The Notch signalling pathway has recently been implicated in the development and patterning of the sensory epithelium in the cochlea, the organ of Corti. As part of an ongoing large-scale mutagenesis programme to identify new deaf or vestibular mouse mutants, we have identified a novel mouse mutant, slalom, which shows abnormalities in the patterning of hair cells in the organ of Corti and missing ampullae, structures that house the sensory epithelia of the semicircular canals. We show that the slalom mutant carries a mutation in the Jagged1 gene, implicating a new ligand in the signalling processes that pattern the inner ear neuro-epithelium.

The semi-dominantly inherited mouse mutation pardon (Pdo) was isolated due to the lack of a Preyer reflex (ear flick) in response to sound from a large-scale N -ethyl- N -nitrosourea (ENU) mutagenesis programme. Dissection of the middle ear revealed malformations in all three ossicles, rendering the ossicular chain incomplete. Hair cell counts in the apical turn of the organ of Corti revealed a significant 22.7% increase in the number of outer hair cells. Raised compound action potential thresholds in Pdo/+ mutants suggested a combined sensorineural/conductive hearing loss. We show that a missense mutation in the homeobox gene Emx2 is responsible for these defects, identifying a new function for this gene in the development of specific structures in the ear.

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