Taste buds are chemosensory organs consisting of collections of spindle-shaped modified epithelial cells that transduce chemical stimuli into neural signals. Large numbers of taste buds reside in three taste areas of the tongue; fungiform, foliate

Taste buds are chemosensory organs consisting of collections of spindle-shaped modified epithelial cells that transduce chemical stimuli into neural signals. Large numbers of taste buds reside in three taste areas of the tongue; fungiform, foliate and circumvallate papillae. The lingual epithelium also contains numerous non-gustatory, filiform papillae which may also serve as tactile endorgans (Nosrat et al., 1996). Although lingual taste buds receive specific gustatory innervation from the 7th and 9th cranial nerves, the surrounding epithelium, as well as the filiform papillae receive an extensive somatosensory type of innervation from cranial nerves V and IX. While the taste buds and the taste receptor cells within them are innervated by gustatory nerves, conveying mainly gustatory stimuli, many fibers conveying general somatosensory stimuli are found immediately surrounding the taste buds and in the lingual epithelium. In a gustatory papilla, those fibers that enter taste buds at their base and have synaptic contacts with taste cells are denoted intragemmal fibers while nerve fibers found outside the taste bud proper are denoted perigemmal fibers. In addition, some perigemmal fibers enter the taste buds without synapsing on taste cells (Nagy et al., 1982; Finger et al., 1990; Yamasaki et al., 1984). It is assumed that taste buds in mammals are induced in local epithelium by ingrowing nerves (Vintschgau and Hönigschmied, 1876; Torrey, 1940; Farbman and Mbiene, 1991). Recent studies indicate that taste buds indeed form from local epithelium (Barlow and Northcutt, 1995; Stone et al., 1995). However, in urodele, taste buds develop in the absence of proper innervation (Stone, 1940; Wright, 1964; Barlow et al., 1996). The capacity for differentiation into taste buds resides in certain epithelia and not in others (Zalewski, 1974) and indicates that lingual epithelium is specialized for taste buds before nerves arrive. In addition, we have also shown that the gustatory, but not the surrounding epithelium of the developing rat tongue expresses brain-derived neurotrophic factor (BDNF) which therefore probably selectively supports the gustatory innervation (Nosrat and Olson, 1995). Cell turnover studies in mature taste buds (Beidler and Smallman, 1965; Farbman, 1980), indicate that renewal and maturation of taste cells, as well as formation of synaptic contacts, are continuous processes also in the adult. Low levels of neurotrophin expression may thus be needed in taste buds throughout life. In rats, BDNF and neurotrophin 3 (NT3) mRNAs are expressed in specific patterns in the lingual epithelium suggesting gustatory and somatosensory functions, respectively (Nosrat et al., 1996). The neurotrophins BDNF and NT3 (see Barde, 1989; Johnson and Oppenheim, 1994; Barbacid, 1995) play roles in the innervation of sensory organs, such as the inner ear (Ernfors et al., 1995), vibrissae (Ibanez et al., 1993), carotid body (Hertzberg et al., 1994; Erickson et al., 1996) and muscle spindles (Ernfors et al., 1994b). They are expressed in the target areas that are going to become innervated. Neuronal loss has been observed in sensory but not motor neurons in mice 1333 Development 124, 1333-1342 (1997) Printed in Great Britain © The Company of Biologists Limited 1997 DEV1149