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PubMed1988

Gustatory stimulating technique for glossopharyngeal neurophysiological recordings.

A new stimulating technique for studying the gustatory sensitivity of the glossopharyngeal nerve is presented. Among its other functions the glossopharyngeal nerve relays multisensory information from the posterior tongue region. Study of the gustatory function of this nerve is therefore complicated by concomitant tactile and thermal components. Additionally taste buds innervated by the glossopharyngeal nerve are located in trenches within the foliate and circumvallate papillae which results in slow delivery and removal of tastant solutions. By delivering the stimulus or adapting solution through an infusion pump connected to a double-barrelled pipette placed into the trench of the circumvallate papilla, quick delivery and removal of the stimulus is possible. Moreover the continuous flow of solution, switched from adapting solution to stimulus and back, allows tactile components to completely adapt. Due to both the proximity of the stimulus to the receptors and the continuous flow of the solution this technique produces gustatory responses with large phasic components, as is commonly observed in other gustatory nerves. The temporal characteristics of the gustatory portion of the glossopharyngeal nerve can therefore now be more readily studied.

Action Potentials↗
PubMed1996

Ultrastructural study of the dorsal lingual epithelium of the soft-shell turtle, Trionyx cartilagineus (Chelonia, Trionychidae).

BACKGROUND: The soft-shell turtle, Trionyx cartilagineus, is classified phylogenetically to the family Trionychidae, whose members live in small rivers or ponds. The purpose of the present study was to examine the ultrastructure of the dorsal epithelium of the tongue of the soft-shell turtle and to compare the results of the observations with those reported for the tongue of other freshwater turtles. METHODS: Light microscopy, transmission electron microscopy, and scanning electron microscopy were used to examine the dorsal epithelium of the tongue of the soft-shell turtle. RESULTS: The tongue is triangular with a slightly round apex when viewed dorsally but it appears flattened when viewed laterally. Lingual papillae were visible on the dorsal surface of the tongue with some localized variations. Irregular, dome-shaped or ridge-like papillae were observed on the anterior part of the dorsal lingual surface. Large, cylindrical papillae were located along the midline of the posterior part of the tongue. Low, disk-like papillae were located on both sides of the dorsal surface of the posterior part of the tongue. Taste pores were recognizable in the center of the disk-like papillae. At higher magnification, scanning electron microscopy revealed microridges on the surface of cells located on the outermost side of the anterior part of the tongue, and the thickenings of cell margins were clearly seen. Microvilli were distributed compactly over the entire posterior part of the tongue. Light microscopy revealed that the mucosal epithelium of the anterior part of the tongue was of the keratinized, stratified squamous type, while the mucosal epithelium of the posterior part of the tongue was of the nonkeratinized, stratified cuboidal type. In the latero-posterior part of the tongue, taste buds were recognized. Transmission electron microscopy revealed that the epithelium of the anterior part of the tongue was of a typical keratinized type. Small numbers of keratohyalin granules and membrane-coating granules appeared in the cytoplasm of the shallow intermediate layer. On the apical side of the lingual papillae located on the posterior side of the tongue, cells from the intermediate layer to the surface layer of the non-keratinized epithelium contained many fine, discoidal granules. A large part of the epithelium consisted of mucous cells in the concave area on the posterior side. CONCLUSIONS: The dorsal surface and epithelium of the tongue of the soft-shell turtle differed significantly from those of other freshwater turtles, in spite of the similarity in terms of gross morphology among the tongues of such turtles.

Animals↗
PubMed1996

Effects of chorda tympani transection on long-term salt preference in hamsters.

The effects of denervation of fungiform taste buds on preference behavior were studied. Presurgery, 0.01, 0.1, 0.2, and 0.4 M NaCl, and 0.1, 0.2, and 0.4 M KCl were strongly avoided relative to water as measured with 48-h intakes. Bilateral cuts of the chorda tympani nerve (CT) resulted in a decrease in the strength of aversions to all concentrations of NaCl and to 0.2 and 0.4 M KCl. Only 0.4 M NaCl and KCl were strongly avoided after CT section. After CT section, when the animals were allowed to choose between equimolar (0.1 or 0.2 M) KCl and NaCl, they drank equal amounts, whereas intact hamsters preferred KCl to NaCl. In golden hamsters, unlike most laboratory rat strains, intake-based long-term preference behavior for salts is significantly affected by selective gustatory deafferentation.

