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PubMed1994

Application of serial sectioning and three-dimensional reconstruction to the study of taste bud ultrastructure and organization.

The lingual taste buds of mammals are complex organs containing dozens of cells of varying morphology and numerous nerve fibers that are intermingled among the cellular processes. Some of the taste bud cells form synaptic contacts with these nerve fibers. Important questions remain to be answered regarding the structure and function of the cells of various types within taste buds and the means by which responses to gustatory stimuli are transmitted to the nerve fibers that communicate with the brain. Using both conventional and high voltage electron microscopy, we have examined serially sectioned taste buds from the tongues of mice and rabbits in order to address these issues and to obtain more complete information than that available from sampling of sections. The technique of computer-assisted 3-D reconstruction was used to generate models of whole taste buds and individual cellular and neural elements within taste buds from the serial sections. Analysis of serially sectioned taste buds from mice and rabbits has revealed that in both of these species relatively few (30% or less) of the cells within the taste buds form synaptic contacts with nerve fibers. In the foliate taste buds of rabbits, all of the cells that are presynaptic to nerve fibers are of a single morphological type (type III). The cells that are presynaptic to nerve fibers within the taste buds of mice are morphologically diverse. A pattern of synaptic connectivity exists within murine taste buds such that a given nerve fiber receives synaptic input only from taste cells that are ultrastructurally similar. In the taste buds of both mice and rabbits, we have observed both divergence and convergence of synaptic input from the putative taste receptor cells onto nerve fibers, suggesting that at the level of the taste bud there is some integration of the information generated by individual receptor cells. In addition to typical chemical synapses, other cytoplasmic specializations (such as subsurface cisternae and atypical mitochondria) may be involved in interactions between taste bud cells and nerve fibers.

Animals↗
PubMed1986

Variation in human fungiform taste bud densities among regions and subjects.

Taste sensitivity is known to vary among regions of the tongue and between subjects. The distribution of taste buds on the human tongue is examined in this report to determine if interregional and intersubject variation of taste bud density might account for some of the variation in human taste sensitivity. The subjects were ten males, aged 22-80 years, who died from acute trauma or an acute cardiovascular episode. Specimens were obtained as anatomical gifts or from autopsy. A sample of tissue about 1 cm2 was taken from the tongue tip and midlateral region; frozen sections were prepared for light microscopy; and serial sections were examined by light microscopy to count the taste buds. The average taste bud (tb) density on the tongue tip was 116 tb/cm2 with a range from 3.6 to 514 among subjects. The number of gustatory papillae on the tip averaged 24.5 papillae/cm2 with a range from 2.4 to 80. Taste bud density in the midregion averaged 25.2 tb/cm2 (range: 0-85.9), and the mean number of gustatory papillae was 8.25/cm2 (range: 0-28). The mean number of taste buds per papilla was 3.8 +/- 2.2 (s.d.) on the tip and 2.6 +/- 1.5 (s.d.) on the midregion. Subjects with the highest taste bud densities on the tip also had the highest densities in the midregion and the highest number of taste buds per papilla. Taste bud density was 4.6 times higher on the tip than the midregion, which probably accounts for some of the regional difference in taste sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1993

Persistence and calcium-dependent ATPase staining of denervated fungiform taste buds in the hamster.

Some fungiform taste buds in the hamster have been previously shown to persist for indefinite periods when deprived of their gustatory, chorda tympani (CT), innervation or both their CT and their trigeminal, lingual nerve, innervation (CT-L). The properties and numbers of persisting fungiform taste buds were examined 1 or 3 weeks after permanent CT or combined CT-L nerve cuts. The purpose was to reveal the status of taste buds at a time (3 weeks) when regenerating nerve fibres would normally be expected to reinnervate the epithelium. Denervated taste buds retain many normal characteristics including the pattern of histochemical staining for ectocalcium-dependent ATPase (Ca-ATPase). Taste-bud cells (including basal cells) have an intensely Ca-ATPase stained core surrounded by lightly stained peripheral cells. The Ca-ATPase stain was used to help identify and to define the size of the taste-bud core in denervated taste buds. Following CT-L or CT denervation most taste buds persisted; however the size of the taste-bud core was dramatically reduced. Fungiform taste buds differed in size based on their location in one of three tongue regions. The percentage decrease in size after denervation was also region specific and about the same for CT-L or CT cuts, suggesting that trigeminal fibres have no trophic effect on taste buds. However, trigeminal denervation caused a reduction in the number of persisting taste buds relative to CT denervation alone, which may be due to damage because of the loss of somatosensation.

