Developmental Plasticity of Patterned and Regenerating Oral Organs

In many aquatic vertebrates, including bony and cartilaginous fishes, teeth and taste buds colocalize on jaw elements. In these animals, taste buds are renewed continuously throughout life, whereas teeth undergo cycled whole-organ replacement by various means. Recently, studies of cichlid fishes have yielded new insights into the development and regeneration of these dental and sensory oral organs. Tooth and taste bud densities covary positively across species with different feeding strategies, controlled by common regions of the genome and integrated molecular signals. Developing teeth and taste buds share a bipotent epithelium during early patterning stages, from which dental and taste fields are specified. Moreover, these organs share a common epithelial ribbon that supports label-retaining cells during later stages of regeneration. During both patterning and regeneration stages, dental organs can be converted to taste bud fate by manipulation of BMP signaling. These observations highlight a surprising long-term plasticity between dental and sensory organ types. Here, we review these findings and discuss the implications of developmental plasticity that spans the continuum of craniofacial organ patterning and regeneration.

Multiple Shh signaling centers participate in fungiform papilla and taste bud formation and maintenance

The adult fungiform taste papilla is a complex of specialized cell types residing in the stratified squamous tongue epithelium. This unique sensory organ includes taste buds, papilla epithelium and lateral walls that extend into underlying connective tissue to surround a core of lamina propria cells. Fungiform papillae must contain long-lived, sustaining or stem cells and short-lived, maintaining or transit amplifying cells that support the papilla and specialized taste buds. Shh signaling has established roles in supporting fungiform induction, development and patterning. However, for a full understanding of how Shh transduced signals act in tongue, papilla and taste bud formation and maintenance, it is necessary to know where and when the Shh ligand and pathway components are positioned. We used immunostaining, in situ hybridization and mouse reporter strains for Shh, Ptch1, Gli1 and Gli2-expression and proliferation markers to identify cells that participate in hedgehog signaling. Whereas there is a progressive restriction in location of Shh ligand-expressing cells, from placode and apical papilla cells to taste bud cells only, a surrounding population of Ptch1 and Gli1 responding cells is maintained in signaling centers throughout papilla and taste bud development and differentiation. The Shh signaling targets are in regions of active cell proliferation. Using genetic-inducible lineage tracing for Gli1-expression, we found that Shh-responding cells contribute not only to maintenance of filiform and fungiform papillae, but also to taste buds. A requirement for normal Shh signaling in fungiform papilla, taste bud and filiform papilla maintenance was shown by Gli2 constitutive activation. We identified proliferation niches where Shh signaling is active and suggest that epithelial and mesenchymal compartments harbor potential stem and/or progenitor cell zones. In all, we report a set of hedgehog signaling centers that regulate development and maintenance of taste organs, the fungiform papilla and taste bud, and surrounding lingual cells. Shh signaling has roles in forming and maintaining fungiform papillae and taste buds, most likely via stage-specific autocrine and/or paracrine mechanisms, and by engaging epithelial/mesenchymal interactions.

Separate and distinctive roles for Wnt5a in tongue, lingual tissue and taste papilla development

Although canonical Wnt signaling is known to regulate taste papilla induction and numbers, roles for noncanonical Wnt pathways in tongue and taste papilla development have not been explored. With mutant mice and whole tongue organ cultures we demonstrate that Wnt5a protein and message are within anterior tongue mesenchyme across embryo stages from the initiation of tongue formation, through papilla placode appearance and taste papilla development. The Wnt5a mutant tongue is severely shortened, with an ankyloglossia, and lingual mesenchyme is disorganized. However, fungiform papilla morphology, number and innervation are preserved, as is expression of the papilla marker, Shh. These data demonstrate that the genetic regulation for tongue size and shape can be separated from that directing lingual papilla development. Preserved number of papillae in a shortened tongue results in an increased density of fungiform papillae in the mutant tongues. In tongue organ cultures, exogenous Wnt5a profoundly suppresses papilla formation and simultaneously decreases canonical Wnt signaling as measured by the TOPGAL reporter. These findings suggest that Wnt5a antagonizes canonical Wnt signaling to dictate papilla number and spacing. In all, distinctive roles for Wnt5a in tongue size, fungiform papilla patterning and development are shown and a necessary balance between non-canonical and canonical Wnt paths in regulating tongue growth and fungiform papillae is proposed in a model, through the Ror2 receptor.

Factors that regulate embryonic gustatory development

Numerous molecular factors orchestrate the development of the peripheral taste system. The unique anatomy/function of the taste system makes this system ideal for understanding the mechanisms by which these factors function; yet the taste system is underutilized for this role. This review focuses on some of the many factors that are known to regulate gustatory development, and discusses a few topics where more work is needed. Some attention is given to factors that regulate epibranchial placode formation, since gustatory neurons are thought to be primarily derived from this region. Epibranchial placodes appear to arise from a pan-placodal region and a number of regulatory factors control the differentiation of individual placodes. Gustatory neuron differentiation is regulated by a series of transcription factors and perhaps bone morphongenic proteins (BMP). As neurons differentiate, they also proliferate such that their numbers exceed those in the adult, and this is followed by developmental death. Some of these cell-cycling events are regulated by neurotrophins. After gustatory neurons become post-mitotic, axon outgrowth occurs. Axons are guided by multiple chemoattractive and chemorepulsive factors, including semaphorins, to the tongue epithelium. Brain derived neurotrophic factor (BDNF), functions as a targeting factor in the final stages of axon guidance and is required for gustatory axons to find and innervate taste epithelium. Numerous factors are involved in the development of gustatory papillae including Sox-2, Sonic hedge hog and Wnt-β-catenin signaling. It is likely that just as many factors regulate taste bud differentiation; however, these factors have not yet been identified. Studies examining the molecular factors that regulate terminal field formation in the nucleus of the solitary tract are also lacking. However, it is possible that some of the factors that regulate geniculate ganglion development, outgrowth, guidance and targeting of peripheral axons may have the same functions in the gustatory CNS.

Structural diversification of the gustatory organs during metamorphosis in the alpine newt Triturus alpestris

Gustatory organs of the taste bud type occur in the epithelial lining of the oropharyngeal cavity of alpine newt larvae. They resemble the taste buds of bony fish, both in appearance (as revealed by scanning electron microscopy) and in detailed internal structure (seen on transmission electron micropscopy). During metamorphosis, at stage 55 of development, the secondary tongue (i.e. the soft tongue) is well formed and the anlages of taste discs are clearly apparent. Somewhat later, taste discs also appear in the epithelial lining outside the tongue, paralleling the disappearance of the taste buds. Well-developed taste discs of the newt differ from taste buds mainly by their structurally diversified set of 'associate cells' (mucous, wing and glial cells), which have no synaptic contact with nerve fibres. These cells accompany the neurosensory cellular components of the taste disc, i.e. the taste receptor cells and basal cells. This indicates that gustatory organs in metamorphosed newts, regardless of their small dimensions, fulfil the criteria established for taste discs previously defined in other Caudata and Anura species. Therefore, in the development of the newt there are two subsequent types of gustatory organs and two generations of the tongue: primary, in the larvae, and secondary, in metamorphosed animals.

