Persistence of taste buds in denervated fungiform papillae

Taste buds in hamster fungiform papillae persist in an atrophic state for as long as 330 days after chorda tympani denervation or 50 days after combined chorda tympani-lingual nerve resection. Although taste bud structure depends on innervation, there is no absolute neural requirement for taste bud survival.     

Morphological changes of taste buds and fungiform papillae following long-term neurectomy

Long-term neurectomy of chorda tympani-lingual nerves results in a complete disappearance of taste buds from rabbit fungiform papillae. This supports the view that taste buds of mammalian fungiform papillae are neurally dependent. Furthermore, the covering epithelium of denervated fungiform papillae develops a characteristic keratinization pattern corresponding to that of filiform papillae.     

Development of fungiform papillae,taste buds,and their innervation in the hamster

Fungiform taste buds in mature hamsters are less subject to neurotrophic influences than those of other species. This study evaluates taste-bud neurotrophism during development in hamsters by examining the relation between growing nerves and differentiating fungiform papillae. Chorda tympani (CT) or lingual (trigeminal) nerve (LN) fibers were labelled with Lucifer Yellow as they grew into (CT fibers) or around (LN fibers) developing taste buds. Developing fungiform papillae and taste pores were counted with the aid of a topical tongue stain. The tongue forms on embryonic days (E) 10.5–11 and contains deeply placed CT and LN fibers but no papillae. By E12, the tongue epithelium develops scattered elevations. These “eminences” selectively become innervated by LN fibers that grow to the epithelium earlier and in larger numbers than CT fibers. Definitive fungiform papillae form rapidly during E13–14 and become heavily innervated by LN fibers. Intraepithelial CT fibers, rare at E13, invariably innervate fungiform papillae containing nascent taste buds at E14. During E14–15 (birth = E15–16), most papillae contain taste buds with pores, extensive perigemmal LN innervation, and extensive intragemmal CT innervation. At birth, numbers of fungiform papillae and taste pores are adultlike. The results show that fungiform eminences begin forming in the absence of innervation. The subsequent differentiation of definitive fungiform papillae and their innervation by LN fibers occur synchronously, prior to the differentiation of taste buds and their CT innervation. The hamster is precocious (e.g., compared to rat) in terms of LN development and the structural maturity of the anterior tongue at birth. © Wiley-Liss, Inc.     

Rapid quantitative assessment of fungiform papillae density in the human tongue

Fungiform papillae density, which can be used in a variety of circumstances as an indicator of taste function [L.M. Bartoshuk, V.B. Duffy, I.J. Miller, PTC/PROP tasting: anatomy, psychophysics and sex effects, Physiol. Behav. 56 (1994) 1165-1171; I.J. Miller, F.E. Reedy, Variation in human taste bud density and taste intensity perception, Physiol. Behav. 47 (1990) 1213-1219; J.R. Zuniga, N. Chen, C.L. Phillips, Chemosensory and somatosensory regeneration after lingual nerve repair in humans, J. Oral Maxillofac. Surg. 55 (1997) 2-13], was measured on the dorsal surface of the anterior tongue of living humans using a digital camera and a videomicroscope. Both procedures provided similar results, with the camera providing a more rapid, portable and flexible imaging procedure. Subsequently, the camera was successfully used to identify small regions of the anterior tongue which provide reliable measures of fungiform papillae density that correlate highly with the total number of fungiform papillae on the anterior tongue.     

Subtype-dependent postnatal development of taste receptor cells in mouse fungiform taste buds

Taste buds contain two types of taste receptor cells, inositol 1,4,5-triphosphate receptor type 3-immunoreactive cells (type II cells) and synaptosomal-associating protein-25-immunoreactive cells (type III cells). We investigated their postnatal development in mouse fungiform taste buds immunohistochemically and electrophysiologically. The cell density, i.e. the number of cells per taste bud divided by the maximal area of the horizontal cross-section of the taste bud, of type II cells increased by postnatal day (PD)49, where as that of type III cells was unchanged throughout the postnatal observation period and was equal to that of the adult cells at PD1. The immunoreactivity of taste bud cell subtypes was the same as that of their respective subtypes in adult mice throughout the postnatal observation period. Almost all type II cells were immunoreactive to gustducin at PD1, and then the ratio of gustducin-immunoreactive type II cells to all type II cells decreased to a saturation level, ～60% of all type II cells, by PD15. Type II and III cells generated voltage-gated currents similar to their respective adult cells even at PD3. These results show that infant taste receptor cells are as excitable as those of adults and propagate in a subtype-dependent manner. The relationship between the ratio of each taste receptor cell subtype to all cells and taste nerve responses are discussed.     

