Previous work suggested qualitatively different effects of neurotrophin 3 (NT-3) in cochlear innervation patterning in different null mutants. We now show that all NT-3 null mutants have a similar phenotype and lose all neurons in the basal turn of the cochlea. To understand these longitudinal deficits in neurotrophin mutants, we have compared the development of the deficit in the NT-3 mutant to the spatial-temporal expression patterns of brain-derived neurotrophic factor (BDNF) and NT-3, using lacZ reporters in each gene and with expression of the specific neurotrophin receptors, trkB and trkC. In the NT-3 mutant, almost normal numbers of spiral ganglion neurons form, but fiber outgrowth to the basal turn is eliminated by embryonic day (E) 13.5. Most neurons are lost between E13.5 and E15.5. During the period preceding apoptosis, NT-3 is expressed in supporting cells, whereas BDNF is expressed mainly in hair cells, which become postmitotic in an apical to basal temporal gradient. During the period of neuronal loss, BDNF is absent from the basal cochlea, accounting for the complete loss of basal turn neurons in the NT-3 mutant. The spatial gradients of neuronal loss in these two mutants appear attributable to spatial-temporal gradients of neurotrophin expression. Our immunocytochemical data show equal expression of their receptors, TrkB and TrkC, in spiral sensory neurons and thus do not relate to the basal turn loss. Mice in which NT-3 was replaced by BDNF show a qualitative normal pattern of innervation at E13.5. This suggests that the pattern of expression of neurotrophins rather than their receptors is essential for the spatial loss of spiral sensory neurons in NT-3 null mutants.

Interactions between FGF10 and the IIIb isoform of FGFR-2 appear to be crucial for the induction and growth of several organs, particularly those that involve budding morphogenesis. We determined their expression patterns in the inner ear and analyzed the inner ear phenotype of mice specifically deleted for the IIIb isoform of FGFR-2. FGF10 and FGFR-2(IIIb) mRNAs showed distinct, largely nonoverlapping expression patterns in the undifferentiated otic epithelium. Subsequently, FGF10 mRNA became confined to the presumptive cochlear and vestibular sensory epithelia and to the neuronal precursors and neurons. FGFR-2(IIIb) mRNA was expressed in the nonsensory epithelium of the otocyst that gives rise to structures such as the endolymphatic and semicircular ducts. These data suggest that in contrast to mesenchymal-epithelial-based FGF10 signaling demonstrated for other organs, the inner ear seems to depend on paracrine signals that operate within the epithelium. Expression of FGF10 mRNA partly overlapped with FGF3 mRNA in the sensory regions, suggesting that they may form parallel signaling pathways within the otic epithelium. In addition, hindbrain-derived FGF3 might regulate otocyst morphogenesis through FGFR-2(IIIb). Targeted deletion of FGFR-2(IIIb) resulted in severe dysgenesis of the cochleovestibular membraneous labyrinth, caused by a failure in morphogenesis at the otocyst stage. In addition to the nonsensory epithelium, sensory patches and the cochleovestibular ganglion remained at a rudimentary stage. Our findings provide genetic evidence that signaling by FGFR-2(IIIb) is critical for the morphological development of the inner ear.

The spinal nerves in amphioxus are compared with the spinal and cranial nerves in lampreys. The dorsal spinal roots in amphioxus are similar to the mixed sensory and motor dorsal roots of many cranial nerves in lampreys but not to the purely sensory dorsal spinal roots in lampreys and gnathostomes. Likewise, cranial nerves V, VII, IX and X in lampreys, and all spinal nerves in amphioxus, lack a separate ventral motor root which is a constant feature of all spinal motor roots in lampreys and other vertebrates. Based on these similarities and differences, it is proposed that cranial and spinal nerves in craniates are independently derived serial homologs of elements of an amphioxus-like ancestral pattern. Further evolution involved the addition of neural crest-derived ganglia to most cranial and all spinal nerves, and the addition of placodally derived ganglia to many cranial nerves. The possible homology of ocular motor nerves is discussed but cannot be resolved owing to the absence of these nerves in hagfishes, which are the only relevant outgroup.

