The pattern of innervation at E14.5 and E15.5. Wild-type animals show afferents (in green) to the growing cochlear spiral (A). Efferent fibers, shown in red, begin to form the intraganglionic spiral bundle. There is no cochlear spiral or spiral ganglion in Pax2 null mice (E). Instead there is an aggregation of neurons near the reduced fibers leading to the posterior canal crista (B), the presumed spiral ganglion (SG). In mutants, efferent fibers reroute from the spiral ganglion to the facial nerve (B). The innervation to the anterior canal crista (AC in C, D) is reduced, but not that to the horizontal crista (C, D). The saccule shows near normal organization of fibers in the sensory epithelium (F) as well as an overshooting projection at a different focal plane (E). At E15.5, sensory neurons that can be filled with brain injections of lipophilic dyes form a tear drop arrangement of spiral sensory neurons (G, H). Multiple nerve fibers radiate away for a short distance toward the cochlear expansion (CO), but no pattern reminiscent of radial fibers in the cochlea can be observed (G, H, J, K). There is a variable reduction of nerve fibers to the anterior and posterior canal crista (D, G) and the innervation of the saccule is variable (G-L). A focal series (I-L) shows that spiral ganglion neurons are distinct in their distribution from vestibular ganglion cells as is evidenced by. AC, anterior canal crista; HC, horizontal canal crista; Co, cochlear duct/sack; Gen Ggl, geniculate ganglion; PC, posterior canal crista; VG, vestibular ganglion; IX, glossopharyngeal nerve. Bar indicates 100 um.