Innervation of single fungiform taste buds during development in rat

To determine whether the innervation of taste buds changes during postnatal development, the number of geniculate ganglion cells that innervated single fungiform taste buds were quantified in the tip- and midregions of the tongue of adult and developing rats. There was substantial variation in both the size of individual taste buds and number of geniculate ganglion cells that innervated them. Importantly, taste bud morphology and innervation were highly related. Namely, the number of labeled geniculate ganglion cells that innervated a taste bud was highly correlated with the size of the taste bud (r = 0.91, P < .0003): The larger the taste bud, the more geniculate ganglion cells that innervated it. The relationship between ganglion cell number and taste bud volume emerged during the first 40 days postnatal. Whereas there was no difference in the average number of ganglion cells that innervated individual taste buds in rats aged 10 days postnatal through adulthood, taste bud volumes increased progressively between 10 and 40 days postnatal, at which age taste bud volumes were similar to adults. The maturation of taste bud size was accompanied by the emergence of the relationship between taste bud volume and number of innervating neurons. Specifically, there was no correlation between taste bud size and number of innervating geniculate ganglion cells in 10-, 20-, or 30-day-old rats, whereas taste bud size and the number of innervating ganglion cells in 40-day-old rats were positively correlated (r = .80, P < .002). Therefore, the relationship between taste bud size and number of innervating ganglion cells develops over a prolonged postnatal period and is established when taste buds grow to their adult size. J. Comp. Neurol. 398:13–24, 1998. © 1998 Wiley-Liss, Inc.     

Machado-Joseph disease associated with an absence of fungiform papillae on the tongue

The authors report the case of a family with Machado-Joseph disease (MJD) associated with sensory and autonomic disturbances-particularly the absence of fungiform papillae on the tongue and taste buds. Sural nerve biopsy showed a loss of myelinated fibers. Autonomic function tests showed bladder-bowel dysfunction, hypohidrosis, and low coefficients of variation of R-R intervals on electrocardiogram. These findings may be another possible variant or previously unrecognized symptoms in MJD.     

A quantitative study of fungiform papillae and taste pore density in adults and children

Male children (8-9 years) are reported to have a higher sensitivity than male adults to the sweet tastant sucrose when small regions of the anterior tongue are stimulated. The present study investigated the hypothesis that the higher sensitivity was due to a greater density of fungiform papillae and taste pores (buds), since it has been reported in adults that increased densities of these two structures correlates with increased taste suprathreshold sensitivity [Physiol. Behav. 47 (1990) 1213]. Quantitative measures of the number and size of papillae and pores in two areas of the tongue that had been shown to have a higher sensitivity for sucrose were achieved in 20 male children 8-9 years of age and 20 adults 18-30 years of age, using videomicroscopy and NIH Image software. Customized templates and a red food dye were used to define the equivalent tongue locations across the 40 subjects and taste pores were stained with methylene blue. Children were found to have substantially smaller papillae than adults but significantly higher papilla densities in both areas. Similar numbers of taste pores per papilla were found for both groups, resulting in children having much higher taste pore densities in each area than adults. Other differences included smaller taste pore diameters in children compared to adults, and the papillae tended to be rounder in children. Overall, the results support the hypothesis that the higher densities of fungiform papillae and taste pores in children underlie their greater sensitivity for sucrose in the two areas. In addition, the anatomical differences between adults and children indicate the sense of taste is in a state of development during mid-childhood.     

Early development and innervation of taste bud-bearing papillae on the rat tongue

Early development of fungiform papillae on the fetal rat tongue was examined: (1) to determine whether morphogenesis of the taste bud-bearing fungiform papillae is induced by nerve and (2) to study the growth pattern of the two sensory nerves that innervate the papilla. The papillae first appear on the 15th day of gestation (E15; E1 is the day when the dam is sperm positive) in rows parallel to the midline sulcus. There appears to be a medial-lateral and an anterior-posterior gradient in the sequence of papilla differentiation. The epithelium of the early papilla resembles a multilayered placode topped by a flattened surface periderm. Close examination of the peridermal cells at the apex of the papillae reveals that the cells have fewer surface microvilli and their cytoplasm is more electron opaque than that of similar cells in interpapillary regions. The basal cells in the placode-like epithelium differ from those in interpapillary regions in that they are postmitotic and have more mitochondria. At later stages, the papilla acquires a mesenchymal core and nerves grow into the core. Results from organ culture experiments of tongue fragments taken from E14 fetuses indicate that morphogenesis of fungiform papillae is initiated in the absence of sensory nerve influence, but the nerve exerts a trophic effect on their maintenance. The two sensory nerves of the tongue, the chorda tympani and the lingual branch of the trigeminal nerve, enter the tongue mesenchyme at E14 and grow toward the epithelium. By E15 the chorda tympani branches have reached the developing fungiform papillae, by E16 many have entered the papilla, and by E17 they have penetrated the epithelium at the papilla apex. Their fibers are associated exclusively with the cells at the papilla apex, where the taste bud will develop. The trigeminal nerve ramifies beneath the surface of the entire epithelium by E15. Later, it, too, sends branches into fungiform papillae; these ascend along the trunk of the chorda tympani and at E17 terminate in the connective tissue core around the chorda tympani field. The results are compatible with the notion that the tongue epithelium exerts a general tropic effect on growing axons of both sensory nerves, and the epithelial cells of the fungiform papilla apex exert a similar effect to which only the chorda tympani axons are responsive.     

