Monoamine-containing basal cells in the taste buds of the newt Triturus pyrrhogaster

Neurotransmitters and synaptic components in the Merkel cell-neurite complex, a gentle-touch receptor

Merkel cells are an enigmatic group of rare cells found in the skin of vertebrates. Most make contacts with somatosensory afferents to form Merkel cell-neurite complexes, which are gentle-touch receptors that initiate slowly adapting type I responses. The function of Merkel cells within the complex remains debated despite decades of research. Numerous anatomical studies demonstrate that Merkel cells form synaptic-like contacts with sensory afferent terminals. Moreover, recent molecular analysis reveals that Merkel cells express dozens of presynaptic molecules that are essential for synaptic vesicle release in neurons. Merkel cells also produce a host of neuroactive substances that can act as fast excitatory neurotransmitters or neuromodulators. Here, we review the major neurotransmitters found in Merkel cells and discuss these findings in relation to the potential function of Merkel cells in touch reception.

Developing a sense of taste

Taste buds are found in a distributed array on the tongue surface, and are innervated by cranial nerves that convey taste information to the brain. For nearly a century, taste buds were thought to be induced by nerves late in embryonic development. However, this view has shifted dramatically. A host of studies now indicate that taste bud development is initiated and proceeds via processes that are nerve-independent, occur long before birth, and governed by cellular and molecular mechanisms intrinsic to the developing tongue. Here we review the state of our understanding of the molecular and cellular regulation of taste bud development, incorporating important new data obtained through the use of two powerful genetic systems, mouse and zebrafish.

Neurochemical characterization of sea lamprey taste buds and afferent gustatory fibers: presence of serotonin, calretinin, and CGRP immunoreactivity in taste bud bi-ciliated cells of the earliest vertebrates

Neuroactive substances such as serotonin and other monoamines have been suggested to be involved in the transmission of gustatory signals from taste bud cells to afferent fibers. Lampreys are the earliest vertebrates that possess taste buds, although these differ in structure from taste buds in jawed vertebrates, and their neurochemistry remains unknown. We used immunofluorescence methods with antibodies raised against serotonin, tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA), glutamate, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), calretinin, and acetylated alpha-tubulin to characterize the neurochemistry and innervation of taste buds in the sea lamprey, Petromyzon marinus L. For localization of proliferative cells in taste buds we used bromodeoxyuridine labeling and proliferating cell nuclear antigen immunohistochemistry. Results with both markers indicate that proliferating cells are restricted to a few basal cells and that almost all cells in taste buds are nonproliferating. A large number of serotonin-, calretinin-, and CGRP-immunoreactive bi-ciliated cells were revealed in lamprey taste buds. This suggests that serotonin participates in the transmission of gustatory signals and indicates that this substance appeared early on in vertebrate evolution. The basal surface of the bi-ciliated taste bud cells was contacted by tubulin-immunoreactive fibers. Some of the fibers surrounding the taste bud were calretinin immunoreactive. Lamprey taste bud cells or afferent fibers did not exhibit TH, GABA, glutamate, or NPY immunoreactivity, which suggests that expression of these substances evolved in taste buds of some gnathostomes lines after the separation of gnathostomes and lampreys.

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.

Evolution of the structure and function of the vertebrate tongue

Studies of the comparative morphology of the tongues of living vertebrates have revealed how variations in the morphology and function of the organ might be related to evolutional events. The tongue, which plays a very important role in food intake by vertebrates, exhibits significant morphological variations that appear to represent adaptation to the current environmental conditions of each respective habitat. This review examines the fundamental importance of morphology in the evolution of the vertebrate tongue, focusing on the origin of the tongue and on the relationship between morphology and environmental conditions. Tongues of various extant vertebrates, including those of amphibians, reptiles, birds and mammals, were analysed in terms of gross anatomy and microanatomy by light microscopy and by scanning and transmission electron microscopy. Comparisons of tongue morphology revealed a relationship between changes in the appearance of the tongue and changes in habitat, from a freshwater environment to a terrestrial environment, as well as a relationship between the extent of keratinization of the lingual epithelium and the transition from a moist or wet environment to a dry environment. The lingual epithelium of amphibians is devoid of keratinization while that of reptilians is keratinized to different extents. Reptiles live in a variety of habitats, from seawater to regions of high temperature and very high or very low humidity. Keratinization of the lingual epithelium is considered to have been acquired concomitantly with the evolution of amniotes. The variations in the extent of keratinization of the lingual epithelium, which is observed between various amniotes, appear to be secondary, reflecting the environmental conditions of different species.