Animals↗
PubMed1980

Ultrastructural and histochemical studies on the taste buds in some reptiles.

The taste buds in tortoises (Clemmys japonica and Geoclemys reevesii), lizards (Takydromus tachydromoides) and snakes (Elaphe quadrivirgata) were examined by both ultrastructural and histochemical methods. The taste buds consisted of at least three types of cells: the type I, II and III cells. The type I cells were characterized by the presence of secretory dense granules containing polysaccharides which were demonstrated by periodic acid-chromic acid-silver methenamine technique. The type II cells contained numerous tubular, vesicular and lamellated structures. The type III cells were characterized by dense cored vesicles and afferent synaptic contacts. Besides these cells, basally located cells which resembled the basal cells of other lower vertebrates were sometimes found in the tortoises. After administration of L-DOPA following nialamide, some taste bud cells of the tortoises, Clemmys japonica, showed weak yellowish green fluorescence by monoamine fluorescence histochemistry and the dense-cored vesicles in the type III cells increased in number. Acetylcholine esterase activity was not observed in tortoise taste buds. It is suggested that the three types of cells which compose the taste buds of the reptiles may correspond to the three types of cells in mammalian, and the type III cells represent the gustatory cells which are able to potentially produce biogenic monoamines. From these results, the taste buds of the reptiles may hold an intermediate position between those of mammals and amphibia or fishes.

5-Hydroxytryptophan↗
PubMed2005

Taste bud development in the channel catfish.

Taste bud formation in channel catfish is first seen to occur in stage 39 embryos, when taste bud primordia (stage 1), consisting of three to five cells, including a single calretinin-positive cell, can be recognized within the oropharyngeal cavity and maxillary barbels. Within a short time (stage 40), stage 2 taste bud primordia are apparent and include two or three calretinin-positive cells. The number of calretinin-positive cells continues to increase (stage 3), and the primordia begin to erupt as mature taste buds (stage 4) by embryonic stage 48. This same pattern of taste bud development characterizes other regions of the head, with calretinin-positive cells first detected around the mouth and on the other barbels by stage 41 and on the rest of the head by stage 48. The development of trunk taste buds lags far behind that of the head, with the first calretinin-positive cells occurring on the lobes of the caudal fin by stage 48 and on the remaining fins by stage 50. Taste bud primordia on the trunk proper do not begin to appear until stage 53, when the larvae begin to feed, and these receptors begin to erupt only in 1-week-old larvae. Fibers of the facial nerve, which innervate all external taste buds, ramify within the ectoderm prior to the first appearance of taste bud primordia or their precursors.

Animals↗
PubMed2001

Maintenance of rat taste buds in primary culture.

The differentiated taste bud is a complex end organ consisting of multiple cell types with various morphological, immunocytochemical and electrophysiological characteristics. Individual taste cells have a limited lifespan and are regularly replaced by a proliferative basal cell population. The specific factors contributing to the maintenance of a differentiated taste bud are largely unknown. Supporting isolated taste buds in culture would allow controlled investigation of factors relevant to taste bud survival. Here we describe the culture and maintenance of isolated rat taste buds at room temperature and at 37 degrees C. Differentiated taste buds can be sustained for up to 14 days at room temperature and for 3-4 days at 37 degrees C. Over these periods individual cells within the cultured buds maintain an elongated morphology. Further, the taste cells remain electrically excitable and retain various proteins indicative of a differentiated phenotype. Despite the apparent health of differentiated taste cells, cell division occurs for only a short period following plating, suggesting that proliferating cells in the taste bud are quickly affected by isolation and culture.

