Transcellular labeling by DiI demonstrates the glossopharyngeal innervation of taste buds in the lingual epithelium of the axolotl.

Innervation of the axolotl lingual epithelium by the glossopharyngeal nerve was examined to reveal its sensory target cells. The carbocyanine dye diI was applied to the nerve stump in the tongue fixed with paraformaldehyde. After a diffusion period of several months, the tongues were examined with a conventional epifluorescence microscope and a confocal laser scanning microscope (LSM) in wholemounts or preparations sectioned with a vibratome. Beneath the epithelium the labeled nerve fibers spread horizontally to form a meshwork of fibers, from which fascicles of fibers extended upward perpendicularly to the epithelium to innervate taste buds. Numerous taste buds were labeled by possible transcellular diffusion of diI. At the base of the taste bud, the nerve fibers branched and formed a basal plexus of fine fibers, on which numerous varicosities were seen. One or at most several taste cells were labeled in a taste bud. In the basal part of taste buds, the cell without an apical process, the basal cell, was also labeled. In the epithelium, between the taste buds, a few solitary cells were labeled. In some cases, a single fascicle of fibers innervating these cells was clearly shown by the LSM. In addition, fine fibers apparently formed free nerve endings in the epithelial cell layer. The results showed that the IX nerve innervated not only taste cells, but also presumed mechanosensory basal cells in the taste bud and the solitary cells of unknown function in the non-taste lingual epithelium. Afferent nerve responses to mechanical stimulation of the tongue may be explained by these non-taste cellular elements in the epithelium.