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.

Recent progress in studies on Merkel cell biology

Merkel cells ubiquitously distribute in the skin of vertebrates, from cyclostomes to mammals. It is well known that mammalian Merkel cells coupled with axon terminals of type I sensory nerve fibers form slowly adapting mechanoreceptors, Merkel endings, within the epidermis. However, there are still many unresolved problems in the biology of Merkel cells. We reviewed recently acquired knowledge about the histochemical nature of Merkel cell granules, the morphological heterogeneity of Merkel cells and the roles of neurotrophins and their receptors for the development and survival of the cells. We discuss the functional significance of Merkel cell granules and the heterogeneity of Merkel cell populations.

Merkel cells are responsible for the initiation of taste organ morphogenesis in the frog

Taste organs in the frog have a distinctive cell type located exclusively in the basal portion. In the same fashion as type III cells in mammalian taste buds, these basal cells show immunoreactivity for serotonin antibody. Further, these cells are morphologically similar to epidermal Merkel cells. To determine the significance of these serotonergic basal cells, we examined the early development of taste organs during metamorphosis of the frog by focusing on the origin and possible roles of serotonergic basal cells. For convenience of description, five stages of development (metamorphic stage to climax stages A-D) are defined. In the metamorphic stage, a few noninnervated Merkel cells appear at the upper layer of the lingual epithelium. No neuronal elements are seen in the epithelium at this stage. At climax stages A-B, immature fungiform papillae become discernible in the dorsal surface of the tongue, where the Merkel cells are located. Merkel cells then move downward and extend their cytoplasmic processes toward the basal lamina. These cells are identified by their intense immunoreactivity for serotonin. During the later stages, many nerve fibers in the subepithelial connective tissue approach the epithelium containing Merkel cells. At climax stages C-D, Merkel cells extend cytoplasmic processes along the basal lamina toward the center of the newly forming fungiform papillae. The morphology of these Merkel cells exactly coincides with that of serotonergic basal cells in adult taste organs. Profuse exocytotic release of dense-cored granules of Merkel cells toward the nerve fibers through the basal lamina is frequently seen in these stages. The present study indicates that serotonergic basal cells are derived from intraepithelial Merkel cells, which act as target sites for growing nerves and may be responsible for the initiation of taste organ morphogenesis.

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

BACKGROUND

Taste buds are assemblies of slender epithelial cells that receive chemical stimuli from the outer (oral) environment. In contrast to the large and well documented information on the morphology of taste buds in adult humans and animals, there are only a few reports on fetal ones, and ultrastructural studies of prenatal human taste buds are lacking completely. Therefore, the present investigation has been carried out to study the taste bud primordium, its morphological changes including synaptogenesis, cell differentiation, and taste pore formation from the time of the onset of taste bud formation around the 8th week until the 15th postovulatory week.

METHODS

Taste bud primordia of 42 human embryonic/fetal tongues have been examined by means of transmission electron microscopy.

RESULTS

Nerve fibers approach the lingual epithelium between the 6th and 7th postovulatory week. They penetrate the basal lamina during the 8th week and form synapses with poorly differentiated, elongated, epithelial cells. By the 12th week, more differentiated cell types are seen: 1) electron-dense cells resembling type III cells of the adult taste bud containing large numbers of dense-cored vesicles (80-150 nm in diameter); 2) electron-dark cells with well developed endoplasmic reticulum and many apical mitochondria, being candidates for type II cells. Basally, these cells have foot-like processes containing dense-cored vesicles (120-200 nm in diameter), but they do not synapse to nerve fibers. Type I cells, characterized by apically located dense secretory granules, are not observed. First shallow grooves above the taste bud primordium are found around the 10th week. Untypically differentiated apical cellular processes extend onto the surface. Most of the taste pores develop around the 14th to 15th week. In the taste pit, mucous material is not present during the first 15 weeks of gestation. Synapses between cells and afferent nerve fibers were found by the 8th week, reaching a maximum around the 12th to 13th week.