Embryonic origin of gustatory cranial sensory neurons

Cranial nerves VII, IX and X provide both gustatory (taste) and non-gustatory (touch, pain, temperature) innervation to the oral cavity of vertebrates. Gustatory neurons innervate taste buds and project centrally to the rostral nucleus of the solitary tract (NTS), whereas neurons providing general epithelial innervation to the oropharynx project to non-gustatory hindbrain regions, i.e., spinal trigeminal nucleus. In addition to this dichotomy in function, cranial ganglia VII, IX and X have dual embryonic origins, comprising sensory neurons derived from both cranial neural crest and epibranchial placodes. We used a fate mapping approach to test the hypothesis that epibranchial placodes give rise to gustatory neurons, whereas the neural crest generates non-gustatory cells. Placodal ectoderm or neural crest was grafted from Green Fluorescent Protein (GFP) expressing salamander embryos into unlabeled hosts, allowing us to discern the postembryonic central and peripheral projections of each embryonic neuronal population. Neurites that innervate taste buds are exclusively placodal in origin, and their central processes project to the NTS, consistent with a gustatory fate. In contrast, neural crest-derived neurons do not innervate taste buds; instead, neurites of these sensory neurons terminate as free nerve endings within the oral epithelium. Further, the majority of centrally directed fibers of neural crest neurons terminate outside the NTS, in regions that receive general epithelial afferents. Our data provide empirical evidence that embryonic origin dictates mature neuron function within cranial sensory ganglia: specifically, gustatory neurons derive from epibranchial placodes, whereas neural crest-derived neurons provide general epithelial innervation to the oral cavity.

Development of fungiform papillae: patterned lingual gustatory organs

The fungiform papilla is a gustatory organ that provides a specific tissue residence for taste buds on the anterior tongue. Thus, during development there must be a progressive differentiation to acquire papilla epithelium, then taste cell progenitor epithelium, and finally taste cells within the papilla apex. Arranged in rows, the patterned distribution of fungiform papillae requires molecular regulation not only to induce papillae, but also to suppress papilla formation in the between-papilla tissue. Intact sensory innervation is not required to initiate papilla development or pattern. However, members of several molecular families have now been identified with specific localization in developing papillae. These may participate in papilla development and pattern formation, and subsequently in taste progenitor and taste cell differentiation. This review focuses on development of fungiform papillae in embryonic rat and mouse. Basic morphology, cell biology and molecular phenotypes of developing papillae are reviewed. Regulatory roles for molecules in several families are presented, and a broad schema is proposed for progressive epithelial differentiation to form taste cell progenitors in parallel with the temporal course, and participation of lingual sensory innervation.

Immunohistochemical localization of aromatic l-amino acid decarboxylase in mouse taste buds and developing taste papillae

Aromatic L-amino acid decarboxylase (AADC) catalyses the decarboxylation of all aromatic L-amino acids. In mammals, AADC is expressed in many tissues besides the nervous system, and is associated with additional regulatory roles of dopamine and serotonin in a wide range of tissues. We examined the expression of AADC by using reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. RT-PCR analysis showed that mRNA of AADC was detected in the taste bud-containing epithelium of the circumvallate papilla of mice. By immunohistochemical analyses, AADC was detected in a subset of taste bud cells of fungiform, foliate, and circumvallate papillae. Double-label studies showed that AADC colocalized with serotonin, NCAM, PLCbeta2, and PGP9.5. On the other hand, AADC never colocalized with alpha-gustducin. Our results of double staining with AADC and taste cell markers indicate that only the type III cells could convert 5-hydroxytryptophan (5-HTP) to serotonin within taste buds. Taken together with previous studies, the properties of the type III cell of taste buds exactly fit into the APUD (amine and amine precursor uptake and decarboxylation) cell scheme. Furthermore, in the developing circumvallate papilla, AADC are first detected in a small number of papillary epithelial cells at E14.5. By E18.5, AADC-positive epithelial cells also express PGP9.5, which is one of marker of taste cells, and these cells have been contacted by developing nerve fibers. These results suggest that AADC expression begins at early stages of taste bud cell differentiation, and biogenic amines may act on taste bud differentiation of tongue epithelial cells, and further may regulate innervation of taste bud progenitor cells.

Sox2 is required for development of taste bud sensory cells

Sox2 is expressed in basal epithelial cells of the tongue, with high levels in taste bud placodes, fungiform papillae, and mature taste cells, and low levels in filiform papillae. High Sox2 expression appears to lie downstream from canonical Wnt signaling. In hypomorphic Sox2(EGFP/LP) embryos, placodes form but no mature taste buds develop. In contrast, transgenic overexpression of Sox2 in the basal cells inhibits differentiation of filiform keratinocytes. Together, our loss-of-function and gain-of-function studies suggest that Sox2 functions in a dose-dependent manner to regulate the differentiation of endodermal progenitor cells of the tongue into taste bud sensory cells versus keratinocytes.

Angiogenesis and developmental expression of vascular endothelial growth factor in rat lingual papillae

We used an embryological approach to investigate development and microvasculature of lingual papillae, and expression of vascular endothelial growth factor (VEGF) in the rat tongue. Temporal changes in the rat tongue at each developmental stage from embryonic day 13 (E13) to postnatal day 7 (P7) were observed by intravascular injection of India ink and immunohistochemistry using a VEGF antibody. At E13, the primordium of circumvallate papilla was observed among various lingual papillae. VEGF was widely expressed at E16 on the proliferated epithelium and the connective tissue core of circumvallate papilla. Invasion by capillary sprouts forming the lingual papillae was observed at E17. The primordium of fungiform papillae was observed at E14. VEGF was strongly expressed around the basal cells of proliferated epithelial tissues of fungiform papillae at E17. At E18, blind-ended capillary sprouts invaded into connective tissue cores from subepithelial sinusoidal capillaries by sprout angiogenesis. At P1, the invading capillary sprouts formed loops by vascular remodeling. The primordium of foliate papillae was observed at E16. VEGF was slightly expressed, but uniformly at E17 on the epithelium, muscle cells, and fibroblasts of foliate papillae. At E18, vascular density was increased by angiogenesis. The primordium of filiform papillae was observed at E17. It was the last to develop among the lingual papillae. VEGF was expressed in the cytoplasm of grown epithelial cells of filiform papillae at E19, and in blind-ended capillary sprouts formed by angiogenesis in the connective tissue cores at E20. The capillary sprouts formed loops by vascular remodeling at P1. Consequently, VEGF was expressed on the papillary epithelium and connective tissue cores of papillae during development of the papillary epithelium, and invasion by capillary sprouts into each papillae was observed thereafter. These results suggest a close relationship between expression of VEGF and angiogenesis of lingual papillae in the rat.

Wnt-beta-catenin signaling initiates taste papilla development

Fungiform taste papillae form a regular array on the dorsal tongue. Taste buds arise from papilla epithelium and, unusually for epithelial derivatives, synapse with neurons, release neurotransmitters and generate receptor and action potentials. Despite the importance of taste as one of our five senses, genetic analyses of taste papilla and bud development are lacking. We demonstrate that Wnt-beta-catenin signaling is activated in developing fungiform placodes and taste bud cells. A dominant stabilizing mutation of epithelial beta-catenin causes massive overproduction of enlarged fungiform papillae and taste buds. Likewise, genetic deletion of epithelial beta-catenin or inhibition of Wnt-beta-catenin signaling by ectopic dickkopf1 (Dkk1) blocks initiation of fungiform papilla morphogenesis. Ectopic papillae are innervated in the stabilizing beta-catenin mutant, whereas ectopic Dkk1 causes absence of lingual epithelial innervation. Thus, Wnt-beta-catenin signaling is critical for fungiform papilla and taste bud development. Altered regulation of this pathway may underlie evolutionary changes in taste papilla patterning.

Refinement of innervation accuracy following initial targeting of peripheral gustatory fibers

During development, axons of the chorda tympani nerve navigate to fungiform papillae where they penetrate the lingual epithelium, forming a neural bud. It is not known whether or not all chorda tympani axons initially innervate fungiform papillae correctly or if mistakes are made. Using a novel approach, we quantified the accuracy with which gustatory fibers successfully innervate fungiform papillae. Immediately following initial targeting (E14.5), innervation was found to be incredibly accurate: specifically, 94% of the fungiform papillae on the tongue are innervated. A mean of five papillae per tongue were uninnervated at E14.5, and the lingual tongue surface was innervated in 17 places that lack fungiform papillae. To determine if these initial errors in papillae innervation were later refined, innervation accuracy was quantified at E16.5 and E18.5. By E16.5 only two papillae per tongue remained uninnervated. Innervation to inappropriate regions was also removed, but not until later, between E16.5 and E18.5 of development. Therefore, even though gustatory fibers initially innervate fungiform papillae accurately, some errors in targeting do occur that are then refined during later embryonic periods. It is likely that trophic interactions between gustatory neurons and developing taste epithelium allow appropriate connections to be maintained and inappropriate ones to be eliminated.

