A Taste for Development

mice. In particular, these types of studies have been performed using animals that have disrupted gustatory University of Denver Denver, Colorado 80208 innervation due to the lack of a particular neurotrophin, brain-derived neurotrophic factor (BDNF), or its principal receptor, trkB. BDNF is expressed in developing taste For the past century, studies of the development of taste papillae (Nosrat et al., 1996) and is necessary for devel-buds have focused on the role of nerves. Specifically, opment of gustatory innervation. In BDNF or trkB knock-the conventional view has held not only that mature taste out mice, the majority of neurons within cranial ganglia buds are dependent on intact innervation for survival but die during embryogenesis, and their axons do not reach also that embryonic taste buds are induced by nerve the lingual epithelium (Conover and Yancopoulos, 1997; contact (Oakley, 1993). The neural induction of taste Fritzsch et al., 1997). When these animals are examined buds has been a convenient model that incorporates in several weeks after birth, taste buds and taste papillae a single mechanism many aspects of the development are severely disrupted (Fritzsch et al., 1997; Nosrat et of taste buds: contact by nerves could explain how taste al., 1997; Oakley et al., 1998). However, since innervated buds are induced, how they are patterned, and how taste buds are already present at birth in normal mice, they become specifically innervated. Support for this assessing mutant mice 2 weeks later does not address hypothesis, however, has come primarily from regenera-directly the role of nerves in taste bud induction, i.e., tion studies in adult or postembryonic animals where the acquisition of a taste bud fate. Rather, these studies an intact nerve supply is typically necessary for mainte-indicate that, as has been demonstrated by surgical nance or elaboration of taste bud morphology. The role denervation in rat pups (Hosley et al., 1987), normal of nerves in embryonic taste bud induction, in contrast, postnatal innervation is necessary for maintenance and has been extrapolated from the repeated observation elaboration of taste buds. In fact, when trkB knockout in most vertebrate species that nerve fibers arrive before mice are examined at or around birth, taste buds are embryonic taste buds differentiate (Figure 1, normal de-not disrupted but only become progressively disorga-velopment of amphibian taste buds). However, recent nized in the absence of normal postnatal innervation experimental work has revealed that much of the embry-(Fritzsch et al., 1997). …