Cutaneous taste buds in cod

The distribution of cutaneous taste buds was determined quantitatively in larvae, juveniles and young adults of cod, using scanning electron microscopy. Changes in these distributions associated with development were followed in laboratory reared fish. Taste buds were first seen on the snout and lips of cod at a total length of 8 mm, and on the barbel at a length of 22 mm. The highest taste bud densities were seen at a length of around 90 mm, and subsequently declined on the barbel and pelvic fins with further growth. In these late 0-group fish, mean taste bud densities over much of the head, e.g. throat, dentary and sides of the snout were <100 mm⁻². On the tip of the snout and the lips, mean densities were in the region of 350–400 mm⁻², while on projecting parts of the fish, especially the barbel, anterior naris flap and extremities of the fins, spot densities occasionally exceeded 1000 mm⁻² at some sites. Mean taste bud diameter increased rapidly from 2.23μ± 0.35 μm (S.D.) at a length of 22 mm to 7.19 ± 0.23 μm at 90 mm length, with a much slower increase to about 8 μm associated with a further doubling in body length. These changes indicate a phase of rapid proliferation and growth in size of cutaneous taste buds in the period preceding the adoption of a benthic habit in their first summer. The presence of high taste bud densities on the barbel and pelvic fins in particular appears to correlate with the known feeding behaviour of cod.     

Intragemmal spaces in taste buds

Summary Empty spaces are seen under both light and electron microscopes inside the taste buds of the dog lingual circumvallate papillae. They average 10 in diameter and 20 in length. Lacking endothelial lining, they are bordered directly by cell membranes of neighboring bud cells, and thus represent enlarged intercellular spaces. Intergemmal blood capillaries encircle the buds in close proximity to these intragemmal spaces. It is suggested that these spaces act as reservoirs for tissue fluid which may flow from them to the exterior via the intercellular spaces and the gustatory pores. This provides an effective mechanism whereby taste buds may be flushed of stimulating agents.Supported by Emory University Research Funds; Publication No. 650 of Division of Basic Health Sciences.     

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.     

Cutaneous taste buds in gadoid fishes

Cutaneous taste buds occurred on the head and fins in five species of juvenile gadoid fishes from the west of Scotland, but there were significant differences in their density between regions on the fish and between species for individual regions. The highest taste bud densities were recorded on the edge of the anterior naris flap, the barbel, pelvic fin rays, snout tip and upper lip. Cod Gadus morhua and poor cod Trisopterus minutus had significantly higher taste bud densities on the first two pelvic fin rays than the other species. This appears to correspond with their more benthic lifestyle, in which the pelvic fins are frequently trailed over the sea bed when searching for prey.     

The effects of beta-bungarotoxin on the morphogenesis of taste papillae and taste buds in the mouse

Although it has been long accepted that innervation by a tastenerve is essential for maintenance of taste buds, it is notclear what role, if any, innervation plays in the morphogenesis oftaste papillae and taste bud development. The following studywas undertaken to determine what effects lack of sensory innervationhave on the development of taste papillae and the formationof taste buds in the mouse. Timed-pregnant female mice (n =3) at gestational day 12 (gd12) were anesthetized and a 1 µlsolution (1 µg/µl) of ß-bungarotoxin (ß-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 atgd18, 1 day before birth, and four ß-BTX-injected embryos,two shams and two controls were fixed in buffered paraformaldehyde.Serial sections were examined for the presence and morphologyof taste papillae and taste buds. No nerve profiles were observedin ß-BTX-injected tongues. Although circumvallate papillaewere present on ß-BTX tongues, only five fungiform papillaecould be identified. Taste buds were present on a large percentageof fungiform papillae profiles (24% and on circumvallate papillaein sham and control fetuses; in contrast, no taste buds wereassociated with taste papillae in ß-BTX fetuses. Theseresults implicate a significant role for innervation in tastepapillae and taste bud morphogenesis.     

Apoptosis in mouse taste buds after denervation

Apoptotic cells in the taste buds of mouse circumvallate papillae after the sectioning of bilateral glossopharyngeal nerves were examined by the method of DNA nick-end labeling (TUNEL), together with standard electron microscopy. The taste buds decreased in number and size 3–11 days after denervation and disappeared at 11 days. The TUNEL method revealed only a few positively stained nuclei in normal taste buds but, in those of mice 1–5 days after denervation, the number of positive nuclei had increased to 3–5 times that of taste buds from normal mice. Electron-microscopic observation after denervation demonstrated taste bud cells containing condensed and fragmentary nuclei in a cytoplasm with increased density. The results show that taste bud cells under normal conditions die by apoptosis at the end of their life span, and that gustatory nerve sectioning causes apoptosis of taste bud cells with taste buds decreasing in number and ultimately disappearing. Received: 20 November 1995 / Accepted: 15 May 1996     

Membrane-bounded intratubular bodies in rat taste buds

The ultrastructural features of membrane-bounded bodies contained in the tubulo-vesicular system in the outer segment of taste bud cells are described. Each body showed a round, fusiform or oval shape, was surrounded by a trilaminar membrane and enclosed an electron dense matrix sometimes containing inclusions. These bodies were found at all ages studied. Similar structures were also found embedded in the material plugging the taste pore. Our finding suggest that these bodies could be secreted at the free surface of the cells and be involved in the concentration of divalent cations.     