Expression of tryptophan hydroxylase in developing mouse taste papillae

Gustatory papillae and associated taste buds receive and process chemical information from the environment. In mammals, their development takes place during the late phase of embryogenesis. However, the cellular factors that regulate the differentiation of taste papillae remain largely unknown. Here, we show by quantitative real time RT-PCR that both isoforms of tryptophan hydroxylase (TPH1 and TPH2), the first and rate limiting enzyme of serotonin (5-HT) synthesis, are expressed in developing circumvallate papillae. Immuno-staining experiments further indicated that TPH is localized both in gustatory fibers and in differentiated taste receptor cells. These results point to the synthesis of 5-HT in gustatory papillae, and allow one to hypothesize that the development of taste buds might be modulated by serotonin.

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

Chick taste bud (gemmal) primordia normally appear on embryonic day (E) 16 and incipient immature, spherical-shaped buds at E17. In ovo injection of beta-bungarotoxin at E12 resulted in a complete absence of taste buds in lower beak and palatal epithelium at developmental ages E17 and E21. However, putative gemmal primordia (solitary clear cells; small, cell groupings) remained, lying adjacent to salivary gland duct openings as seen in normal chick gemmal development. Oral epithelium was immunonegative to neural cell adhesion molecule (NCAM) suggesting gemmal primordia are nerve-independent. Some NCAM immunoreactivity was evident in autonomic ganglion-like cells and nerve fibers in connective tissue. After unilateral geniculate ganglion/otocyst excision on E2.5, at developmental ages E18 and posthatching day 1, approximately 12% of surviving ipsilateral geniculate ganglion cells sustained approximately 54% of the unoperated gemmal counts. After E18, proportional stages of differentiation in surviving developing buds probably reflect their degree of innervation, as well as rate of differentiation. Irrespective of the degree of geniculate ganglion damage, the proportion of surviving buds can be sustained at the same differentiated bud stage as on the unoperated side, or may differentiate to a later bud stage, consistent with the thesis that bud maturation, maintenance, and survival are nerve-dependent.

Association of Shh and Ptc with keratin localization in the initiation of the formation of circumvallate papilla and von Ebner's gland

The development of gustatory papillae in mammalian embryos requires the coordination of a series of morphological events, such as proliferation, differentiation and innervation. In mice, the circumvallate papilla (CVP) is a specialized structure that develops in a characteristic spatial and temporal pattern in the posterior region of the tongue dorsal surface. The distinct expression patterns of Shh and Ptc, which play important roles in the development of other epithelial appendages, have been localized in the trench wall that gives rise to von Ebner's gland (VEG). To define the cellular mechanisms responsible for morphogenesis and differentiation during early development of CVP and VEG, the localization patterns of keratins (cytokeratins) K7, K8, K18, K19, K14 and connexin-43, which are dependent on Shh expression in other developmental systems, have been examined in detail. The distinct localization of keratins K7, K8, K18, K19, K14 and connexin-43 in the epithelium giving rise to the CVP and VEG suggests that cytodifferentiation is established prior to morphological changes. Interestingly, the localization of proliferating cell nuclear antigen, a marker for cell proliferation, is similar to that of Shh. An understanding of the regulatory roles of cell-cell interactions and signalling molecules in orchestrating a mutual network will bring us nearer to defining the molecular and cellular mechanisms underlying morphogenesis in mammalian taste bud development.

Modularity and sense organs in the blind cavefish, Astyanax mexicanus

Mexican tetra (Astyanax mexicanus) exist as two morphs: a sighted (surface) form and a blind (cavefish) form. In the cavefish, some modules are lost, such as the eye and pigment modules, whereas others are expanded, such as the taste bud and cranial neuromast modules. We suggest that modularity can be viewed as being nested in a manner similar to Baupläne so that modules express unique sets of genes, cells, and processes. In terms of evolution, we conclude that natural selection can act on any of these hierarchical levels within modules or on all the sensory modules as a whole. We discuss interactions within and between modules with reference to the blind cavefish from both genetic and developmental perspectives. The cavefish represents an illuminating example of module interaction, uncoupling of modules, and module expansion.

Building sensory receptors on the tongue

Neurotrophins, neurotrophin receptors and sensory neurons are required for the development of lingual sense organs. For example, neurotrophin 3 sustains lingual somatosensory neurons. In the traditional view, sensory axons will terminate where neurotrophin expression is most pronounced. Yet, lingual somatosensory axons characteristically terminate in each filiform papilla and in each somatosensory prominence within a cluster of cells expressing the p75 neurotrophin receptor (p75NTR), rather than terminating among the adjacent cells that secrete neurotrophin 3. The p75NTR on special specialized clusters of epithelial cells may promote axonal arborization in vivo since its over-expression by fibroblasts enhances neurite outgrowth from overlying somatosensory neurons in vitro. Two classical observations have implicated gustatory neurons in the development and maintenance of mammalian taste buds--the early arrival times of embryonic innervation and the loss of taste buds after their denervation in adults. In the modern era more than a dozen experimental studies have used early denervation or neurotrophin gene mutations to evaluate mammalian gustatory organ development. Necessary for taste organ development, brain-derived neurotrophic factor sustains developing gustatory neurons. The cardinal conclusion is readily summarized: taste buds in the palate and tongue are induced by innervation. Taste buds are unstable: the death and birth of taste receptor cells relentlessly remodels synaptic connections. As receptor cells turn over, the sensory code for taste quality is probably stabilized by selective synapse formation between each type of gustatory axon and its matching taste receptor cell. We anticipate important new discoveries of molecular interactions among the epithelium, the underlying mesenchyme and gustatory innervation that build the gustatory papillae, their specialized epithelial cells, and the resulting taste buds.

Taste placodes are primary targets of geniculate but not trigeminal sensory axons in mouse developing tongue

Tongue embryonic taste buds begin to differentiate before the onset of gustatory papilla formation in murine. In light of this previous finding, we sought to reexamine the developing sensory innervation as it extends toward the lingual epithelium between E 11.5 and 14.5. Nerve tracings with fluorescent lipophilic dyes followed by confocal microscope examination were used to study the terminal branching of chorda tympani and lingual nerves. At E11.5, we confirmed that the chorda tympani nerve provided for most of the nerve branching in the tongue swellings. At E12.5, we show that the lingual nerve contribution to the overall innervation of the lingual swellings increased to the extent that its ramifications matched those of the chorda tympani nerve. At E13.0, the chorda tympani nerve terminal arborizations appeared more complex than those of the lingual nerve. While the chorda tympani nerve terminal branching appeared close to the lingual epithelium that of the trigeminal nerve remained rather confined to the subepithelial mesenchymal tissue. At E13.5, chorda tympani nerve terminals projected specifically to an ordered set of loci on the tongue dorsum corresponding to the epithelial placodes. In contrast, the lingual nerve terminals remained subepithelial with no branches directed towards the placodes. At E14.5, chorda tympani nerve filopodia first entered the apical epithelium of the developing fungiform papilla. The results suggest that there may be no significant delay between the differentiation of embryonic taste buds and their initial innervation.

