Location of taste buds in intact taste papillae by a selective staining method

Human biology of taste

Taste or gustation is one of the 5 traditional senses including hearing, sight, touch, and smell. The sense of taste has classically been limited to the 5 basic taste qualities: sweet, salty, sour, bitter, and umami or savory. Advances from the Human Genome Project and others have allowed the identification and determination of many of the genes and molecular mechanisms involved in taste biology. The ubiquitous G protein-coupled receptors (GPCRs) make up the sweet, umami, and bitter receptors. Although less clear in humans, transient receptor potential ion channels are thought to mediate salty and sour taste; however, other targets have been identified. Furthermore, taste receptors have been located throughout the body and appear to be involved in many regulatory processes. An emerging interplay is revealed between chemical sensing in the periphery, cortical processing, performance, and physiology and likely the pathophysiology of diseases such as diabetes.

Gustatory impairment in patients undergoing head and neck irradiation

OBJECTIVES

To determine whether radiation alters taste function and structure.

RESEARCH DESIGN

Prospective, longitudinal study.

METHODOLOGY

Testing prior to starting radiation, and 2 weeks, 2 months, and 6 months after completing radiation.

RESULTS

Relative to controls, patients had lower taste identification test scores for bitter, salty, and sour tastes. Sour taste also showed a significant group-by-time interaction (P = .03). Taste pores were decreased in the irradiated group, with a significant group-by-time interaction (P = .03).

CONCLUSION

Head and neck cancer patients have decreased taste function, and radiation adversely affected sour taste and taste pores.

Glycoconjugate in rat taste buds

The taste buds of the fungiform papillae, circumvallate papilla, foliate papillae, soft palate and epiglottis of the rat oral cavity were examined by lectin histochemistry to elucidate the relationships between expression of glycoconjugates and innervation. Seven out of 21 lectins showed moderate to intense staining in at least more than one taste bud. They were succinylated wheat germ agglutinin (s-WGA). Dolichos biflorus agglutinin (DBA), Bandeiraea simplicifolia lectin-I (BSL-I), Ricinus communis agglutinin-I (RCA-I), peanut agglutinin (PNA), Ulex europaeus agglutinin-I (UEA-I) and Phaseolus vulgaris agglutinin-L (PHA-L). UEA-I and BSL-I showed moderate to intense staining in all of the taste buds examined. They strongly stained the taste buds of the epiglottis, which are innervated by the cranial nerve X. UEA-I intensely stained the taste buds of the fungiform papillae and soft palate, both of which are innervated by the cranial nerve VII. The taste buds of circumvallate papilla and foliate papillae were innervated by the cranial nerve IX and strongly stained by BSL-I. Thus, UEA-I and BSL-I binding glycoconjugates, probably alpha-linked fucose and alpha-D-galactose, respectively, might be specific for taste buds. Although the expression of these glycoconjugates would be related to the innervation of the cranial nerve X, the differential expression of alpha-linked fucose and alpha-D-galactose might be related to the innervation of the cranial nerve VII and IX, respectively.

Scanning electron microscopy of denervated taste buds in hamster: morphology of fungiform taste pores

BACKGROUND

Taste pores of fungiform papillae are critical for taste function. Taste nerve injury affects the pore, rendering it refractory to staining with vital dyes. Whether pores of denervated fungiform papillae disappear or undergo more modest structural changes to account for diminished staining was the subject of the present study.

METHODS

The chorda tympani in the hamster was served unilaterally and the anterior tongue prepared for scanning electron microscopy after 31 days of survival.

RESULTS

Taste pores were found on 92% of control fungiform papillae. They were round openings formed by the free margins of keratinocytes, and centered in hillock-shaped elevations of the papillary surface. Hillocks were encircled by an indentation which, in turn, was surrounded by a circular epithelial rim. These structures associated with fungiform pores distinguish pores on the anterior tongue from those on the posterior tongue. The pores led to a channel that penetrated into the papilla. The experimental side of the tongue had markedly fewer pores. Definitive pores were present on only 53% of denervated papillae. The papillae that lacked pores either exhibited a small hillock and a subtle depression in place of the pore, or had entirely flat apical surfaces. The denervated papillae that retained pores exhibited structural changes. The pores had smaller diameters and led to shallower channels than control pores. Moreover, these persistent pores were associated with hillocks, indentations and rims that were more variable and less distinct than those of control papillae.

CONCLUSIONS

Pores of fungiform papillae in hamster are associated with specialized surfaces features of the papillary epithelium. Denervation results in changes that range from disappearance of the pores to their shrinkage and the atrophy of pore-associated epithelial structures.