Animals↗
PubMed2004

A strong nerve dependence of sonic hedgehog expression in basal cells in mouse taste bud and an autonomous transcriptional control of genes in differentiated taste cells.

The nerve-dependency of gene expression in mouse taste bud was examined through an analysis of changes in gene expression in and around the taste buds in circumvallate papillae after surgery of cranial nerve IXth (glossopharyngeal nerve). The number of cells expressing T1r3, gustducin, Mash1 and Nkx2.2 gradually decreased after denervation. However, the expression intensity of these genes was barely influenced by denervation, and strong expression was observed at 6 days after denervation. In contrast, the basal cell-specific Sonic hedgehog (Shh) expression in the taste buds was decreased markedly at 6 h after denervation. In the regeneration process of taste buds, Shh expression was observed during a very early phase before taste bud formation. These results indicate the autonomous transcriptional control of genes in differentiated taste cells and the strong nerve-dependency of Shh expression in basal cells. Furthermore, in order to reveal the mitotic activity of Shh-expressing cells in taste buds, the BrdU-labeling experiments were performed using a combination of BrdU-immunohistochemistry and in situ hybridization. BrdU-signal was very rarely observed in Shh-expressing cells immediately after BrdU injection, and the signals were noted mainly in Ptc-expressing cells. BrdU signals rapidly increased in Shh-expressing cells in following 12 h and began to decrease after 2 days post-injection. These results suggest that most Shh-expressing cells are not mitotically active, but that Shh-expressing cells may be in the early transient developmental state of taste cells in taste buds.

Animals↗
PubMed1995

Molecular cloning of Ebnerin, a von Ebner's gland protein associated with taste buds.

Salivary secretions modulate taste perception. Taste buds in the circumvallate and foliate papillae are bathed in secretions of unique lingual salivary glands, von Ebner's glands (VEG). We have identified a rat cDNA encoding a novel protein of 1290 amino acids, Ebnerin, that is specifically expressed in VEG and released onto the tongue surface along the apical region of taste buds in the clefts of circumvallate papillae. Ebnerin possesses a putative single transmembrane domain at the C terminus with 17 amino acids in the cytoplasmic area. The extracellular region of Ebnerin contains a number of repeated domains with homology to the scavenger receptor cysteine-rich domain and to a repeated domain of bone morphogenetic protein-I and other related proteins. Western blot analysis reveals that Ebnerin exists in particulate and soluble forms in VEG and is present in secretions from VEG. In situ hybridization and immunohistochemistry demonstrate that Ebnerin is located in secretory duct epithelial cells of VEG and is released onto the tongue surface along the apical region of taste buds in the clefts of circumvallate papillae. The unique structure and localization of Ebnerin suggest that it may function as a binding protein in saliva for the regulation of taste sensation.

Amino Acid Sequence↗
PubMed1996

Embryonic and early fetal development of human taste buds: a transmission electron microscopical study.

