Fig. 8. NT3 cannot support neuronal survival or innervation in the tongue. (A) Schematic drawing of BDNF (red) and NT3 (light blue) mRNA expression in fungiform papilla in rodents. The arrow indicates the taste bud with innervation. The yellow lines represent the BDNF-dependent intragemmal fibres (gustatory fibres) and the dark blue dashed lines represent perigemmal fibres (somatosensory innervation). Gustatory nerve fibres innervate only the taste buds where BDNF is present while somatosensory nerve fibres predominantly innervate the surrounding epithelium where NT3 is present. (B) Typical innervation of a fungiform papilla and its taste bud (arrow) in a wild-type P7 mouse tongue, visualised by immunohistochemistry against protein gene product 9.5 (PGP). The papilla is well-innervated, gustatory fibres innervate the taste bud (arrow) and perigemmal somatosensory nerve fibres innervate the surrounding epithelium. (C) An example of fungiform papilla innervation in BDNF^NT3/NT3 mice. The NT3-dependent somatosensory innervation apparatus of the papilla appears intact, while the taste bud innervation (arrow) is almost non-existent. (D) Another example of fungiform papilla innervation in BDNF^NT3/NT3 mice. Generally, the papilla is malformed. It has filiform papilla morphology but could be distinguished from a normal filiform papilla by the amount of nerve fibres in this papilla. An under-developed taste bud seems to be present in the core part of the papilla (arrow). (E-G) Scanning electron micrographs of the anterior part of the tongue from P7 mice. (E) Fungiform papillae rise above the filiform papillae in a wild-type tongue. (F) Surface morphology of the BDNF^NT3/NT3 tongue reveals major loss of fungiform papillae and the few remaining papillae are reduced in size. (G) Anterior tongue morphology in BDNF^–/– mice is similar to that of the BDNF^NT3/NT3 mice. (H) Quantitative measurements of P7 geniculate ganglion neuronal numbers. Half of the neurons are missing in the BDNF^–/– and BDNF^NT3/NT3 mice compared with wild-type animals. The neuronal numbers show no significant difference between BDNF^–/– and BDNF^NT3/NT3 mice. ANOVA Fischer's ***P<0.001; ±s.e.m. Scale bars: 100 µm in B-D and 200 µm in E-G.