Distribution and innervation of taste buds in the axolotl

Adult axolotls have approximately 1,400 taste buds in the epithelium of the pharyngeal roof and floor and the medial surfaces of the visceral bars. These receptors are most dense on the lingual surfaces of the upper and lower jaws, slightly less dense throughout lateral portions of the pharyngeal roof and floor, and more sparse within medial portions of the pharyngeal roof and floor, except for a median oval patch of receptors located rostrally between the vomerine tooth fields. Each taste bud is a pear-shaped organ, situated at the center of a raised hillock and averaging 80 and 87 microm in height and width, respectively. Each comprises 50 to 80 cells, which can be classified as basal, dark fusiform, or light fusiform, based on differences in their morphology. The distal ends of the apical processes of the fusiform cells reach the surface of each hillock, forming a single taste pore with an average diameter of 15 microm. Each apical process terminates in one of three ways: as short, evenly spaced microvilli; as long clustered microvilli; or as large, stereocilia-like microvilli. The pharyngeal epithelium and associated taste buds in axolotls are innervated solely by rami of the facial, glossopharyngeal and vagal nerves. Approximately, the rostral one half of the pharyngeal roof is innervated by the palatine rami of the facial nerve, whereas the caudal one half of the pharyngeal roof is innervated by the pharyngeal rami of the glossopharyngeal and vagal nerves. The lingual surface of the lower jaw is innervated by the pretrematic (mandibular) ramus of the facial nerve. The dorsal two-thirds of the visceral arches, and the ventral one-third of the visceral arches and the pharyngeal floor, are innervated by both the pretrematic and post-trematic rami of the glossopharyngeal and vagal nerves, respectively.