Light and electron microscopical demonstration of methylene blue accumulation sites in taste buds of fish and mouse after supravital dye injection

Electron microscopical data regarding methylene blue staining of taste buds in the epithelia of the goldfish lip and the cirumvallate papilla of the mouse tongue after supravital dye application are presented for the first time. The ultrastructural details were compared with the corresponding light microscopical findings. The dye was applied in different concentrations by injection or in crystalline from directly to the surface of the tissues. Both methylene blue and tissue were simultaneously fixed by immersion in a paraformaldehyde-glutaraldehyde solution with the addition of phosphomolybdic acid. The ensuing dye precipitate was further stabilized by ammonium heptamolybdate. On the light microscopical level, the taste bud's receptive structures, i.e. the receptor area (fish) and the taste pit (mouse), exhibited the highest affinity for the dye. Additionally, the mucous material within the trenches around the circumvallate papillae in mice was intensely stained. On the electron microscopical level, the cationic phenothiazine dye bound to the receptor villi or to the mucus coating the receptive structures. In the case of higher dye concentrations, a staining of single taste bud cells took place starting apically and proceeding down to the base. Dye accumulations within the intercellular clefts between the epithelial cells or within other structures were observed only if the dye concentration was further increased. Since similar results were also obtained with the cationic phenazo dye Janus green, dye accumulation in the mucus covering the receptor villi may be representative of the general binding of organic cations, which are known to induce bitter taste sensations.

Comparative lectin histochemistry on taste buds in foliate, circumvallate and fungiform papillae of the rabbit tongue

Taste buds (TB) in the foliate, circumvallate and fungiform papillae of the rabbit tongue were examined with lectin histochemistry by means of light (LM) and electron (EM) microscopy. Biotin- and gold-labeled lectins were used for the detection of carbohydrate residues in TB cells and subcutaneous salivary glands. At the LM level, the lectins of soybean (SBA) and peanut (PNA) react with material of the foliate and circumvallate taste pores only after pretreatment of the section with neuraminidase. This indicates that the terminal trisaccharide sequences are as follows: Sialic acid-Gal-GalNAc in O-glycosylated glycoproteins or Sialic acid-Gal-GlcNAc in N-glycosylated glycoproteins. In fungi-form taste buds the lectins of Dolichos biflorus (DBA) and Helix pomatia (HPA), also specific to GalNAc residues, are reactive without preincubation with neuraminidase. Wheat germ agglutinin (WGA), specific to GlcNAc, reacts with TBs of all papillae; and the lectin from Ulex europaeus (UEA I), specific to fucose, binds to individual TB cells. The presence of sialic acid may protect mucus or other glycoproteins in TB cells and inside the taste pore from premature enzymatic degradation. In a post-embedding EM procedure on LR-White-embedded tissue sections, only gold-labeled HPA was found to bind especially on membrane surfaces of the microvilli which protrude into the taste pore; however HPA did not bind to the electron-dense mucus inside the taste pore. The mucus situated in the trough and at the top of the adjacent epithelial cells also is strongly HPA-positive, but is of different origin and composition than that found in the taste pore. These results demonstrate distinct carbohydrate histochemical differences between fungiform and circumvallate/foliate taste buds. The different configuration of galactosyl residues and the occurrence of mannose in circumvallate and foliate TBs leads to the suggestion that the lectin reactivities of TBs are not only due to the presence of mucins, but also to N-linked glycoproteins, possibly with a hormone-like paraneuronal function. A possible relationship to v. Ebner glands in these papillae is discussed.

Immunohistochemical localization of neuron-specific enolase and calcitonin gene-related peptide in pig taste papillae

Immunoreactivity to neuron-specific enolase (NSE), a specific neuronal marker, and calcitonin gene-related peptide (CGRP) was localized in lingual taste papillae in the pigs. Sequential staining for NSE and CGRP by an elution technique allowed the identification of neuronal subpopulations. NSE-staining revealed a large neuronal network within the subepithelial layer of all taste papillae. NSE-positive fibers then penetrated the epithelium as isolated fibers, primarily in the foliate and circumvallate papillae, or as brush-shaped units formed by a multitude of fibers, especially in the fungiform papillae and in the apical epithelium of the circumvallate papilla. Taste buds of any type of taste papillae were found to express a dense subgemmal/intragemmal NSE-positive neuronal network. CGRP-positive nerve fibers were numerous in the subepithelial layer of all three types of taste papillae. In the foliate and circumvallate papillae, these fibers penetrated the epithelium to form extragemmal and intragemmal fibers, while in the fungiforms, they concentrated almost exclusively in the taste buds as intragemmal nerve fibers. Intragemmal NSE- and CGRP-positive fiber populations were not readily distinguishable by typical neural swellings as previously observed in the rat. The NSE-positive neuronal extragemmal brushes never expressed any CGRP-like immunoreactivity. Even more surprising, fungiform taste buds, whether richly innervated by or devoid of NSE-positive intragemmal fibers, always harboured numerous intragemmal CGRP-positive fibers. Consequently, NSE is not a general neuronal marker in porcine taste papillae. Our observations also suggest that subgemmal/intragemmal NSE-positive fibers are actively involved in synaptogenesis within taste buds. NSE-positive taste bud cells were found in all three types of taste papillae. CGRP-positive taste bud cells were never observed.

