Fig. 4. Afferent and efferent projections to the cochlea in wild type, Brn3a^-/- and TrkC^-/- mice. (A-C) Afferent projection to the basal turn of cochlea at P0 is demonstrated by labeling the fibers with DiI. This labeling highlights the modiolus (M), spiral ganglion (SG) and inner ear hair cells (HC). In contrast to the densely organized afferent fibers in the wild-type cochlea (A), there is an overall reduction in fiber density in the basal turn of cochlea in Brn3a^-/- mice, including an almost complete absence of the hook region of the ganglion in the basal turn (double arrowheads) and gapping between fiber bundles (B). These abnormalities in the Brn3a^-/- cochlea are similar to those present in TrkC^-/- mice (C). (D-G) The acetylated tubulin antibody labels both afferent (arrows) and efferent (arrowheads) fibers of the cochlea. In contrast to the orderly arrangement of fibers in wild-type mice (D), there is disordered outgrowth of efferent fibers in the basal turn (BT), middle turn (MT) and apical turn (AT) of the Brn3a^-/- cochlea (E-G). Scale bar: 200 µm in A-C; 100 µm in D-G.