Neurogenin 1 null mutant ears develop fewer, morphologically normal hair cells in smaller sensory epithelia devoid of innervation

The proneuronal gene neurogenin 1 (ngn1) is essential for development of th e inner-ear sensory neurons that are completely absent in ngn1 null mutants . Neither afferent, efferent, nor autonomic nerve fibers were detected in t he ears of ngn1 null mutants. We suggest that efferent and autonomic fibers are lost secondarily to the absence of afferents. In this article we show that ngn1 null mutants develop smaller sensory epithelia with morphological ly normal hair cells. In particular, the saccule is reduced dramatically an d forms only a small recess with few hair cells along a duct connecting the utricle with the cochlea. Hair cells of newborn ngn1 null mutants show no structural abnormalities, suggesting that embryonic development of hair cel ls is independent of innervation. However, the less regular pattern of disp ersal within sensory epithelia may be caused by some effects of afferents o r to the stunted growth of the sensory epithelia. Tracing of facial and sta te-acoustic nerves in control and ngn1 null mutants showed that only the di stal, epibranchial, placode-derived sensory neurons of the geniculate gangl ion exist in mutants. Tracing further showed that these geniculate ganglion neurons project exclusively to the solitary tract. In addition to the norm al complement of facial branchial and visceral motoneurons, ngn1 null mutan ts have some trigeminal motoneurons and contralateral inner-ear efferents p rojecting, at least temporarily, through the facial nerve. These data sugge st that some neurons in the brainstem (e.g., inner-ear efferents, trigemina l motoneurons) require afferents to grow along and redirect to ectopic cran ial nerve roots in the absence of their corresponding sensory roots.