Fig. 4. DiI tracing of afferent innervation in wild type (A,D,G,J) BDNF^–/– (B,E,H,K) and BDNF^NT3/NT3 (C,F,I,L), and whole mount immunohistochemistry to detect radial fibres in wild type (M) BDNF^–/– (N) and BDNF^NT3/NT3 (O). Micrographs illustrate the apical (A-F,M-O), middle (G-I), and base (J-L) of the cochlea. Scale bars: 40 µm in A-C and M-O and 10 µm in D-L. There is no reduction in the density of radial fibres (rf) in either BDNF^–/– (B) or BDNF^NT3/NT3 (C) compared to wild-type (A) mice as seen by DiI tracing. This is also shown by the distribution of nerve fibres stained for acetylated tubulin (red) and counterstained with FITC-conjugated phalloidin (green) to visualise the hair cells (M-O). Note the innervation of the three rows of OHCs in wild-type mice (D,G,J), loss of innervation of all three rows of OHCs in BDNF^–/– mice (E,H,K) and rescue of innervation of the OHCs in the BDNF^NT3/NT3 mice (F,I,L). Arrows indicate the three rows of OHC.