The distribution of fungiform papillae and taste buds on the human tongue

Investigations on monkeys have shown that the application of the acidic dye Ponceau S red or the basic dye Alcian blue to the tongue surface facilitates identification of fungiform papillae and taste buds. Both of these dyes were now used in varying degrees of acidity on fixed and unfixed human cadaveric tongues in an attempt to determine the regional distribution of papillae and buds. Satisfactory staining was obtained with acidic Ponceau S red in 10% formalin and 10% trichloracetic acid (pH 2.5). For six tongues, the number of fungiform papillae ranged from 171 to 253 (mean 195) and these were located predominantly at the tip. Of the fungiform papillae, 67% had no staining of taste bud pores. The average number of visible taste pores on the other fungiform papillae was 3 (range 1-21). The correlation between the number of stained taste pores and underlying taste buds was confirmed using serial histological sections of 90 fungiform papillae. This work has shown that a mean of 193 taste buds are carried on fungiform papillae of the human tongue and that 87% of these are located in the anterior 2 cm.

Variations in human taste bud density and taste intensity perception

Some variations in human taste sensitivity may be due to different numbers of taste buds among subjects. Taste pores were counted on the tongue tips of 16 people with videomicroscopy, and the subjects were divided into two groups (N = 8) by the rank order of their taste bud densities. The "higher" density group averaged 374 +/- 134 taste pores/cm2, while the "lower" density group averaged 135 +/- 43 tp/cm2. The higher density group had an average fungiform papilla density which was 1.8 times greater than the lower density group and an average of 1.5 times more taste pores/papilla. The subjects also rated the intensity for 4 suprathreshold concentrations of 5 taste stimuli placed on the same region of the tongue where taste pores were counted. The group with higher taste bud densities gave significantly higher average intensity ratings for sucrose (196%), NaCl (135%) and PROP (142%), but not for citric acid (118%) and quinine HCl (110%) than the lower density group. Thus, the subjects with higher fungiform taste bud densities also reported some tastes as more intense than subjects with fewer fungiform taste buds.

Quantitative study of fungiform papillae and taste buds on the cat's tongue

The number of fungiform papillae has been counted on the tongues of six adult cats and of kittens both at birth and aged 2 and 4 months. Papillae were sampled from different regions of the tongue, and their size and the number of taste buds they contained were determined using histological sections taken parallel to the tongue surface. There were approximately 250 fungiform papillae on the tongues of the adult cats, the papillae were most numerous at the tip of the tongue, and there was no significant difference between the number of papillae on each side. The size of the papillae increased from a mean maximum diameter of 0.28 mm at the tip of the tongue to 0.48 mm at the back; the mean number of taste buds increased correspondingly from 6.9 to 16.6. The kitten tongues had a number and distribution of fungiform papillae similar to that found in the adults. In the neonate, papillae were smaller and contained fewer taste buds; these parameters increased with the corresponding increase in tongue size in the 2- and 4-month-old kittens.

Calcitonin-gene-related-peptide-immunoreactive innervation of the rat head with emphasis on specialized sensory structures