Distinct roles for Sema3A, Sema3F, and an unidentified trophic factor in controlling the advance of geniculate axons to gustatory lingual epithelium

Geniculate ganglion axons arrive in the lingual mesenchyme on embryonic day 13 (E13), 3-4 days before penetrating fungiform papilla epithelium (E17). This latency may result from chemorepulsion by epithelial Sema3A (Dillon et al. (2004) Journal of Comparative Neurology 470, 13-24), or Sema3F, which we report is also expressed in this epithelium. Sema3A and Sema3F repelled or suppressed geniculate neurite outgrowth, respectively, and these effects were stage and neurotrophic factor dependent. BDNF-stimulated outgrowth is repelled by Sema3A until E17, but insensitive to Sema3F from E16. NT-4-stimulated neurite outgrowth is sensitive to Sema3A and Sema3F through E18, but NT-4 has not been detected in E15-18 tongue. E15-18 tongue explants did not exhibit net chemorepulsion of geniculate neurites, but the ability of tongue explants to support geniculate neurite outgrowth fluctuates: E12-13 (Rochlin et al. (2000), Journal of Comparative Neurology, 422, 579-593) and E17-18 explants promote and may attract geniculate neurites, but stages corresponding to intralingual arborization do not. The E18 trophic and tropic effects were evident even in the presence of BDNF or NT-4, suggesting that some other factor is responsible. Intrinsic neurite outgrowth capability (without exogenous neurotrophic factors) fluctuated similarly: ganglia deteriorated at E15, but exhibited moderate outgrowth at E18. The chemorepulsion studies are consistent with a role for Sema3A, not Sema3F, in restricting geniculate axons from the epithelium until E17, when axons penetrate the epithelium. The transient inability of tongue explants to promote geniculate neurite outgrowth may signify an alternative mechanism for restricting geniculate axons from the epithelium: limiting trophic factor access.

Sonic hedgehog exerts distinct, stage-specific effects on tongue and taste papilla development

Taste papillae are ectodermal specializations that serve to house and distribute the taste buds and their renewing cell populations in specific locations on the tongue. We previously showed that Sonic hedgehog (Shh) has a major role in regulating the number and spatial pattern of fungiform taste papillae on embryonic rat tongue, during a specific period of papilla formation from the prepapilla placode. Now we have immunolocalized the Shh protein and the Patched receptor protein (Ptc), and have tested potential roles for Shh in formation of the tongue, emergence of papilla placodes, development of papilla number and size, and maintenance of papillae after morphogenesis is advanced. Cultures of entire embryonic mandible or tongues from gestational days 12 to 18 [gestational or embryonic days (E)12-E18] were used, in which tongues and papillae develop with native spatial, temporal, and molecular characteristics. The Shh signaling pathway was disrupted with addition of cyclopamine, jervine, or the 5E1 blocking antibody. Shh and Ptc proteins are diffuse in prelingual tissue and early tongue swellings, and are progressively restricted to papilla placodes and then to regions of developing papillae. Ptc encircles the dense Shh immunoproduct in papillae at various stages. When the Shh signal is disrupted in cultures of E12 mandible, tongue formation is completely prevented. At later stages of tongue culture initiation, Shh signal disruption alters development of tongue shape (E13) and results in a repatterned fungiform papilla distribution that does not respect normally papilla-free tongue regions (E13-E14). Only a few hours of Shh signal disruption can irreversibly alter number and location of fungiform papillae on anterior tongue and elicit papilla formation on the intermolar eminence. However, once papillae are well formed (E16-E18), Shh apparently does not have a clear role in papilla maintenance, nor does the tongue retain competency to add fungiform papillae in atypical locations. Our data not only provide evidence for inductive and morphogenetic roles for Shh in tongue and fungiform papilla formation, but also suggest that Shh functions to maintain the interpapilla space and papilla-free lingual regions. We propose a model for Shh function at high concentration to form and maintain papillae and, at low concentration, to activate between-papilla genes that maintain a papilla-free epithelium.

Histogenesis of the oropharyngeal cavity taste buds and the relevant nerves and brain centers in substrate-brooding and mouth-brooding cichlid fish (Cichlidae, Teleostei)

This study follows the histogenesis of the oropharyngeal cavity taste buds, along with the development of the relevant neural centers and gustatory nerves, in two cichlid species: the substrate-brooding Cichlasoma cyanoguttatum and the mouth-brooding Astatotilapia flavijosephi, from fertilization to 20-day-old juveniles, grown at a temperature of 26 degrees C. Significant differences in pace of development were shown between the two social types: Substrate-brooders complete embryogenesis and hatch 48 h after fertilization (HAF) and begin to swim 120 HAF, with the yolk sac disappearing 160 HAF, whereas mouth-brooders hatch 84 HAF and begin to swim 196 HAF, with the yolk sac disappearing 360 HAF. Histogenesis of primordial taste buds occurs 75 HAF and 160 HAF in C. cyanoguttatum and A. flavijosephi, respectively. Accordingly, the related sensory ganglia and nerves (VII, IX, and X) develop much earlier in the substrate-brooded larvae and postlarvae. Nerve and brain development in juvenile A. flavijosephi of 13 mm total length (TL) closely resemble those of 8-mm-TL C. cyanoguttatum. These differences in development continue throughout the early stages of growth. Similar differences are observed in the ripening and increase in number of taste buds and dentition on the jaws and pharyngeal bones. The possible triggers and causes of such differences in development, as well as the inductors of taste bud development, are discussed.

Gustatory neurons derived from epibranchial placodes are attracted to, and trophically supported by, taste bud-bearing endoderm in vitro

Taste buds are multicellular receptor organs innervated by the VIIth, IXth, and Xth cranial nerves. In most vertebrates, taste buds differentiate after nerve fibers have reached the lingual epithelium, suggesting that nerves induce taste buds. However, under experimental conditions, taste buds of amphibians develop independently of innervation. Thus, rather than being induced by nerves, the developing taste periphery likely regulates ingrowing nerve fibers. To test this idea, we devised a culture approach using axolotl embryos. Gustatory neurons were generated from cultured epibranchial placodes, and when cultured alone, axon outgrowth was random over 4 days, a time period coincident with axon growth to the periphery in vivo. In contrast, cocultures of placodal neurons with oropharyngeal endoderm (OPE), the normal taste bud-containing target for these neurons, resulted in neurite growth toward the target tissue. Unexpectedly, placodal neurons also grew toward flank ectoderm (FE), which these neurons do not encounter in vivo. To compare further the impact of OPE and FE explants on gustatory neurons, cocultures were extended and examined at 6, 8, and 10 days, when, in vivo, placodal fibers have innervated the epithelium but prior to taste bud formation, when taste buds have differentiated and are innervated, and when the mouth has opened and larvae have begun to feed, respectively. The behavior of placodal axons with respect to target type did not differ between OPE and FE cocultures at 6 days. However, by 8 days, differences in axonal outgrowth were observed with respect to target type, and these differences were enhanced by 10 days in vitro. Most clearly, exuberant placodal fibers grew in 10-day OPE cocultures, and numerous neurites had invaded OPE explants by this time, whereas gustatory neurites were sparse in FE cocultures, and rarely approached and almost never contacted FE explants. Thus, embryonic endoderm destined to give rise to taste buds specifically attracts its innervation early in development, as placodal neurons send out axons. Later, when gustatory axons synapse with differentiated taste buds in vivo, the OPE provides trophic support for cultured gustatory neurons.

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.

Localization of Ulex europaeus agglutinin-I (UEA-I) in the developing gustatory epithelium of the rat

To understand the development of the gustatory structures necessitates a reliable marker for both immature and mature taste buds. It has been reported that the intragemmal cells within the taste buds of adult rats were bound to Ulex europaeus agglutinin-I (UEA-I), a specific lectin for alpha-linked fucose, but it has not been determined whether immature taste buds, i.e. taste buds without an apparent taste pore, are labeled with UEA-I. The present study was conducted to examine the UEA-I binding pattern during the development of the rat gustatory epithelium. In adult animals, UEA-I bound to the membrane of taste buds in all examined regions of the gustatory epithelium. Within the individual taste buds, UEA-I labeled almost all intragemmal cells. The binding of UEA-I was occasionally detected below the keratinized layer of the trench wall epithelium but could not be found in the lingual epithelium of the adult animal. During the development of circumvallate papilla, some cells within the immature taste buds were also labeled with UEA-I. The developmental changes in the UEA-I binding pattern in fungiform papillae were almost identical to those in the circumvallate papilla: both immature and mature taste buds were labeled with UEA-I. The present results indicate that UEA-I is a specific lectin for the intragemmal cells of both immature and mature taste buds and, thus, UEA-I can be used as a reliable marker for all taste buds in the rat.

