their biology and pathology

Neurotrophins (NTFs) and ciliary neurotrophic factor (CNTF) induce the differentiation of neuronal cells, rescue them from naturally occurring death, and trigger neuronal regeneration. The NTFs bind to two classes of cell surface receptors, whereas CNTF receptor is composed of three subunits. The functions of these polypeptide survival factors with trophic action on nerve cells have recently been approached by the targeted disruption of the CNTF, NTF and their receptor genes by the homologous recombination technique. The embryonic growth and morphogenesis of these gene 'knock-out' mice is normal, but they develop with defects in various subsets of the peripheral nervous system, and the homozygous mutant mice often die during the early postnatal period. Disturbances in the biology of NTFs and CNTF have recently been implicated in the pathogenesis of certain common neurodegenerative disorders, such as Parkinson's disease, motor neurone diseases, and Alzheimer's disease. Intensive research on their pharmaceutical perspective has, therefore, been provoked. All neurotrophins and CNTF can now be synthesized on a large scale as biologically active recombinant proteins, and several alternatives for their local applications to the target tissue have been presented. Their therapeutic potential is discussed.

Characterization of different neuron populations in mouse statoacoustic ganglion cultures

Statoacoustic ganglion (SAG) cells were grown in primary culture and the appearance of different neuronal phenotypes was investigated. Analysis criteria were shape, size and staining for the immunocytochemical markers: neurofilament proteins (NF-200 kDa), neuron-specific enolase (NSE), calretinin, a calcium-binding protein and substance P, a neurotransmitter. Cultures were prepared from dissociated SAG cells of 13 gestation-day-old mouse embryos. Neurons were identified and counted after 7 days in vitro. At this stage, neurons were organized in small clusters forming an extensive network of neurites grown on a layer of fibroblasts and glia. Most neurons identified by NF or NSE immunoreactivity showed a typical adult-like bipolar profile. The diameters of the neurons were between 5.62 and 17.00 microns and displayed a normal distribution (mean: 10.6 microns). Two distinct subpopulations were identified by the expression of calretinin and substance P. Calretinin-immunoreactive neurons were large and very rare and had a mean diameter of 11.3 microns; the distribution of substance P was more extensive than that of calretinin and identified a population of small neurons with a mean diameter of 8.9 microns. The distributions of these two markers in SAG cultures were consistent with in vivo results. In conclusion, dissociated SAG cell cultures appear to be a suitable model for analyzing the development of the immunocytochemical and functional characteristics of the neurons of this inner ear ganglion.

Coordinated expression and function of neurotrophins and their receptors in the rat inner ear during target innervation

We show that trkB and trkC mRNAs, encoding the high-affinity receptor tyrosine kinases for brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), respectively, as well as low-affinity nerve growth factor receptor (p75LNGFR) mRNA are expressed in the cochleovestibular ganglion (CVG) before and during innervation of the target fields. Correspondingly, from preinnervation stages onward, BDNF and NT-3, but neither nerve growth factor (NGF) nor neurotrophin-4 (NT-4) mRNAs are expressed in the sensory epithelium of the otic vesicle, the peripheral target field of CVG neurons. No neurotrophin transcripts were detected by in situ hybridization in the medullary central targets. In explant cultures, neuritogenesis from both the cochlear and vestibular part of the CVG was promoted by BDNF, while NT-3 evoked neurites mainly from the cochlear neurons. Also NT-4 stimulated neurite outgrowth from the CVG in vitro. In dissociated neuron-enriched cultures, NT-3 and BDNF promoted survival of overlapping subsets of CVG neurons and, correspondingly, results from in situ hybridization showed that both trkC and trkB mRNAs were expressed in most neurons of this ganglion. The negligible effect of NGF seen in the bioassays agrees well with the expression of only a few trkA transcripts, encoding the high-affinity receptor for NGF, in the CVG. Based on the spatiotemporal expression patterns and biological effects in vitro, peripherally-synthesized BDNF and NT-3 regulate the survival of CVG neurons as well as the establishment of neuron-target cell contacts in the early-developing inner ear. In addition, the expression of trkB mRNA, more specifically its truncated form, and trkC as well as p75LNGFR mRNAs in distinct non-neuronal structures indicates novel roles for these molecules during development.

Expression of BDNF and NT-3 mRNA in hair cells of the organ of Corti: quantitative analysis in developing rats

Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are synthesized by inner and outer hair cells of the developing organ of Corti. This raises the possibility that the reorganization of cochlear innervation patterns that occurs postnatally may be influenced by changing levels of neurotrophin expression. To determine if differential expression of BDNF or NT-3 in the inner and outer hair cells correlates with the reorganization of afferent and efferent innervation, we used in situ hybridization techniques to quantify relative levels of transcript biosynthesis in hair cells of developing rats. BDNF transcripts decreased in inner and outer hair cells from E17 to insignificant levels at P4. NT-3 expression was high at E17 in inner and outer hair cells, decreased in outer hair cells by E21, in inner hair cells by P1, remained low during the first postnatal week and was increased in the adult. The decreases in expression of both neurotrophins at birth precede the retraction of afferent nerve terminals from outer hair cells. BDNF and NT-3 transcription decreases substantially in outer hair cells between E21 and P4 when efferent innervation begins, indicating target biosynthesis of these neurotrophins is not likely to be instrumental in efferent target selection.