Acidic stimuli activates two distinct pathways in taste receptor cells from rat fungiform papillae

A sour taste sensation may be produced when acidic stimuli interact with taste receptor cells (TRCs) on the dorsal surface of the tongue. We have searched for pathways in TRCs that may be activated by acidic stimuli using RT-PCR and changes in intracellular calcium (Ca(2+)(I)) induced by acidic stimuli in rat fungiform papillae. RT-PCR revealed the presence of proton-gated subunits ASIC-beta and VR1. Ca(2+) imaging measurements of the TRCs revealed two distinct responses to acidic stimuli: Ca(2+)(i) was increased in 9% (28/308; Type I) and was decreased in 39% (121/308; Type II). Neither of these responses was affected by the removal of extracellular Ca(2+), indicating that the changes arise from the release and sequestration of Ca(2+) from intracellular stores. These responses were also not inhibited by the vanilloid receptor antagonist, capsazepine, suggesting they do not arise from the activation of vanilloid receptors. The Type I, but not the Type II response was inhibited by amiloride. Dose-response measurements for Types I and II responses yielded pH(50%) of 4.8 and 4.9, respectively. Type II responses were inhibited by pertussis toxin, suggesting G-protein involvement. TRCs that exhibit Type II responses could also be activated by quinine (which increased Ca(2+)(I)) thus suggesting a mechanism by which the addition of acid may be suppressive to other chemical stimuli.     

Expression of the basal cell markers of taste buds in the anterior tongue and soft palate of the mouse embryo

Although embryonic expression of Shh in the fungiform papilla placodes has a critical role in fungiform papilla patterning, it remains unclear whether its appearance indicates the differentiation of the basal cells of taste buds. To examine the embryonic development of the basal cells, the expression of Shh, Prox1, and Mash1 was determined in the anterior tongue and soft palate in mouse embryos by in situ hybridization. In the anterior tongue, Prox1 was coexpressed with Shh from the beginning of Shh expression in the fungiform papilla placodes at E12.5. Shh was expressed in the soft palate in a band-like pattern in the anteriormost region and in a punctate pattern in the posterior region at E14.5. The number (21.4 +/- 4.3, at E14.5) of locations where Shh was observed (i.e., spots) rapidly increased and reached a peak level (54.8 +/- 4.0 at E15.5). Also in the soft palate, Prox1 was coexpressed with Shh from the beginning of Shh expression. These results suggest that basal cell differentiation occurs synchronously with the patterning of Shh spots both in the anterior tongue and in the soft palate. In contrast, Mash1 expression lagged behind the expression of Shh and Prox1 and began after the number of Shh spots had reached its peak level in the soft palate. Furthermore, immunohistochemistry of PGP9.5 and Shh revealed that epithelial innervation slightly preceded Mash1 expression both in the tongue and in the soft palate. This is the first report describing the time courses of the embryonic expression of basal cell markers of taste buds.     

Carbonic anhydrase is associated with taste buds in rat tongue

A modification of Hansson's histochemical technique was used to reveal carbonic anhydrase activity in mounted cryostat sections of the circumvallate papillae from rat tongue. An intensely positive reaction was found at the level of the neck of the papilla, associated with the taste buds. Lingual glands also contained abundant carbonic anhydrrase activity. The presence of carbonic anhydrase in taste buds, as well as our previous observation that it is found in a population of olfactory receptor cells, may indicate a role for the enzyme in gustative and olfactory phenomena.     

Facilitation of the development of fungiform taste buds by early intraoral acesulfame-K stimulation to mice

The gustatory system is susceptible to anatomical modification by postnatal taste stimulations. This study investigated the effects of early intraoral infusion of acesulfame-K solution on the development of fungiform taste buds in mice. It was found that the acesulfame-K infusion increased the number, promoted the maturation, and enlarged the size of taste bud during the postnatal stages, compared with the age-matched controls. This provides fundamental and new information about the development of taste bud under normal and early acesulfame-K-stimulated conditions.     

Erb and c-Kit receptors have distinctive patterns of expression in adult and developing taste papillae and taste buds.