Essential functions of neurotrophin 3 (NT-3) in regulating afferent and efferent innervation of the cochlea have been characterized by comparison of normal and NT-3 mutant mice. NT-3 deficiency has striking, region-specific effects, with complete loss of sensory neurons in the basal turn and dramatic but incomplete neuronal loss in the middle and apical turns. The sensory innervation of inner and outer hair cells was reorganized in mutant animals. Instead of a strictly radial pattern of innervation, the axons of remaining sensory neurons projected spirally along the row of inner hair cells to innervate even the most basal inner hair cells. Innervation of outer hair cells was strongly reduced overall and was not detected in the basal turn.The presence of fibers extending to both inner and outer hair cells suggests that subsets of types I and II sensory neurons survive in the absence of NT-3. Likewise, projections of the cochlea to auditory nuclei of the brainstem were attenuated but otherwise present. Equally striking changes in efferent innervation were observed in mutant animals that closely mimicked the abnormal sensory innervation pattern. Despite these impressive innervation deficiencies, the morphology of the organ of Corti and the development of inner and outer hair cells appeared comparatively normal. Key words: NT-3 mutants; inner ear; cochlea; spiral ganglion; innervation; ear developmentThe inner ear contains vestibular structures responsible for regulation of balance and cochlear structures involved in hearing. Numerous studies have characterized the distribution and documented the importance of the neurotrophins, brain-derived neurotrophic factor (BDN F) and neurotrophin 3 (N T-3), and their respective receptor tyrosine kinases, TrkB and TrkC, in the development of these sensory organs. Sensory epithelia of the inner ear express BDN F and N T-3, and their receptors are expressed by vestibular and cochlear sensory neurons (Pirvola et al., 1994;Schecterson and Bothwell, 1994;Wheeler et al., 1994). Analyses of mice with targeted mutations have shown that these molecules have essential and complementary roles in the development of these systems, with N T-3 being more important for the cochlear system and BDN F for the vestibular system (Ernfors et al., 1994(Ernfors et al., , 1995Fariñas et al., 1994;Jones et al., 1994;Fritzsch et al., 1995Fritzsch et al., , 1997aSchimmang et al., 1995;Bianchi et al., 1996).The cochlea is a spiral duct located within the inner ear that contains the organ of Corti, a specialized epithelium involved in the reception of auditory stimuli. The organ of Corti consists of two types of sensory receptor cells, or hair cells, arranged along the entire extent of the cochlea. The inner hair cells (IHCs) form a single row, whereas the outer hair cells (OHC s) are arranged in three parallel rows. The organ of Corti receives afferent innervation from sensory neurons of the cochlear (spiral) ganglion, which convey the auditory information from the hair cells to auditory (cochlear) nuclei...

The topography of motoneurons supplying each of the six ocular muscles of the lamprey, Lampetra fluviatilis, was studied by selective application of HRP to the cut nerves of identified muscles. In addition, the distributions of motoneuron populations to both eyes were studied simultaneously with fluorescein and rhodamine coupled dextran-amines (FDA and RDA) applied to cut ocular muscle nerves of either side. The motoneuron pool of the caudal oblique muscle is represented bilaterally in the trochlear (N IV) motor nucleus. The dorsal rectus muscle is innervated from a contralateral group of oculomotor (N III) motoneurons and the remaining four muscles exclusively from the ipsilateral side (N III and N VI). The inferior and posterior rectus muscles are both innervated by the abducens nerve. In contrast to all jawed vertebrates, only three eye muscles (the dorsal rectus, rostral rectus, and rostral oblique) are innervated by the oculomotor nerve in lampreys (N III). Lampreys have a motor nucleus similar to the accessory abducens nucleus previously described only in tetrapods. They lack the muscle homologous to the nasal rectus muscle of elasmobranchs and the medial rectus muscle of osteognathostomes. The distribution of the dendrites of different groups of motoneurons was studied and is considered in relation to inputs from tectum and the different cranial nerves.