BACKGROUND: Taste buds are assemblies of slender epithelial cells that receive chemical stimuli from the outer (oral) environment. In contrast to the large and well documented information on the morphology of taste buds in adult humans and animals, there are only a few reports on fetal ones, and ultrastructural studies of prenatal human taste buds are lacking completely. Therefore, the present investigation has been carried out to study the taste bud primordium, its morphological changes including synaptogenesis, cell differentiation, and taste pore formation from the time of the onset of taste bud formation around the 8th week until the 15th postovulatory week. METHODS: Taste bud primordia of 42 human embryonic/fetal tongues have been examined by means of transmission electron microscopy. RESULTS: Nerve fibers approach the lingual epithelium between the 6th and 7th postovulatory week. They penetrate the basal lamina during the 8th week and form synapses with poorly differentiated, elongated, epithelial cells. By the 12th week, more differentiated cell types are seen: 1) electron-dense cells resembling type III cells of the adult taste bud containing large numbers of dense-cored vesicles (80-150 nm in diameter); 2) electron-dark cells with well developed endoplasmic reticulum and many apical mitochondria, being candidates for type II cells. Basally, these cells have foot-like processes containing dense-cored vesicles (120-200 nm in diameter), but they do not synapse to nerve fibers. Type I cells, characterized by apically located dense secretory granules, are not observed. First shallow grooves above the taste bud primordium are found around the 10th week. Untypically differentiated apical cellular processes extend onto the surface. Most of the taste pores develop around the 14th to 15th week. In the taste pit, mucous material is not present during the first 15 weeks of gestation. Synapses between cells and afferent nerve fibers were found by the 8th week, reaching a maximum around the 12th to 13th week. CONCLUSIONS: The early presence of taste bud cells containing dense-cored vesicles suggests an at least dual function of embryonic/ fetal taste buds: First, from the 8th until the 14th week, non-gustatory, paracrine functions should be considered. After the 14th week of gestation, when typical taste pores are present, the taste buds possibly start their gustatory function. Differentiated marginal cells are possibly involved in the formation of the taste pore. The lack of type I cells producing the mucous material in the taste pit indicates that the taste bud has not achieved a fully developed function until the 15th week of gestation.

Aging↗
PubMed1977

Surface morphology of taste buds in catfish barbels.

External taste buds abound on barbels of the adult catfish Corydoras arcuatus. When examined by scanning electron microscopy, they are visualized as a series of punctate, conical elevations projecting from the general surface epithelium. All taste buds were found to be of one type. Both their external and internal surface features could be clearly elucidated on intact barbels and in barbels fractured transversely at various positions along their length. An extensive nerve terminal network penetrates the base of each taste bud. Two populations of elongated cells bearing prominent microvilli project through the central pore at the tip of each bud. One set of microvilli is thicker, longer and more club-shaped than its counterpart. While both are randomly distributed within each central pore, the small, short microvilli appear to outnumber the larger ones. A third population of cells, devoid of any apical microvilli, was also seen in some of the taste buds examined internally. These cells do not project to the external surface and are interpreted as "basal" cells described in previous light and transmission electron microscope studies of taste buds in other vertebrate species. The functional significance of some of these morphological findings is discussed.

Animals↗
PubMed1994

Fine structure of taste buds located on the lamb epiglottis.

BACKGROUND: Taste buds located on the aryepiglottal folds and laryngeal surface of the epiglottis are the principal receptors responsible for the initiation of the laryngeal chemoreflex. In contrast to the wealth of information available concerning the ultrastructure of oral taste buds, little comparable data exists for taste buds located at the entrance to the larynx. Therefore, the present study was designed to investigate the fine structure of taste buds located on the lamb epiglottis. MATERIALS: Stained thick and semi-serial thin sections from taste buds located on the lamb epiglottis were examined with light and electron microscopy. RESULTS: Based on morphological criteria, three types of cells could be identified in the taste bud: Type I, Type II, and basal cells. Both Type I and Type II cells extended into the apical taste pore, but there were differences between these two cell types with regard to nuclear profiles, electron density, and the relative density of ribosomes, apical mitochondria, and rough and smooth endoplasmic reticulum. Basal cells did not extend a process into the taste pore. Nerve processes were observed throughout the taste bud. Synapses were observed between both Type I and Type II cells and nerve fibers. These synapses exhibited membrane thickenings and accumulations of clear and dense-cored vesicles of varying proportions in the taste cell cytoplasm adjacent to membrane specializations. CONCLUSIONS: The taste buds located on the lamb epiglottis share several structural similarities to taste buds located in the oral cavity and other regions of the pharynx and larynx of many mammalian species. The presence of synapses on both Type I and Type II cells of the lamb epiglottal taste bud suggests that both cell types are involved in laryngeal chemoreception.

Animals↗
PubMed1985

Distribution of calmodulin in taste buds.