Twenty four different protein tyrosine kinases (PTKs) were amplified from a taste-enriched cDNA library using PCR. The expression of four protein tyrosine kinase receptors (EGFR, ErbB2, ErbB3, and c-kit) was examined in adult and developing rat taste papillae. All four of these receptors were expressed in overlapping populations of differentiated taste cells within adult taste buds. Taste bud basal cells were ErbB2(+) but did not express the other Erb receptors. During prenatal development, the Erb receptors were expressed extensively in the basal cells around developing papillae, and ErbB2 and c-kit immunoreactive neuronal fibers were seen in close association with taste papillae. In early postnatal stages, ErbB2(+) and c-kit(+) neuronal fibers were often seen entering the taste papillae epithelium, where new taste buds form, and by postnatal day 2 (P2), individual ErbB2(+) and c-kit(+) cells were seen in this region as well. Between P3 and P8, c-kit was highly expressed at the bottom of foliate papillae trenches. The extensive expression of the Erb and c-kit receptors in adult taste buds and in and around developing papillae suggests that these receptors may play a role in the prenatal and postnatal development of gustatory papillae and taste buds.     

Brain-derived neurotrophic factor mRNA is expressed in the developing taste bud-bearing tongue papillae of rat

Brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are expressed in many areas of the nervous system and its target tissues. Using in situ hybridization we have investigated the possible presence of NGF mRNA and BDNF mRNA in the developing fungiform and circumvallate papillae of the rat tongue. BDNF mRNA is present in the epithelium of the developing fungiform papillae in E15, E16, and E17 rat embryos with peak concentration at E16. It starts to diminish after E17 and is almost absent at E21. There is a specific temporospatial change in the expression of BDNF mRNA in developing circumvallate papillae. It is expressed in the epithelium of the superior and posterior surfaces of the papillae at E15, E16, and E17. Already at E17 the BDNF mRNA labeling has started to decrease in the superior epithelium. At E19 and E21, BDNF mRNA is exclusively present in the epithelium of the inner and outer walls of the trench, surrounding the papilla at the posterior and lateral surfaces where the taste buds are located later in life. BDNF mRNA was also detected in the developing palatal taste buds. NGF mRNA was below detection level in the developing papillae. The highly localized expression of BDNF mRNA in areas where taste buds are to be formed suggests that BDNF may be one crucial factor in the formation of the epithelial innervation prior to taste bud formation. It might also participate in the formation and/or maintenance of the papillary and/or taste bud innervation apparatus. We conclude that the neurotrophin BDNF is expressed in early development of taste bud-bearing papillae in the rat tongue in a temporally and spatially controlled manner, presumably to act as a target-derived chemoat tractant for the early nerve fibers. © 1995 Wiley-Liss, Inc.     

Cellular expression of alpha-gustducin and the A blood group antigen in rat fungiform taste buds cross-reinnervated by the IXth nerve.

Although taste buds are trophically dependent on their innervation, cross-reinnervation experiments have shown that their gustatory sensitivities are determined by the local epithelium. Both the gustatory G-protein, alpha-gustducin, and the cell-surface carbohydrate, the A blood group antigen, are expressed by significantly fewer fungiform than vallate taste cells in the rat. In these experiments, one side of the anterior portion of the tongue was cross-reinnervated by the IXth nerve in order to determine whether the molecular expression of taste bud cells is determined by the epithelium from which they arise or by the nerve on which they are trophically dependent. The proximal portion of the IXth nerve was anastomosed to the distal portion of the chorda tympani (CT) nerve using fibrin glue (IX-CT rats). Control animals had the CT cut and reanastomosed using the same technique (CT-CT rats), or had the CT avulsed from the bulla and resected to prevent regeneration (CTX rats). The animals survived for 12 weeks postoperatively, and the tongues were removed, stained with methylene blue, and the fungiform taste pores counted on both sides. Tissue from the anterior 5 mm of the tongue was cut into 50-microm sections, which were incubated with antibodies against alpha-gustducin and the human blood group A antigen. In both CT-CT and IX-CT rats, there was regeneration of fungiform taste buds, although in both groups there were significantly fewer taste buds on the operated side of the tongue. The normal vallate papilla had a mean of 8.37 alpha-gustducin-expressing cells and 5.22 A-expressing cells per taste bud, whereas the fungiform papillae contained 3.06 and 0.23 cells per taste bud, respectively. In both CT-CT and IX-CT rats there was a normal number of cells expressing alpha-gustducin or the A antigen in regenerated taste buds; in the CTX animals there was a significant decrease in the expression of these markers. These results demonstrate that the molecular phenotype of taste bud cells is determined by the local epithelium from which they arise and not by properties of the innervating nerve.