Taste Buds: Development and Evolution

The gustatory system in vertebrates comprises peripheral receptors (taste buds), innervated by three cranial nerves (VII, IX, and X), and a series of central neural centers and pathways. All vertebrates, with the exception of hagfishes, have taste buds. These receptors vary morphologically in different vertebrates but usually consist of at least four types of cells (dark, light, basal, and stem cells). An out-group analysis indicates that taste buds were restricted to the oropharynx, primitively, and that external taste buds, distributed over the head and, in some cases, even the trunk, evolved a number of times independently. The sensory neurons of the cranial nerves that innervate taste buds are believed to arise from epibranchial placodes, which are induced by pharyngeal endoderm, but it has never been demonstrated experimentally that these sensory neurons do, in fact, arise from these placodes. Although many details of the development of the innervation of taste buds are still unknown, it is now clear that taste buds are induced from either ecto- or endodermal epithelia, rather than arising from either placodes or neural crest. At present, there are two developmental models of taste bud induction: The neural induction model claims that peripheral nerve fibers induce taste buds, whereas the early specification model claims that oropharyngeal epithelium is specified by or during gastrulation and that taste buds arise from cell-cell interactions within the specified epithelium. There is now substantial evidence that the early specification model best describes the induction of taste buds.

Spacing patterns on tongue surface-gustatory papilla

The dorsal surface of the mammalian tongue is covered with four kinds of papillae, fungiform, circumvallate, foliate and filiform papillae. With the exception of the filiform papillae, these types of papillae contain taste buds and are known as the gustatory papillae. The gustatory papillae are distributed over the tongue surface in a distinct spatial pattern. The circumvallate and foliate papillae are positioned in the central and lateral regions respectively and the fungiform papillae are distributed on the anterior part of the tongue in a stereotyped array. The patterned distribution and developmental processes of the fungiform papillae indicate some similarity between the fungiform papillae and the other epithelial appendages, including the teeth, feathers and hair. This is because 1) prior to the morphological changes, the signaling molecules are expressed in the fungiform papillae forming area with a stereotyped pattern; 2) the morphogenesis of the fungiform papillae showed specific structures in early development, such as epithelial thickening and mesenchymal condensation and 3) the fungiform papillae develop through reciprocal interactions between the epithelium and mesenchymal tissue. These results led us to examine whether or not the early organogenesis of the fungiform papillae is a good model system for understanding both the spacing pattern and the epithelial-mesenchymal interaction during embryogenesis.

Cell contact-dependent mechanisms specify taste bud pattern during a critical period early in embryonic development

After gastrulation, the pharyngeal endoderm is specified to give rise to taste receptor organs without further signaling from other embryonic tissues. We hypothesized that intercellular signaling might be responsible for the specification of taste buds. To test if and when this signaling was occurring, intercellular contacts were transiently disrupted in cultures of pharyngeal endoderm from axolotl embryos, and the number, size, and distribution of taste buds analyzed. Disruption of cell contacts at progressive time points, from neurula to late tail bud stages, revealed a critical period, during mid-tail bud stages, when disruption of cell contacts resulted in a significant increase in taste bud number and size. The spatial distribution of taste buds was also altered; taste buds were more clustered in explants disrupted during the critical period. These effects were not due to general alterations in mitosis and apoptosis. Rather, at least three aspects of taste bud patterning, i.e., number, size, and distribution, are governed by mechanisms dependent on normal cell contacts during a concise time window. Furthermore, our findings are consistent with specification of taste buds by means of lateral inhibitory signaling, which we hypothesize results from cell contact-dependent or short-range diffusible signals.

Morphological evidence of the importance of epithelial tissue during mouse tongue development

The morphogenesis of fungiform papillae occurs in a stereotyped pattern on the dorsal surface of the tongue in mice from embryonic day 12 (E12) to E17. The histological results and ultrastructural observations showed the development of specific structures in the epithelium into fungiform papillae. Prior to the morphological changes, the Bmp-4 and Shh transcripts are expressed in a restricted area on the dorsal surface. These results suggest that the development of fungiform papillae requires an epithelium and mesenchyme interaction during morphogenesis. In order to obtain direct evidence of the epithelium and mesenchyme interaction during tongue papillae morphogenesis, the formation of fungiform papillae was examined after a recombination assay. In order to confirm the epithelium and mesenchyme interactions during the early development of the mouse tongue, a recombination assay was conducted after the recombination assay at E12.5 and E13.5 for 2 days using an in vitro organ culture. From the recombination assay results, the E13.5 epithelial portion of the fungiform papillae could determine the position of the newly formed fungiform papillae with the epithelial signaling molecules. E13.5 was a critical stage for fungiform papillae morphogenesis. Fungiform papillae can be considered to be small epithelial appendages, which are formed via the epithelium and mesenchyme interactions.

Notch-associated gene expression in embryonic and adult taste papillae and taste buds suggests a role in taste cell lineage decisions

The Notch signaling pathway is involved in cell fate decisions during development. To explore the role of this signaling cascade in the taste system, we investigated the expression patterns of Notch signaling genes in fetal and adult mouse tongues using in situ hybridization. Three of the four murine Notch receptors, their ligands, Delta-like 1 (Dll-1), Jagged1, and Jagged2, as well as three transcription factors, Hes1, Hes6, and Mash1, are expressed in the embryonic taste epithelium. Expression is first detected in the circumvallate papilla at embryonic day E14.5, when Notch1, Jagged1, and Jagged2 are expressed broadly in the papilla and general lingual epithelium. In contrast, Mash1 and Hes6 are restricted to only a few epithelial cells in the apical region of the developing papilla. By E18.5, many of the genes now exhibit a bimodal expression pattern in the papillary epithelium: apically and dorsally they are expressed in sparse clusters of cells, while more ventrally expression typically occurs throughout the lower regions of the trenches. The extent of papilla innervation was compared with Mash1 and Hes6 expression. At E14.5, when Hes6 and Mash1 are already expressed in small numbers of epithelial cells, PGP9.5 immunoreactive fibers have not yet invaded the epithelium, consistent with the specification of taste bud primordia prior to nerve contact. All of the genes examined (except Notch2) are also expressed in subsets of cells within circumvallate taste buds in adult mice, although Notch1 is restricted to basal cells adjacent to taste buds. The onset of embryonic Notch associated gene expression after the morphological differentiation of the circumvallate papilla argues that this signaling cascade may specify taste receptor cell lineages within an already specified taste papilla. Similarly, Notch gene expression in adult taste buds suggests continued roles in cell lineage determination and cell turnover.