CONCLUSIONS

The early presence of taste bud cells containing dense-cored vesicles suggests an at least dual function of embryonic/ fetal taste buds: First, from the 8th until the 14th week, non-gustatory, paracrine functions should be considered. After the 14th week of gestation, when typical taste pores are present, the taste buds possibly start their gustatory function. Differentiated marginal cells are possibly involved in the formation of the taste pore. The lack of type I cells producing the mucous material in the taste pit indicates that the taste bud has not achieved a fully developed function until the 15th week of gestation.

Enkephalin immunoreactivity in the paraneurons of the tiger salamander (Ambystoma tigrinum) tongue

Immunohistochemical tests have demonstrated the presence of leu-5-enkephalin and other bioactive compounds (serotonin and neuron-specific enolase) in the basal cells of lingual taste buds in Ambystoma tigrinum; there was also a weak reaction for met-5-enkephalin. Similar reactions were obtained from particular cells dispersed within the lingual epithelium, which are provisionally identified as Merkel cells.

Ultrastructure of Merkel-like cells labeled with carbocyanine dye in the non-taste lingual epithelium of the axolotl

Fluorescent carbocyanine dye (diI), applied to the glossopharyngeal (IX) nerve of the axolotl, transneuronally labeled solitary cells in the non-taste lingual epithelium. With diaminobenzidine (DAB), the diI was photoconverted to a dark, electron-dense product. The labeled cell had a large nucleus with invaginations, dense-cored vesicles in the cytoplasm, and finger-like processes. These are reminiscent of morphological features of cutaneous Merkel cells, suggesting that solitary cells innervated by the IX nerve are associated with mechanosensory function of the IX nerve system.

Aspects of vertebrate gustatory phylogeny: morphology and turnover of chick taste bud cells

The taste bud is a receptor form observed across vertebrates. The present report compares chick taste buds to those of other vertebrates using light and electron microscopy. Unlike mammals, but common to many modern avians, the dorsal surface of chick anterior tongue lacks taste papillae and taste buds. Ultrastructurally, chick buds located in the anterior floor of the mouth (as in some reptiles and amphibians) and palate contain dark, intermediate, light, and basal cell types. Dark, intermediate, and light cells extend microvilli into intragemmal lumina and pores communicating with the oral cavity. As specialized features, dark cell apices lack dense granules and exhibit short microvilli relative to light and intermediate cells. Dark cell cytoplasmic fingers envelop intragemmal nerve fibers and cells as in other species, and sometimes contain abundant clear vesicles. Nerve profile expansions often are located adjacent to dark, intermediate, and light cell nuclei. Classical afferent synaptic contacts are rarely observed. Taste cell turnover is suggested by mitotic and degenerating figures in chick buds. In addition, tritiated thymidine injected into hatchlings, whose anterior mandibular oral taste bud population approximates that in adults, reveals a turnover rate of about 4.5 days. This is about half that observed in altricial mammals, reflecting a species difference or developmental factor in precocial avians. It is concluded that chick taste buds exhibit morphologic features common to other vertebrate buds with specializations reflecting the influences of niche, glandular relations, and/or age.

Merkel-like basal cells in Necturus taste buds contain serotonin

Several types of cells have been identified in vertebrate taste buds, including dark cells, light cells, intermediate cells, type III cells, and basal cells. The physiological roles of these cell types are not well understood, especially those of basal cells. In this paper we show that there are two types of basal cells in taste buds from Necturus maculosus. One type of basal cell is an undifferentiated cell, presumably a stem cell. By combining light microscopic immunocytochemistry with electron microscopy, we show that the other type of basal cell is positive for serotonin-like immunoreactivity and that these cells have ultrastructural features similar to those found in cutaneous Merkel cells. Based on these findings, and the fact that the Merkel-like taste cells have been shown to make synaptic contacts with adjacent taste cells and with innervating nerve fibers, we conclude that these Merkel-like basal taste cells are serotonergic interneurons.