Electron microscopy of taste buds of the rat

Summary The taste buds of the circumvallate papillae have been examined by electron microscopy in OsO₄-fixed, PTA stained material or after KMnO₄ fixation. The microvilli of the receptor cells have terminal dilatations which presumably give an increased surface area for transduction. The extracellular spaces at the necks of the receptor cells near the bases of the microvilli are interrupted by closed contacts.The synapses have a well defined synaptic cleft suggesting a chemical rather than an electrical mode of transmission. Synaptic membrane specialisations differ from the membrane thickenings of other types of synapse. Presynaptic dense projections are present but there is no well define postsynaptic thickening. Vesicles occur in both pre- and postsynaptic components, but it is debatable whether or not they should be termed synaptic vesicles. Acknowledgements. We are indebted to Professor J. Z. Young, F. R. S., for his stimulating support, and to Mr. S. Waterman for skilled photography.     

Development of the taste buds in rat embryogenesis

Electron microscopic studies have been made on the developing taste buds in fungiform and vallate papillae of prenatal rats. Three stages of differentiation of these buds are described. The first stage is characterized by presence of the nervous fibers in the connective tissue of the papillae and dense granules of various size, as well as dense-cored vesicles (500-700 A in diameter) in the basal parts of some epithelial cells at the top of the papillae (16-17th days of gestation). The second stage is characterized by nerve processes entering the epithelium and by formation of afferent synaptic contacts between the differentiating epithelial cells and the nervous fibers (19th day of gestation). At the third stage, the cluster of differentiating epithelial cells attains a form which is similar to mature taste buds (21-22nd days of gestation). Thus, to the birthday of rats, differentiation of the basal parts of the taste buds takes place, whereas the apical parts of the taste buds remain undeveloped and do not communicate with the oral cavity. Peculiarities of fine structure of differentiating epithelial cells at the three stages are discussed.     

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.     

Surface morphology of taste buds in catfish barbels

Summary 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.Supported by grants from the Medical Research Council of Canada and the Muscular Dystrophy Association of CanadaThe excellent technical assistance of Mr. F.T. McConnell is gratefully acknowledged     

A2BR adenosine receptor modulates sweet taste in circumvallate taste buds

In response to taste stimulation, taste buds release ATP, which activates ionotropic ATP receptors (P2X2/P2X3) on taste nerves as well as metabotropic (P2Y) purinergic receptors on taste bud cells. The action of the extracellular ATP is terminated by ectonucleotidases, ultimately generating adenosine, which itself can activate one or more G-protein coupled adenosine receptors: A1, A2A, A2B, and A3. Here we investigated the expression of adenosine receptors in mouse taste buds at both the nucleotide and protein expression levels. Of the adenosine receptors, only A2B receptor (A2BR) is expressed specifically in taste epithelia. Further, A2BR is expressed abundantly only in a subset of taste bud cells of posterior (circumvallate, foliate), but not anterior (fungiform, palate) taste fields in mice. Analysis of double-labeled tissue indicates that A2BR occurs on Type II taste bud cells that also express Gα14, which is present only in sweet-sensitive taste cells of the foliate and circumvallate papillae. Glossopharyngeal nerve recordings from A2BR knockout mice show significantly reduced responses to both sucrose and synthetic sweeteners, but normal responses to tastants representing other qualities. Thus, our study identified a novel regulator of sweet taste, the A2BR, which functions to potentiate sweet responses in posterior lingual taste fields.     

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

Summary Taste buds were found to stain strongly and selectively in intact papillae with highly acidic dyes such as ponceau S. In intact tongues the taste buds in the fungiform, circumvallate and foliate papillae of the cynomolgus monkey and in the fungiform papillae of the rat as well as the taste discs in the fungiform papillae of the frog could be visualized. This method enables a rapid location and counting of taste buds in taste papillae without preparing histological sections. In cynomolgus tongue material fixed in formalin, the dyes penetrate into the buds. In fresh tongues only the taste pore region of the buds stains, which suggests that in vivo taste buds are impenetrable underneath the pore.     

Mosaic analysis of the embryonic origin of taste buds

The embryonic origins of taste receptor cells have not beenestablished experimentally. Although related receptor cells(e.g. hair cells of the inner ear, lateral line receptors) areknown to arise from neurogenic ectoderm (e.g. neural crest orplacodes), taste buds are described as arising from local epithelialcells. Also unknown is whether or not each taste bud is a cloneof cells, i.e. arising from a single progenitor. To addressthese problems, mosaic and chimeric analyses of lingual epitheliumand taste buds have been undertaken. This paper describes thetheory of chimeric and mosaic cell lineage analyses, the advantagesand disadvantages, and the preliminary results obtained fromthe examination of the taste buds and lingual epithelium of:1) mosaic Xenopus, 2) chimeric mice and 3) X-inactivation mosaicmice.