Embryonic geniculate ganglion neurons in culture have neurotrophin-specific electrophysiological properties

Geniculate ganglion neurons provide a major source of innervation to mammalian taste organs, including taste buds in the soft palate and in fungiform papillae on the anterior two thirds of the tongue. In and around the fungiform papillae, before taste buds form, neurotrophin mRNAs are expressed in selective spatial and temporal patterns. We hypothesized that neurotrophins would affect electrophysiological properties in embryonic geniculate neurons. Ganglia were explanted from rats at gestational day 16, when growing neurites have entered the papilla core, and maintained in culture with added brain-derived neurotrophic factor (BDNF), neurotrophin 4 (NT4), nerve growth factor (NGF) or neurotrophin 3 (NT3). Neuron survival with BDNF or NT4 was about 80%, whereas with NGF or NT3 less than 15% of neurons survived over 6 days in culture. Whole cell recordings from neurons in ganglion explants with each neurotrophin condition demonstrated distinctive neurophysiological properties related to specific neurotrophins. Geniculate neurons cultured with either BDNF or NT4 had similar passive-membrane and action potential properties, but these characteristics were significantly different from those of neurons cultured with NGF or NT3. NGF-maintained neurons had features of increased excitability including a higher resting membrane potential and a lower current threshold for the action potential. About 70% of neurons produced repetitive action potentials at threshold. Furthermore, compared with neurons cultured with other neurotrophins, a decreased proportion had an inflection on the falling phase of the action potential. NT3-maintained neurons had action potentials that were of relatively large amplitude and short duration, with steep rising and falling slopes. In addition, about 20% responded with a repetitive train of action potentials at threshold. In contrast, with BDNF or NT4 repetitive action potential trains were not observed. The data demonstrate different neurophysiological properties in developing geniculate ganglion neurons maintained with specific neurotrophins. Therefore, we suggest that neurotrophins might influence acquisition of distinctive neurophysiological properties in embryonic geniculate neurons that are fundamental to the formation of peripheral taste circuits and a functioning taste system.

Disruption of sonic hedgehog signaling alters growth and patterning of lingual taste papillae

Taste buds on the anterior part of the tongue develop in conjunction with epithelial-mesenchymal specializations in the form of gustatory (taste) papillae. Sonic hedgehog (Shh) and Bone Morphogenetic Protein 4 (BMP4) are expressed in developing taste papillae, but the roles of these signaling molecules in specification of taste bud progenitors and in papillary morphogenesis are unclear. We show here that BMP4 is not expressed in the early tongue, but is precisely coexpressed with Shh in papillary placodes, which serve as a signaling center for both gustatory and papillary development. To elucidate the role of Shh, we used an in vitro model of mouse fungiform papillary development to determine the effects of two functional inhibitors of Shh signaling: anti-Shh (5E1) antibody and cyclopamine. Cultured E11.5 tongue explants express Shh and BMP4(LacZ) in a pattern similar to that of intact embryos, localizing to developing papillary placodes after 2 days in culture. Tongues cultured with 5E1 antibody continue to express these genes in papillary patterns but develop more papillae that are larger and closer together than in controls. Tongues cultured with cyclopamine have a dose-dependent expansion of Shh and BMP4(LacZ) expression domains. Both antibody-treated and cyclopamine-treated tongue explants also are smaller than controls. Taken together, these results suggest that, although Shh is not involved in the initial specification of papillary placodes, Shh does play two key roles during pmcry development: (1) as a morphogen that directs cells toward a nonpapillary fate, and (2) as a mitogen, causing expansion of the interplacodal epithelium and underlying mesenchyme.

Distribution of keratin 8-containing cell clusters in mouse embryonic tongue: evidence for a prepattern for taste bud development

The initiation of the morphogenesis of gustatory papillae is independent of innervation. To address the question of whether taste bud formation is associated with gustatory papilla morphogenesis, we examined developing tongues in mouse embryos from embryonic day 11 to birth. Despite the smooth morphological appearance of the lingual dorsal surface at 13 days of gestation, we observed embryonic taste bud primordia as discrete collections of cytokeratin 8-positive and elongated cells in epithelial placodes in the anterior tongue. In subsequent stages until birth, cytokeratin 8 continues to be expressed in embryonic taste buds distributed in punctuate patterns at regular intervals along rows that are symmetrically located on both sides of the median sulcus in the dorsal anterior developing tongue. Embryonic taste buds were observed in the developing circumvallate papillae from 15.5 days of gestation until birth. The dorsal epithelium of the anterior tongue is not innervated when embryonic taste buds first occur. The increased numbers of embryonic taste buds in developing fungiform papillae until birth are not correlated with the neural invasion of the epithelium. Thus, taste buds occur prenatally more likely independently of the innervation.

Cyclopamine and jervine in embryonic rat tongue cultures demonstrate a role for Shh signaling in taste papilla development and patterning: fungiform papillae double in number and form in novel locations in dorsal lingual epithelium

From time of embryonic emergence, the gustatory papilla types on the mammalian tongue have stereotypic anterior and posterior tongue locations. Furthermore, on anterior tongue, the fungiform papillae are patterned in rows. Among the many molecules that have potential roles in regulating papilla location and pattern, Sonic hedgehog (Shh) has been localized within early tongue and developing papillae. We used an embryonic, tongue organ culture system that retains temporal, spatial, and molecular characteristics of in vivo taste papilla morphogenesis and patterning to study the role of Shh in taste papilla development. Tongues from gestational day 14 rat embryos, when papillae are just beginning to emerge on dorsal tongue, were maintained in organ culture for 2 days. The steroidal alkaloids, cyclopamine and jervine, that specifically disrupt the Shh signaling pathway, or a Shh-blocking antibody were added to the standard culture medium. Controls included tongues cultured in the standard medium alone, and with addition of solanidine, an alkaloid that resembles cyclopamine structurally but that does not disrupt Shh signaling. In cultures with cyclopamine, jervine, or blocking antibody, fungiform papilla numbers doubled on the dorsal tongue with a distribution that essentially eliminated inter-papilla regions, compared with tongues in standard medium or solanidine. In addition, fungiform papillae developed on posterior oral tongue, just in front of and beside the single circumvallate papilla, regions where fungiform papillae do not typically develop. The Shh protein was in all fungiform papillae in embryonic tongues, and tongue cultures with standard medium or cyclopamine, and was conspicuously localized in the basement membrane region of the papillae. Ptc protein had a similar distribution to Shh, although the immunoproduct was more diffuse. Fungiform papillae did not develop on pharyngeal or ventral tongue in cyclopamine and jervine cultures, or in the tongue midline furrow, nor was development of the single circumvallate papilla altered. The results demonstrate a prominent role for Shh in fungiform papilla induction and patterning and indicate differences in morphogenetic control of fungiform and circumvallate papilla development and numbers. Furthermore, a previously unknown, broad competence of dorsal lingual epithelium to form fungiform papillae on both anterior and posterior oral tongue is revealed.

Two generations of the tongue and gustatory organs in the development of Hynobius dunni Tago

In the development of Hynobius dunni there are two consecutive generations of the tongue and two generations of gustatory organs (taste buds and taste disks). The anlage of the developing secondary tongue appears just in front of the free ending of the primary tongue beginning at the larval developmental stage 62. From stage 67, a gradual reduction in the anterior part of the gill skeleton that supports the primary tongue occurs as the developing secondary tongue replaces the primary one. The lining of the entire oropharyngeal cavity of larvae contains only gustatory organs of the taste bud (TB) type. In younger larvae, the sensory area of a TB has a diameter of between 10 and 13 microm, while in older larvae, TBs reach 16-18 microm in diameter. After metamorphosis, some gustatory organs in the secondary tongue with a sensory area of 26-36 microm in diameter appear. In older animals they may reach as much as 56-71 microm. In other regions of the oropharyngeal epithelium than the tongue, these organs have an ellipsoid shape with a major axis of about 50 microm. On the basis of the cytomorphological criteria established previously, these organs were designated as taste disks. Thus, the presence of two generations of gustatory organs is characteristic of some urodeles, as well as frogs.

Fibroblast and epidermal growth factors modulate proliferation and neural cell adhesion molecule expression in epithelial cells derived from the adult mouse tongue

Lingual epithelial cells, including those of the taste buds, are regularly replaced by proliferative stem cells. We found that integrin beta(1), a keratinocyte stem cell marker, was expressed at the basal layer and taste buds of adult mouse tongue epithelium. We purified and cultured integrin beta(1)-positive cells (termed KT-1 cells), whose growth was stimulated by epidermal growth factor (EGF) and basic fibroblast growth factor (FGF-2). FGF-2 stimulation induced translocation of the FGF type I receptor (FGFR1) into nuclei, suggesting that the growth-stimulating effect of FGF-2 was mediated through FGFR1. EGF and FGF-2 also regulated cell surface expression of the neural cell adhesion molecule (N-CAM) in KT-1 cells. Anti-N-CAM antibody immunoreactivity was restricted to the gustatory epithelium and the nerves in the tongue epithelium, giving rise to the possibility that KT-1 may contain gustatory epithelial cells. KT-1 cells may thus be useful for analyzing the factors that regulate the growth and differentiation of lingual and gustatory epithelial cells in vitro.