The distribution of calcitonin-gene-related peptide-like immunoreactivity (CGRP-IR) was studied in sections of decalcified rat head and selected whole-mount preparations in order to address the complex peptidergic innervation patterns in peripheral cephalic specialized zones and to examine neuronal ganglia in situ. Labeled neuron somata in trigeminal, glossopharyngeal, and vagal ganglia comprised a large proportion of small to medium size type B ganglion cells. Parasympathetic ganglia (ciliary, otic, sphenopalatine, submandibular) revealed a small population of labeled somata and numerous perisomatic IR axons, whereas sympathetic ganglion cells (superior cervical) were devoid of label though richly innervated by perisomatic IR axons. The gustatory geniculate ganglion contained only a few labeled neurons and axons. Coarse peripheral CGRP-IR axons were traced to skeletal muscle motor end plates (e.g., lingual, tensor tympani, etc.), and thin sensory axons most densely innervated the cornea, iris, general integument, all mucosal epithelia lining the tympanic, nasal, sinus and oropharyngeal cavities, and the cerebral meninges. Blood vessels, glands, ducts, and their orifices were often heavily innervated, and specific specializations and exceptions are discussed. Distinctive patterns of IR innervation characterized the various specialized sensory systems, including 1) cochlear and vestibular hair cells; 2) lingual, palatal, oropharyngeal, and laryngoepiglottal taste buds; 3) main olfactory epithelium and axons projecting to glomeruli in specific sectors of main olfactory bulb; 4) septal-olfactory organ; 5) vomeronasal organ; and 6) the nervus terminalis system. Secretory epithelia (ciliary body, choroid plexus, and stria vascularis) were notably lacking in CGRP-IR. Despite the multiplicity of functionally distinct CGRP neuronal and axonal populations, certain generalizations merit consideration. The extensive innervation of chemosensory nasal and oral epithelia may contribute to specific chemical sensitivities (e.g., relating to olfactory and gustatory senses) as well as evoking "nociceptive" responses to chemical irritants as part of a "common chemical sense." An efferent role for some of these peptidergic afferent axons may also be inferred from their specific distributions. Sites involved in regulating access to and sensitivity of sense organs to external stimuli (e.g., cochlear and vestibular hair cells, taste bud orifices, and main olfactory epithelium) are heavily innervated. Other IR axons are in position to exert control over airflow through nasal turbinates, glandular secretion, blood circulation, and duct transport systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Photoaffinity labeling of thaumatin-binding protein in monkey circumvallate papillae

Thaumatin I is an intensely sweet-tasting protein. It was photo-crosslinked with taste papillae of crab-eating monkey by using a conjugated photo-affinity reagent [3H]azidobenzoylthaumatin I. Serial sections of SDS-polyacrylamide gel electrophoresis of the 0.1 M sodium phosphate buffer-soluble fraction from taste papillae had a large peak of radioactivity at the Mr region of approx. 70,000; fractions from non-taste papillae did not. Excess unlabeled thaumatin I reduced the photo-crosslinking at the 70 kDa region; acetylated thaumatin I (which is not sweet) did not. The results show that taste papillae of the monkey contain a protein of Mr approx. 50,000, which binds to thaumatin I (Mr 22,209) but not to completely acetylated thaumatin I. The possibility that the thaumatin-binding protein is a sweet receptor protein is discussed.

Age does not affect numbers of taste buds and papillae in adult rhesus monkeys

Taste buds and papillae in tongues of rhesus monkeys were examined and counted to determine if there are age-related differences in general morphology or numbers of receptor organs. Tongues from 15 monkeys in five groups aged 4-31 years were studied with light microscopy. Fungiform, circumvallate, and foliate papillae were examined and taste buds in each papilla type were counted. Numbers of papillae did not differ with age through 31 years; however, at 24 years and older, fungiform papillae were reduced in number in some animals that had lost tongue tips due to trauma. There were no age-related differences in numbers of taste buds in any of the three gustatory papilla types, nor did taste bud diameter alter with age. From data on each papilla type, estimates were made of total numbers of lingual taste buds. Totals ranged from about 8,000 to 10,000 and there were no age-related differences. These results support other recent reports that taste buds are not decreased in number in old rats or humans. Since taste bud numbers and general morphology are maintained even in old age, any age-related differences in taste behavior cannot be attributed to gross degenerative changes in lingual taste buds.

The nature of the substance P-containing nerve fibres in taste papillae of the rat tongue

The nature of the association of substance P (SP) with taste buds in the rat tongue was investigated by immunohistochemical and radioimmunoassay techniques. Both the circumvallate and fungiform papillae were found to receive a rich innervation by substance P-containing fibres. Although these fibres were closely associated with the taste buds in these structures, they assumed a perigemmal rather than an intragemmal location. Bilateral lesions of the glossopharyngeal nerve resulted in the depletion of taste buds from the vallate papilla and a large reduction in substance P immunoreactive fibres in this area. Lesions of the chorda tympani, which led to the degeneration of taste buds in fungiform papillae, had no effect on the immunohistochemical appearance of substance P in these papilla or on the substance P levels in the anterior part of the tongue. Lesions of the mandibular division of the trigeminal nerve or neonatal capsaicin treatment had no effect on the structural integrity of taste buds in fungiform papillae but led to the depletion of substance P-immunoreactive fibres from these papillae. Both of these procedures caused a 71% reduction in the substance P content of the anterior tongue, ipsilaterally after the nerve lesion and bilaterally after capsaicin treatment. The results are discussed in relation to the possible functional role of substance P-containing fibres within nerves supplying taste structures of the tongue.