Ultrastructure of the taste disc in the red-bellied toad Bombina orientalis (Discoglossidae, Salientia)

The taste disc of the red-bellied toad Bombina orientalis (Discoglossidae) has been investigated by light and electron microscopy and compared with that of Rana pipiens (Ranidae). Unlike the frog, B. orientalis possesses a disc-shaped tongue that cannot be ejected for capture of prey. The taste discs are located on the top of fungiform papillae. They are smaller than those in Ranidae, and are not surrounded by a ring of ciliated cells. Ultrastructurally, five types of cells can be identified (mucus cells, wing cells, sensory cells, and both Merkel cell-like basal cells and undifferentiated basal cells). Mucus cells are the main secretory cells of the taste disc and occupy most of the surface area. Their basal processes do not synapse on nerve fibers. Wing cells have sheet-like apical processes and envelop the mucus cells. They contain lysosomes and multivesicular bodies. Two types of sensory cells reach the surface of the taste disc; apically, they are distinguished by either a brush-like arrangement of microvilli or a rod-like protrusion. They are invaginated into lateral folds of mucus cells and wing cells. In contrast to the situation in R. pipiens, sensory cells of B. orientalis do not contain dark secretory granules in the perinuclear region. Synaptic connections occur between sensory cells (presynaptic sites) and nerve fibers. Merkel cell-like basal cells do not synapse onto sensory cells, but synapse-like connections exist between Merkel cell-like basal cells (presynaptic site) and nerve fibers.

Immunocytochemical survey of putative neurotransmitters in taste buds from Necturus maculosus

To investigate synaptic mechanisms in taste buds and collect information about synaptic transmission in these sensory organs, we have examined taste buds of the mudpuppy, Necturus maculosus for the presence of neurotransmitters and neuromodulators. Immunocytochemical staining at the light microscopic level revealed the presence of serotonin-like and cholecystokinin-like (CCK) immunoreactivity in basal cells in the taste bud. Nerve fibers innervating taste buds were immunoreactive for vasoactive intestinal peptide-like (VIP), substance P-like, and calcitonin gene-related peptide-like (CGRP) or compounds closely related to these substances. Immunoreactivity for tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) in the taste cells and nerve fibers was absent. These data suggest that serotonin, CCK, VIP, substance P, and CGRP are involved in synaptic transmission or neuromodulation in the peripheral organs of taste. No evidence was found for cholinergic or adrenergic mechanisms on the basis of the absence of immunocytochemical staining for key enzymes involved in these two transmitter systems.

Development of taste organs in Rana temporaria. Transmission and scanning electron microscopic study

In tadpoles of Rana temporaria two successive generations of taste organs: taste buds (TBs) and taste disks were studied. The TBs are located in the apices of oral premetamorphic papillae. Each TB consists of vertically elongated receptor cells (probably taste) and supporting cells. No basal (Merkel) cells were observed there. Morphologically, two types of receptor cells have been distinguished in a TB. Complete synaptic structures were observed in the TBs between the 30th and 41st of Gosner's (1960) developmental stages, though synaptic vesicles occur already at stage 26. The taste disks consist of taste cells, supporting cells and basal cells. Dense-core synaptic vesicles occur in the gustatory cells at stage 41, but complete synaptic structures were not observed earlier than at stage 45. Synaptic junctions observed at some of the gustatory cells seem to be reciprocal (bidirectional).

HVEM serial-section analysis of rabbit foliate taste buds: I. Type III cells and their synapses

Serially sectioned rabbit foliate taste buds were examined with high voltage electron microscopy (HVEM) and computer-assisted, three-dimensional reconstruction. This report focuses on the ultrastructure of the type III cells and their synapses with sensory nerve fibers. Type III cells have previously been proposed to be the primary gustatory receptor cells in taste buds of rabbits and other mammals. Within rabbit foliate taste buds, type III cells constitute a well-defined, easily recognizable class and are the only taste bud cells observed to form synapses with intragemmal nerve fibers. Among 18 type III cells reconstructed from serial sections, 11 formed from 1 to 6 synapses each with nerve fibers; 7 reconstructed type III cells formed no synapses. Examples of both convergence and divergence of synaptic input from type III cells onto nerve fibers were observed. The sizes of the active zones of the synapses and numbers of vesicles associated with the presynaptic membrane specializations were highly variable. Dense-cored vesicles 80-140 nm in diameter were often found among the 40-60 nm clear vesicles clustered at presynaptic sites. At some synapses, these large dense-cored vesicles appeared to be the predominant vesicle type. This observation suggests that there may be functionally different types of synapses in taste buds, distinguished by the prevalence of either clear or dense-cored vesicles. Previous investigations have indicated that the dense-cored vesicles in type III cells may be storage sites for biogenic amines.