Fig. 6. Failure of terminal nerve in-growth in BDNF^–/– and BDNF^NT3/NT3 mice. (A-I) Immunohistochemistry for p75 to detect afferent innervation (red) and FITC-conjugated phalloidin (green), which visualises the hair cells. (A-C) Already at E16 when the fibres have reached the epithelium of the utricular and saccular maculae in wild-type mice (A), BDNF^NT3/NT3 mice showed a comparable failure of innervation (C) to BDNF^–/– mice (B). (D-F) Similar results were obtained at P0. Note that only a few fibres have reached the hair cells in the BDNF^NT3/NT3 mice (F) and none has reached the hair cells in BDNF^–/– mice (E). Nerve fibres are seen at all stages in the subepithelial layer, suggesting that BDNF is only required for terminal innervation and formation of functional nerve endings but that it is not required for nerve fibres to project to the utricular and saccular compartments. (G-I) The ampullar cristae completely lack innervation in both BDNF^–/– (H) and BDNF^NT3/NT3 mice (I). Scale bars: 50 µm in A-I.