Neurotrophin and neurotrophin receptor mRNA expression in developing inner ear

Receptors which bind the neurotrophins NGF, BDNF, NT-3 and NT-4/5 were shown to be present in cochlear and vestibular ganglion cells during development, implying a neurotrophic role for these molecules in the inner ear. We have found by in situ hybridization that cochlear and vestibular sensory epithelial cells express BDNF and NT-3 mRNAs, but neither NGF or NT-4 mRNAs, in mouse embryos from embryonic day (E)11.5 through postnatal day (P)1. NT-3 mRNA was expressed throughout the sensory epithelium whereas BDNF mRNA appeared to be localized in hair cells (vestibular) and epithelial precursors of hair cells (cochlea). BDNF mRNA was also expressed in a subpopulation of cells in the cochleovestibular ganglion at E11.5 and E12.5. Additionally, cochlear and vestibular neurons contained mRNAs encoding the neurotrophin receptors p75 and trkB. TrkA mRNA was transiently expressed in cochleovestibular ganglion cells at E12.5. These data suggest that BDNF and NT-3 play a role in cochleovestibular neuron survival and neurite outgrowth during development in the inner ear.

Neurotrophins and the development of cochlear innervation

The rapid progress in the past few years concerning neurotrophic factor research, has greatly stimulated advances in developmental neurobiology of hearing. We have summarized evidence that neurotrophins are expressed by auditory sensory epithelia during the time at which ganglion cells with neurotrophin receptors send their processes to these epithelia. Recent findings have led to the identification of BDNF and NT3 as responsible substances. Since no NGF mRNA nor the NGF high affinity receptor component trkA mRNA were detectable during the development of cochlear structures, this factor is not likely to be an important neurotrophin at this level. By their biological activity, neurotrophins could be responsible for chemotrophic, differentiation, survival, and maintenance functions at the afferent as well as at the efferent level of the inner ear development.

Acidic and basic FGF mRNA expression in the adult and developing rat cochlea

In situ hybridization was used to document the distribution of mRNA encoding acidic and basic fibroblast growth factor (aFGF and bFGF) in the rat cochlea from embryonic day (E) 16 to postnatal day (P) > 60. bFGF mRNA was not detected in the cochlea at any age. In the adult, aFGF mRNA was strongly expressed in spiral ganglion (SG) neurons, and this expression increased from base to apex. The stria vascularis (SV) and spiral prominence (SP) showed lesser expression which was equal in all turns. Developmentally, low level expression of aFGF mRNA was first seen in the SG at E-20, and remained low until P-4. Expression increased from P-6 to P-14, when adult levels were reached. aFGF mRNA was also observed in the developing hair cells of all turns at E-20. This expression increased after birth but disappeared after P-6. Expression in the SV and SP was first noted at E-20 and reached adult levels by P-16 and P-10, respectively. High levels of aFGF mRNA in the adult SG suggest that aFGF is important for the maintenance of SG neuron function and structure. aFGF in hair cells during the first postnatal week may be involved in the establishment of cochlear innervation.

An original organotypic culture method to study the organ of Corti of the newborn rat in vitro

The inner ear growth is related to several factors which are not yet well known. In order to study the effect of growth factors on the development of the auditory receptor cells, we have chosen first to establish an in vitro model of the inner ear. Newborn rats were selected as the source of tissue because the inner ear is immature enough at this stage of development for studying some relations between receptors and their innervation. Until recently, the Maximov slide assembly technique was the only organ culture system available and silver neurofibrillary methods were used to stain the nervous structures. These are difficult and time-consuming techniques. With the use of a collagen gel drop floating in the culture medium, we have developed a simple and reliable method. Furthermore, an immunohistochemical fiber-staining technique with anti-neurofilament and histochemical staining technique with phalloidin allows us to check in a few days the organotypy at the spiral ganglion and hair cell levels. This floating drop method gave us some preliminary information about the spiral neuron cells which survived.

