The Effects of beta-Bungarotoxin on the Morphogenesis of Taste Papillae and Taste Buds in the Mouse

Abstract

Although it has been long accepted that innervation by a taste nerve is essential for maintenance of taste buds, it is not clear what role, if any, innervation plays in the morphogenesis of taste papillae and taste bud development. The following study was undertaken to determine what effects lack of sensory innervation have on the development of taste papillae and the formation of taste buds in the mouse. Timed-pregnant female mice (n = 3) at gestational day 12 (gd12) were anesthetized and a 1 microl solution (1 microg/microl) of beta-bungarotoxin (beta-BTX), a neurotoxin that disrupts sensory and motor neuron development, was injected into the amniotic cavity of two embryos per dam. Two shams were injected with PBS. Fetuses were harvested at gd18, 1 day before birth, and four beta-BTX-injected embryos, two shams and two controls were fixed in buffered paraformaldehyde. Serial sections were examined for the presence and morphology of taste papillae and taste buds. No nerve profiles were observed in beta-BTX-injected tongues. Although circumvallate papillae were present on beta-BTX tongues, only five fungiform papillae could be identified. Taste buds were present on a large percentage of fungiform papillae profiles (24%) and on circumvallate papillae in sham and control fetuses; in contrast, no taste buds were associated with taste papillae in beta-BTX fetuses. These results implicate a significant role for innervation in taste papillae and taste bud morphogenesis.

Role of Brain-Derived Neurotrophic Factor in Target Invasion in the Gustatory System

“…The majority of vallate taste buds develop postnatally , and thus when the glossopharyngeal nerve was transected during this period, the majority of vallate taste buds failed to develop . When mouse embryos were treated with ␤-bungarotoxin, disrupting sensory and motor neuron development, no taste buds were observed in the few remaining fungiform papillae present in the midportion of the tongue or associated with the circumvallate papillae (Morris-Wiman et al, 1997).…”

Neuron/target matching between chorda tympani neurons and taste buds during postnatal rat development

“…That is, the number of ganglion cells that innervate a particular taste bud by P10 could direct taste bud growth. Indeed, taste buds depend on neural innervation in order to maintain their structure and integrity in adulthood (Whiteside, 1928: Guth, 1957: Fugimoto and Murray, 1970Oakley et al, 1990;Seta et al, 1999) and for the completion of their normal development (Hosley et al, 1987;Oakley et al, 1998;Morris-Wiman et al, 1999). Although it appears that cellular proliferation rates in fungiform papillae are unchanged following loss of innervation (Oliver and Whitehead, 1992), it is apparent that either taste bud cells die and/or dedifferentiate after denervation.…”

The effect of β-bungarotoxin, or geniculate ganglion lesion on taste bud development in the chick embryo

“…On chronologic (gestation) day E12 (see Morris-Wiman et al 1999 in mouse whose gestation period is about 19 days), eggs were removed singly. After localized swabbing with 70% alcohol and the blunt side of the eggshell pierced so as to permit greater stabilization of the embryo, the shell was windowed ($3-4 mm 2) with a #10 scalpel blade, and the underlying outer shell membrane incised thereby exposing the vascularized chorioallantoic membrane.…”

“…Between chronologic day E20 and Table 1), eggs were individually removed from the incubator, rapidly opened and the embryo removed. Embryos were observed and recorded for spontaneous body movement and response to gentle body pinch (Morris-Wiman et al 1999), anesthetized (6% sodium pentobarbital, 0.05 ml, i.p. ), their chest cavity opened to verify a heartbeat, and then decapitated.…”

“…Interestingly, however, peripheral sensory auditory and vestibular hair cells as well as olfactory epithelium receptor neurons were refractory to the neurotoxic effects of b-BT (Hirokawa 1977(Hirokawa , 1978. The sole report on the effect of b-BT in the developing peripheral taste system indicated that mice injected with b-BT (1.0 lg/ll, into the amniotic cavity) on E12 showed a complete loss of sensory (lingual and glossopharyngeal nerves) and motor (hypoglossal nerve) nerves in the tongue including complete loss of normally identifiable, anterior tongue fungiform papillae, posterior tongue circumvallate papillae and palatal taste buds of 'adult' morphology at E18, 1 day prior to birth (Morris-Wiman et al 1999). Intriguingly, signs were evident of fungiform and palatal taste bud-like primordia (Morris-Wiman et al 1999).…”