Schematic depiction of the major effects of loss of Schwann cells in Sox10 conditional deletion mutants and ErbB2 mutants.

<p>The single row of inner hair cells (IHC) is innervated by type I afferents, the three rows of outer hair cells (OHC) are innervated by type II afferents. All spiral ganglion neurons are in parallel to the organ of Corti in Rosenthal’s canal (grey). In addition, two types of efferent fibers (Medial and Lateral Olivocochlear fibers; MOC, LOC) form intraganglionic spiral bundles (IGSB) and join the afferents in radial fibers (RF) to the organ of Corti. Conditional deletion of Sox10 or ErbB2 leads to excessive migration of spiral ganglion neurons beyond Rosenthal’s canal into the modiolus that normally carries only afferent and efferent fibers. Some spiral ganglion neurons leave the ear to intermingle with vestibular neurons that always migrate outside the otic capsule. Instead of forming an IGSB, efferent fibers crisscross randomly along the greatly lengthened radial fibers. Many, but not all afferents, bypass the organ of Corti and extend to the lateral wall (LW). The overall pattern of migration and disorganization of fibers is similar in ErbB2 mutants. However, at E18.5 there are hardly any fibers to the lateral wall left in ErbB2 mutants; fewer fibers reach the organ of Corti and those that do are very thin. It appears that elimination of Schwann cells by either Sox10 conditional deletion or ErbB2 mutation has an overall similar phenotype of excessive migration and overshooting afferent projection. Differences seem to relate to additional effects of ErbB2 on NT3 expression and are not specific to Schwann cells. Blue indicates BDNF expression in hair cells, yellow indicates NT3 expression in inner hair cells. Abbreviations: IGSB, intraganglionic spiral bundles; IHC, inner hair cells; OHC, outer hair cells; RF, radial fibers; MOC, medial olivocochlear fibers; LOC, lateral olivocochlear fibers; LW, lateral wall.