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.

The development of the tongue and morphological and cytological changes in taste discs of Rana esculenta

From the 38th developmental stage of the tadpole of Rana esculenta the process of tongue formation consists in the fast growth of the lining of the oral cavity floor anteriorly and faucially. This process is accompanied by the development of taste organs on the dorsal side of the tongue. At developmental stages 39-42 taste disc anlages are covered by a layer of ordinary epithelial cells. At these stages, in some cells of a taste disc single synaptic-like vesicles with an electron-dense core appear. Apart from that, as early as at stage 42 differentiation of the cells of a taste disc can be observed at the ultrastructural level. It is only at the 44th stage that all cell types characteristic for the mature TD can be distinguished in TEM (i.e., taste cells, basal cells and three kinds of associate cells: mucous, wing and sustentacular). Starting from that stage changes in the cell membrane can be observed indicating the presence of afferent synaptic junctions. The antibody used in the experiment was raised against neuron-specific enolase (NSE). At each of the developmental stages investigated (38, 42, 45) nerve fibres within the connective tissue beneath the epithelium of a taste disc anlage were immunopositive for NSE. From stage 42 onwards neural elements present in the basal part of the epithelium of a taste disc anlage were also NSE-positive. Basal cells did not show immuno-reactivity for NSE at any of the developmental stages investigated.

Ultrastructural study of the precursor to fungiform papillae prior to the arrival of sensory nerves in the fetal rat

The structure of precursors to fungiform papillae without taste buds, prior to the arrival of sensory nerve fibers at the papillae, was examined in the fetal rat on embryonic day 13 (E13) and 16 (E16) by light and transmission electron microscopy in an attempt to clarify the mechanism of morphogenesis of these papillae. At E13, a row of rudiments of fungiform papillae was arranged along both sides of the median sulcus of the lingual dorsal surface, and each row consisted of about 10 rudiments. There was no apparent direct contact between papillae rudiments and sensory nerves at this time. Bilaterally towards the lateral side of the tongue, adjacent to these first rudiments of fungiform papillae, a series of cord-like invaginations of the dorsal epithelium of the tongue into the underlying connective tissue, representing additional papillary primordia parallel to the first row, was observed. The basal end of each invagination was enlarged as a round bulge, indented at its tip by a mound of fibroblasts protruding into the bulge. At E16 there was still no apparent direct contact between rudiments of fungiform papillae and sensory nerves. Each rudiment apically contained a spherical core of aggregating cells, which consisted of a dense assembly of large, oval cells unlike those in other areas of the lingual dorsal epithelium. The differentiation of these aggregated cells was unclear. The basal lamina was clearly recognizable between the epithelium of the rudiment of fungiform papillae and the underlying connective tissue. Spherical structures, which appeared to be sections of the cord-like invaginations of the lingual epithelium that appeared on E13, were observed within the connective tissue separated from the dorsal lingual epithelium. Transverse sections of such structures revealed four concentric layers of cells: a central core, an inner shell, an outer shell, and a layer of large cells. Bundles of fibers were arranged in the central core, and the diameters of bundles varied somewhat depending on the depth of the primordia within the connective tissue and their distance from the median sulcus. Ultrastructural features of cells in the outer shell differed significantly in rudiments close to the lingual epithelium as compared to those in deeper areas of connective tissue. Around the outer shell there was a large-cell layer consisting of one to three layers of radially elongated, oval cells that contained many variously sized, electron-dense, round granules. Large numbers of fibroblasts formed dense aggregates around each spherical rudiment, and were separated by the basal lamina from the large-cell epithelial layer. Progressing from deep-lying levels of the rudiments of the papillae to levels close to the lingual surface epithelium, the central core, inner shell, and outer shell gradually disappeared from the invaginated papillary cords.

Specification of pharyngeal endoderm is dependent on early signals from axial mesoderm

The development of taste buds is an autonomous property of the pharyngeal endoderm, and this inherent capacity is acquired by the time gastrulation is complete. These results are surprising, given the general view that taste bud development is nerve dependent, and occurs at the end of embryogenesis. The pharyngeal endoderm sits at the dorsal lip of the blastopore at the onset of gastrulation, and because this taste bud-bearing endoderm is specified to make taste buds by the end of gastrulation, signals that this tissue encounters during gastrulation might be responsible for its specification. To test this idea, tissue contacts during gastrulation were manipulated systematically in axolotl embryos, and the subsequent ability of the pharyngeal endoderm to generate taste buds was assessed. Disruption of both putative planar and vertical signals from neurectoderm failed to prevent the differentiation of taste buds in endoderm. However, manipulations of contact between presumptive pharyngeal endoderm and axial mesoderm during gastrulation indicate that signals from axial mesoderm (the notochord and prechordal mesoderm) specify the pharyngeal endoderm, conferring upon the endoderm the ability to autonomously differentiate taste buds. These findings further emphasize that despite the late differentiation of taste buds, the tissue-intrinsic mechanisms that generate these chemoreceptive organs are set in motion very early in embryonic development.

Scanning electron-microscopic view of the oral and basal epithelial surfaces of the rat soft palate

We studied corresponding structures on the oral and basal surfaces of the oral epithelial layer, focusing on the microanatomy of gustatory papillae. Specimens for scanning electron microscopy were obtained from the rat soft palates and fixed with a mixture of 2% paraformaldehyde and 1% glutaraldehyde. They were first prepared without postfixation and conductive-staining to study the oral surface. After examination, the epithelium was reinforced by additional sputter coating and treated with 6 N NaOH at 60 degrees C to exfoliate the basal epithelial surfaces without any significant artifacts. The papillae, showing circular, elliptical or fusiform protuberances on the oral surface, were classified into two types: types I and II. The type I and type II papillae contained one and two taste pores, respectively. On the basal epithelial surface, the basal portions of the taste buds were associated with concentrically arranged nerve fibers and Schwann cells. Another characteristic finding on the basal epithelial surface was the presence of excretory ducts of minor salivary glands in a close spatial relationship to taste buds. It is suggested that saliva coming out through the duct is mixed with food, thus enabling intimate contact with the taste pores of the papillae.

The distribution of PGP9. 5, BDNF and NGF in the vallate papilla of adult and developing mice

The development and innervation of vallate papillae and taste buds in mice were studied using antibodies against the neuronal marker, protein gene product 9.5 (PGP 9.5), and against nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). PGP 9.5 immunohistochemical studies revealed that the earliest sign of median vallate papilla formation was an epithelial bulge at embryonic day 13 (E13), and at E14, a dense nerve plexus was found within the connective tissue core of the papilla. Thin nerve fibers penetrated the apical and medial trench wall epithelium of the papilla at E16 and a few of these began to invade the lateral trench wall epithelium at E17. At postnatal day 1 (P1), the newly formed taste buds were recognizable and a small number of PGP 9.5-immunoreactive (IR) cells appeared on the medial trench wall epithelium. The number of PGP 9.5-IR taste bud cells then increased gradually and reached the adult level at postnatal week 2. PGP 9.5 immunoreactivity increased systematically with age. NGF and BDNF immunoreactivity was first seen at the boundary between the columnar cells in the apical epithelium of the developing vallate papilla at E13, then in the medial and lateral trench walls at E15 (BDNF) or E18 (NGF). At P1, BDNF immunoreactivity was exclusively present in the newly formed taste buds of the medial trench wall. The number of BDNF-IR taste bud cells then increased gradually, reaching the adult level at P7. Similar degrees of NGF and BDNF immunoreactivity were seen in the developing vallate papilla. In the present study, we found that the vallate papilla was formed prior to its innervation, and we propose that initiation of papilla formation does not require any direct influence from the specific gustatory nerve. We also suggest that neurotrophins in the early developing vallate papillae might act as local tropic factors for the embryonic growth of nerve fibers to induce differentiation of the taste buds.

