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Published September 2000 | public
Journal Article

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 the inner-ear sensory neurons that are completely absent in ngn1 null mutants. Neither afferent, efferent, nor autonomic nerve fibers were detected in the 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 morphologically normal hair cells. In particular, the saccule is reduced dramatically and 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 cells is independent of innervation. However, the less regular pattern of dispersal within sensory epithelia may be caused by some effects of afferents or to the stunted growth of the sensory epithelia. Tracing of facial and stato-acoustic nerves in control and ngn1 null mutants showed that only the distal, epibranchial, placode-derived sensory neurons of the geniculate ganglion exist in mutants. Tracing further showed that these geniculate ganglion neurons project exclusively to the solitary tract. In addition to the normal complement of facial branchial and visceral motoneurons, ngn1 null mutants have some trigeminal motoneurons and contralateral inner-ear efferents projecting, at least temporarily, through the facial nerve. These data suggest that some neurons in the brainstem (e.g., inner-ear efferents, trigeminal motoneurons) require afferents to grow along and redirect to ectopic cranial nerve roots in the absence of their corresponding sensory roots.

Additional Information

© 2000 Springer International Publishing AG. Received: 3 May 2000; Accepted: 11 May 2000; Online publication: 15 August 2000. This work was supported by NIDCD (2P01 DC00215-14A1; BF), the Howard Hughes Medical Institute (DA), and an NIMH Center Grant (PI, H. Lester, DA, QM). Dr. D.H. Nichols helped with the English; Mrs. M. Christensen and Ms. C. Miller provided excellent technical assistance. We express our gratitude to two referees whose detailed criticism helped to improve the manuscript, and to Dr. D. Fekete for her comments on parts of this paper.

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October 20, 2023