Expression patterns of neurotrophin and their receptor mRNAs in the rat inner ear

In situ hybridization was used to study the expression of mRNAs of nerve growth factor (NGF), brain-derived neutrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-5 (NT-5) and the components of their high-affinity receptors in the early postnatal and adult rat inner ears. NGF or NT-5 transcripts were not detected in the inner ear neuroepithelium or in the innervating neurons. NT-3 mRNA was intensely expressed over the one-week-old and adult inner hair cells (IHCs) but in the outer hair cells (OHCs) and vestibular maculae only during the early postnatal period. BDNF mRNA was expressed in the IHCs and OHCs of the early postnatal cochlea but not in the adult organ of Corti. High levels of BDNF transcripts were observed in the sensory epithelia of all vestibular end organs. mRNAs of low affinity NGF receptor, trkB and trkC, but not of trk, were expressed in the spiral and vestibular ganglia. In addition, the non-catalytic form of trkB mRNA localized to the sensory epithelia of maculae utriculi and sacculi. The present results show that of the neurotrophins examined, NT-3 is the predominant neurotrophin in the adult organ of Corti and BDNF is that in vestibular organs. The expression patterns of NT-3 and BDNF mRNAs suggest that these neurotrophins may participate in the maintenance of mature cochleovestibular neurons and they may be involved in the survival response of injured neurons.

Cellular localization of brain-derived neurotrophic factor and neurotrophin-3 mRNA expression in the early chicken embryo

Degenerate primers from conserved regions in nerve growth factor, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) were used in the polymerase chain reaction to isolate DNA fragments from the chicken BDNF and NT-3 genes. A genomic clone coding for chicken NT-3 was isolated and the structure of the chicken NT-3 mature protein was subsequently deduced from nucleotide sequence analysis of the isolated chicken NT-3 gene. Comparison of the chicken BDNF and NT-3 with the corresponding rat molecules showed that the avian molecules are very similar to their mammalian homologues. Northern blot analyses of messenger RNA (mRNA) from chicken embryos from embryonic day 3.5 (E3.5), E4.5, E8, E12 and E18 showed that expression of both BDNF and NT-3 mRNA peaked at E4.5 and decreased at later stages of development. Both probes revealed two transcripts; larger mRNAs of 4.5 kilobases (kb) for BDNF and 4.0 kb for NT-3 predominated over the smaller transcripts of 1.4 and 1.3 kb, respectively. The cellular localization of BDNF and NT-3 mRNA in the E4 and E6 embryos was studied by in situ hybridization. In the E4 embryo, labelling for BDNF was seen over cells in restricted parts of the epithelium of the otic vesicle. Analysis of adjacent sections for the low-affinity nerve growth factor receptor mRNA showed that regions in the otic vesicle epithelium which labelled for BDNF mRNA also labelled for low-affinity nerve growth factor receptor mRNA. No labelling for NT-3 was detected in the otic vesicle. Labelling for BDNF mRNA was also found over mesenchyme dorsal to the wing bud, in the wing bud and in the splanchnopleural lining of the stomach. Labelling for NT-3 mRNA was found at E4 over the epidermis on the ventral side in the region of the branchial arches. The labelling extended up the maxillary processes to Rathke's pouch. The closely located infundibulum was weakly labelled for NT-3 mRNA. NT-3 mRNA was also detected in the mesenchyme surrounding the oesophagus and lung buds. The regional expression pattern is in agreement with the established role for BDNF and NT-3 as target-derived neurotrophic factors, but the results also suggest that BDNF may be an intrinsic factor important for the development of the inner ear. The results support the emerging view that neurotrophic factors can play a role in early differentiation of both neuronal and non-neuronal tissues.

Localization of two genes for Usher syndrome type I to chromosome 11

The Usher syndromes (USH) are autosomal recessive diseases characterized by congenital sensorineural hearing loss and progressive pigmentary retinopathy. While relatively rare in the general population, collectively they account for approximately 6% of the congenitally deaf population. Usher syndrome type II (USH2) has been mapped to chromosome 1q (W. J. Kimberling, M. D. Weston, C. Möller, et al., 1990, Genomics 7: 245-249; R. A. Lewis, B. Otterud, D. Stauffer, et al., 1990, Genomics 7: 250-256), and one form of Usher syndrome type I (USH1) has been mapped to chromosome 14q (J. Kaplan, S. Gerber, D. Bonneau, J. Rozet, M. Briord, J. Dufier, A. Munnich, and J. Frezal, 1990. Cytogenet. Cell Genet. 58: 1988). These loci have been excluded as regions of USH genes in our data set, which is composed of 8 French-Acadian USH1 families and 11 British USH1 families. Both of these sets of families show linkage to loci on chromosome 11. Linkage analysis demonstrates locus heterogeneity between these sets of families, with the French-Acadian families showing linkage to D11S419 (Z = 4.20, theta = 0) and the British families showing linkage to D11S527 (Z = 6.03, theta = 0). Genetic heterogeneity of the data set was confirmed using HOMOG and the M test (log likelihood ratio > 10(5)). These results confirm the presence of two distinct USH1 loci on chromosome 11.