Intrauterine dentistry: an integrated model of prevention

Intrauterine Dentistry is a highly relevant subject of our time. The use of preventive measures in the intrauterine stage can avoid several diseases, among these, dental caries. The WHO advises that from the 4th month of pregnancy, women should avoid the intake of sugar, so that the fetus, future child, does not develop an exaggerated attraction for these types of foods, thus being susceptible to caries. Through questionnaires sent to gynecologist-obstetricians and dentists, this research investigated the information they have about this subject and how they instruct their patients. Questionnaires were also sent to pregnant women requesting information about the instructions they had received for the prevention of oral diseases of their fetus. Seventy-one percent of the dentists and 80% of the gynecologist-obstetricians reported having instructed the pregnant women to reduce the intake of sugar. However, only 13.6% of the dentists and no gynecologist-obstetrician instructed the reduction of sugar intake between the 12th and 18th week of pregnancy. A total of 42.2% of the pregnant women referred to these instructions, but none received instruction as to the specific period of the 12th and 18th week. An ideal model of treatment for pregnant women must include integrated and multiprofessional treatment, in which general dentists and gynecologist-obstetricians work together with the participation of the patient.

Development and maturation of taste buds of the palatal epithelium of the rat: histological and immunohistochemical study

Palatal taste buds are intriguing partners in the mediation of taste behavior and their spatial distribution is functionally important for suckling behavior, especially in the neonatal life. Their prenatal development has not been previously elucidated in the rat, and the onset of their maturation remains rather controversial. We delineated the development and frequency distribution of the taste buds as well as the immunohistochemical expression of alpha-gustducin, a G protein closely related to the transduction of taste stimuli, in the nasoincisor papilla (NIP) and soft palate (SP) from the embryonic day 17 (E17) till the postnatal day 70 (PN70). The main findings in the present study were the development of a substantial number of taste pores in the SP of fetal rats (60.3 +/- 1.7 out of 122.8 +/- 5.5; mean +/- SD/animal at E19) and NIP of neonatal rats (9.8 +/- 1.0 out of 44.8 +/- 2.2 at PN4). alpha-gustducin-like immunoreactivity (-LI) was not expressed in the pored taste buds of either prenatal or newborn rats. The earliest expression of alpha-gustducin-LI was demonstrated at PN1 in the SP (1.5 +/- 0.5 cells/taste bud; mean +/- SD) and at PN4 in the NIP (1.4 +/- 0.5). By age the total counts of pored taste buds continuously increased and their morphological features became quite discernible. They became pear in shape, characterized by distinct pores, long subporal space, and longitudinally oriented cells. Around the second week, a remarkable transient decrease in the total number of taste buds was recorded in the oral epithelium of NIP and SP, which might be correlated with the changes of ingestive behaviors. The total counts of cells showing alpha-gustducin-LI per taste bud gradually increased till the end of our investigation (14.1 +/- 2.7 in NIP and 12.4 +/- 2.5 in SP at PN70). We conclude that substantial development of taste buds began prenatally in the SP, whereas most developed entirely postnatal in the NIP. The present study provides evidence that the existence of a taste pore which is considered an important criterion for the morphological maturation of taste buds is not enough for the onset of the taste transduction, which necessitates also mature taste cells. Moreover, the earlier maturation of palatal taste buds compared with the contiguous populations in the oral cavity evokes an evidence of their significant role in the transmission of gustatory information, especially in the early life of rat.

Epithelial overexpression of BDNF or NT4 disrupts targeting of taste neurons that innervate the anterior tongue

Brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4) are essential for the survival of geniculate ganglion neurons, which provide the sensory afferents for taste buds of the anterior tongue and palate. To determine how these target-derived growth factors regulate gustatory development, the taste system was examined in transgenic mice that overexpress BDNF (BDNF-OE) or NT4 (NT4-OE) in basal epithelial cells of the tongue. Overexpression of BDNF or NT4 caused a 93 and 140% increase, respectively, in the number of geniculate ganglion neurons. Surprisingly, both transgenic lines had severe reduction in fungiform papillae and taste bud number, primarily in the dorsal midregion and ventral tip of the tongue. No alterations were observed in taste buds of circumvallate or incisal papillae. Fungiform papillae were initially present on tongues of newborn BDNF-OE animals, but many were small, poorly innervated, and lost postnatally. To explain the loss of nerve innervation to fungiform papillae, the facial nerve of developing animals was labeled with the lipophilic tracer DiI. In contrast to control mice, in which taste neurons innervated only fungiform papillae, taste neurons in BDNF-OE and NT4-OE mice innervated few fungiform papillae. Instead, some fibers approached but did not penetrate the epithelium and aberrant innervation to filiform papillae was observed. In addition, some papillae that formed in transgenic mice had two taste buds (instead of one) and were frequently arranged in clusters of two or three papillae. These results indicate that target-derived BDNF and NT4 are not only survival factors for geniculate ganglion neurons, but also have important roles in regulating the development and spatial patterning of fungiform papilla and targeting of taste neurons to these sensory structures.

Distinctive spatiotemporal expression patterns for neurotrophins develop in gustatory papillae and lingual tissues in embryonic tongue organ cultures

Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) mRNAs are expressed in the developing rat tongue and taste organs in specific spatiotemporal patterns. BDNF mRNA is present in the early lingual gustatory papilla epithelium, from which taste buds eventually arise, prior to the arrival of gustatory nerve fibers at the epithelium, whereas NT-3 initially distributes in the mesenchyme. However, a direct test for neural dependence of neurotrophin expression on the presence of innervation in tongue has not been made, nor is it known whether the patterns of neurotrophin expression can be replicated in an in vitro system. Therefore, we used a tongue organ culture model that supports taste papilla formation while eliminating the influence from sensory nerve fibers, to study neurotrophin mRNAs in lingual tissues. Rat tongue cultures were begun at embryonic day 13 or 14 (E13, E14), and BDNF, NT-3, nerve growth factor (NGF) and neurotrophin-4 (NT-4) mRNAs were studied at 0, 2, 3 and 6 days in culture. BDNF transcripts were localized in the gustatory epithelium of both developing fungiform and circumvallate papillae after 2 or 3 days in culture, and NT-3 transcripts were in the subepithelial mesenchyme. The neurotrophin distributions were comparable to those in vivo at E13-E16. In 6-day tongue cultures, however, BDNF transcripts in anterior tongue were not restricted to fungiform papillae but were more widespread in the lingual epithelium, while the circumvallate trench epithelium exhibited restricted BDNF labeling. The NT-3 expression pattern shifted in 6-day organ cultures in a manner comparable to that in the embryo in vivo, and was expressed in the lingual epithelium as well as mesenchyme. NGF mRNA expression was subepithelial throughout 6 days in cultures. NT-4 mRNA was not detected. The neurotrophin mRNA distributions demonstrate that temporospatial localization of neurotrophins observed during development in vivo is retained in the embryonic tongue organ culture system. Furthermore, initial neurotrophin expression in the developing lingual epithelium, mesenchyme, and/or taste papillae is not dependent on intact sensory innervation. We suggest that patterns of lingual neurotrophin mRNA expression are controlled by the influence of local tissue interactions within the tongue at early developmental stages. However, the eventual loss of restricted BDNF mRNA localization from fungiform papillae in anterior tongue suggests that sensory innervation may be important for restricting the localized expression of neurotrophins at later developmental stages, and for maintaining the unique phenotypes of gustatory papillae.