Calmodulin is higher in particulate fractions from bovine taste buds containing taste bud membranes which specifically bind sweet tastants compared to corresponding fractions from control non-taste bud bearing lingual epithelial tissue. As biochemical purity (i.e., membrane enzyme marker activity) of these membrane enriched fractions increased (P4B greater than P3B greater than P2B) calmodulin correspondingly increased (P4B greater than P3B greater than P2B); these increases also correlated with increased membrane purity as demonstrated by electron microscopy. All PB subfractions from taste buds contained a greater membrane concentration than those from PD subfractions and calmodulin was significantly increased in each corresponding subfraction. The presence of calmodulin in taste bud membranes, its correlation with membrane purification and reports that numerous drugs which induce taste loss are potent inhibitors of calmodulin suggest a role for calmodulin in taste function.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1988

Intermediate filaments in mouse taste bud cells.

The intermediate filaments in mouse taste bud cells were studied by immunocytochemistry using antikeratin antibodies, and by conventional electron microscopy. Taste bud cells (types I, II, and III) possessed less densely aggregated bundles of intermediate filaments than the surrounding epithelial cells. Type III cells, however, contained more densely aggregated bundles than type I or II cells. Basal cells in the taste buds showed aggregations of filaments as dense as those seen in the epithelial cells, although their bundles were more slender than those of the epithelial cells. The antibodies to keratins from the bovine muzzle and human stratum corneum stained all types of the taste bud cells as well as the surrounding epithelial cells. PKK2 antibody reacted with the surrounding epithelial cells, but did not react with the taste bud cells. These results show that keratins are present in both taste bud and surrounding epithelial cells, although the keratin subtype differs between those cells. This finding has led us to the supposition that all cell types comprising the taste buds--including type III (receptor) cells--originate from the epithelial cells surrounding the taste buds. It is also suggested that both keratin subtypes and aggregation patterns of intermediate filament bundles change during differentiation from surrounding epithelial cells to taste bud cells, and from basal cells in the taste buds to types I, II, or III cells.

Animals↗
PubMed2002

Taste bud development in the zebrafish, Danio rerio.

Taste buds are chemosensory endorgans consisting of modified epithelial cells. Fish and other vertebrates use their taste bud cells to sample potential food, either selecting or rejecting substances according to their edibility. The adult gustatory system in fish has been studied thoroughly, including regeneration experiments. Taste buds occur in the epithelia of the lips, the mouth cavity, the oropharyngeal cavity, and also in the skin of the barbels, the head, and sometimes even all over the body surface. Despite its importance for feeding, little is known about the ontogeny of the fish taste system. We examined the development of taste buds in the zebrafish on the light microscopical and the scanning and transmission electron microscopical levels. Taste buds develop later than the olfactory organ and the solitary chemosensory cells, two other chemosensory systems in aquatic vertebrates. The first few taste bud primordia are visible within the epithelia of lips and gill arches 3 to 4 days after fertilization, and the first few taste buds with open receptor areas appear on the lips and simultaneously on the gill arches 4-5 days after fertilization, which coincides with the onset of feeding. Taste buds in the mouth cavity, on the head, and on the barbels are formed later in development. As seen in other fish, zebrafish taste buds contain elongate dark and light cells, termed according to their electron density. Dark cells with a cell apex of many short microvilli appear first, followed by the light cells with one large microvillus. In addition, the zebrafish has a third fusiform cell type, which appears last. This cell type is low in electron density and has a brush-like apical ending with several small microvilli. This cell type has not been described previously. Furthermore, in zebrafish, the ontogenetic processes of taste bud formation differ from regenerative processes described in the literature.

Animals↗
PubMed2001

Shh and Ptc are associated with taste bud maintenance in the adult mouse.

In mammals, taste receptor cells are organized into taste buds on tongue. Taste buds are trophically maintained by taste neurons and under continuous renewal, even in adults. We found that the receptor for Sonic hedgehog (Shh), Patched1 (Ptc), was expressed around taste buds where cells were proliferating, and that Shh was expressed within basal cells of taste buds. Denervation caused the loss of Shh and Ptc expression before the degeneration of taste buds.

Animals↗
PubMed2004

Acid-sensitive two-pore domain potassium (K2P) channels in mouse taste buds.

Sour (acid) taste is postulated to result from intracellular acidification that modulates one or more acid-sensitive ion channels in taste receptor cells. The identity of such channel(s) remains uncertain. Potassium channels, by regulating the excitability of taste cells, are candidates for acid transducers. Several 2-pore domain potassium leak conductance channels (K(2)P family) are sensitive to intracellular acidification. We examined their expression in mouse vallate and foliate taste buds using RT-PCR, and detected TWIK-1 and -2, TREK-1 and -2, and TASK-1. Of these, TWIK-1 and TASK-1 were preferentially expressed in taste cells relative to surrounding nonsensory epithelium. The related TRESK channel was not detected, whereas the acid-insensitive TASK-2 was. Using confocal imaging with pH-, Ca(2+)-, and voltage-sensitive dyes, we tested pharmacological agents that are diagnostic for these channels. Riluzole (500 microM), selective for TREK-1 and -2 channels, enhanced acid taste responses. In contrast, halothane (< or = approximately 17 mM), which acts on TREK-1 and TASK-1 channels, blocked acid taste responses. Agents diagnostic for other 2-pore domain and voltage-gated potassium channels (anandamide, 10 microM; Gd(3+), 1 mM; arachidonic acid, 100 microM; quinidine, 200 microM; quinine, 100 mM; 4-AP, 10 mM; and TEA, 1 mM) did not affect acid responses. The expression of 2-pore domain channels and our pharmacological characterization suggest that a matrix of ion channels, including one or more acid-sensitive 2-pore domain K channels, could play a role in sour taste transduction. However, our results do not unambiguously identify any one channel as the acid taste transducer.

Animals↗
PubMed1994

Electrophysiological and morphological properties of light and dark cells isolated from mudpuppy taste buds.

Isolated Necturus taste receptor cells were studied by giga-seal whole-cell recording and electron microscopy to correlate electrophysiological properties with taste cell structural features. Dark (type I) cells were identified by the presence of dense granular packets in the supranuclear and apical regions of the cytoplasm. In response to a series of depolarizing voltage commands from a holding potential of -80 mV, these cells exhibited a transient, TTX-sensitive inward Na+ current, a sustained outward K+ current, and a slowly inactivating inward Ca++ current. Light (type II) cells were identified by a lack of granular packets and by an abundance of smooth endoplasmic reticulum distributed throughout the cell. In addition, isolated light cells had clear vesicular inclusions in the cytoplasm and blebs on the plasma membrane. Light cells were divided into two functional populations based upon electrophysiological criteria: cells with inward and outward currents, and cells with outward currents only. Light cells with inward and outward currents had voltage-activated Na+, K+, and Ca++ currents with properties similar to those of dark cells. In contrast, the second group of light cells had only voltage-activated outward K+ currents in response to depolarizing voltage commands. These data suggest that dark cells and light cells with inward and outward currents are capable of generating action potentials and releasing neurotransmitters onto gustatory afferent neurons in response to taste stimulation. In contrast, light cells with outward currents only likely serve a different function in the taste bud.

Animals↗
PubMed1994

Mosaic analysis of the embryonic origin of taste buds.

The embryonic origins of taste receptor cells have not been established experimentally. Although related receptor cells (e.g. hair cells of the inner ear, lateral line receptors) are known to arise from neurogenic ectoderm (e.g. neural crest or placodes), taste buds are described as arising from local epithelial cells. Also unknown is whether or not each taste bud is a clone of cells, i.e. arising from a single progenitor. To address these problems, mosaic and chimeric analyses of lingual epithelium and taste buds have been undertaken. This paper describes the theory of chimeric and mosaic cell lineage analyses, the advantages and disadvantages, and the preliminary results obtained from the examination of the taste buds and lingual epithelium of: 1) mosaic Xenopus, 2) chimeric mice and 3) X-inactivation mosaic mice.

Animals↗