Animals↗
PubMed1994

Amiloride sensitivity in the neonatal rat.

Amiloride-sensitive sodium channels in taste buds appear to play a key role in the response to NaCl stimulation, at least in adult rats. The researchers examined whether neonatal rats, which display an exaggerated preference for hypertonic NaCl solutions, lack functional amiloride-sensitive sodium channels. NaCl intake was significantly reduced by amiloride pretreatment, but water and ammonium chloride, NH4Cl, were unaffected. The researchers assessed whether the early appearance of amiloride sensitivity was mediated by effects on chorda tympani (CT) activity by sectioning the CT before testing. CT transection reduced intake of NaCl solutions and eliminated evidence of amiloride sensitivity. Amiloride sensitivity was also assessed by recording of whole-nerve CT activity at 8-11 days of age; the response to NaCl stimulation was significantly suppressed by amiloride. These data indicate that amiloride-sensitive sodium channels develop earlier than previously believed.

Age Factors↗
PubMed1995

The frog taste disc: a prototype of the vertebrate gustatory organ.

The frog taste disc (TD) is apparently the largest gustatory organ found in vertebrates and seems to differentiate into a specialized variety of the prototypic scheme of the taste bud. An explanation for this unusual organization is lacking although it is possible to speculate the existence of environmental and nutritional requirements. Up to the present time, the most common model of the TD was based on two main cell types (sensory and sustentacular). This model may oversimplify the morphology of this structure since more numerous cell types have been described. We now propose a new model of the TD, based on comprehensive data on the ultrastructure of the organ obtained in the last 20 years. The main conclusions are the following: (1) the TD is a pluristratified epithelium with a general organization similar to that of the olfactory and vomeronasal epithelium; (2) it has skeleton composed of three different types of epithelial cells; (3) the chemoreceptorial surface is covered by different microenvironments; (4) three different types of neuro-epithelial systems are present; the type II is an 'open' sensory cell with axonal contacts devoid of vesicles; the type III is an 'open' sensory cell with synaptic-like junctions; the type i.v. is a 'closed' sensory cell with a 'Merkel-neurite complex'; (5) the nerve fibers in the basal plexus are mostly cholinergic while the peridiscal nerve fibers are mostly peptidergic. The presence of several cell types in the TD must be considered using these large receptors in electrophysiological studies or as a source of isolated cells, and their complexity must induce caution in the interpretation of the data. Text books of histology usually describe the peripheral structures associated with taste as very simple: an idea that probably must be revised. A taste organ is a highly complex structure composed of several sensory systems and a comparative approach can aid comprehension of its general organization. The study of the 'large taste organs' present in some species of amphibians can provide useful data for knowledge of the gustatory system of vertebrates.

Animals↗
PubMed1996

Somatotopic organization of the facial lobe of the sea catfish Arius felis studied by transganglionic transport of horseradish peroxidase.

To reveal the somatotopical organization of the facial lobe (FL), a primary medullary gustatory nucleus in the sea catfish Arius felis, the central projections of the peripheral rami of the facial nerve innervating taste buds located across the entire body surface and rostral oral regions were traced by means of horseradish peroxidase neurohistochemistry. The maxillary barbel, lateral mandibular barbel, medial mandibular barbel, and trunk-tail branches project to four different longitudinal columns (i.e., lobules) extending rostrocaudally in the FL. The trunk-tail lobule, which is located dorsolateral to the barbel lobules, lies in the anterior two-thirds of the FL. The tail is represented in a more rostral portion of the trunk-tail lobule than the trunk, indicating that the rostrocaudal trunk axis is represented in the trunk-tail lobule in a posteroanterior axis. The pectoral fin branch ends in an intermediate region of the FL, whereas the hyomandibular, ophthalmic, lower lip, upper lip, and palatine branches terminate in discrete regions of the caudal one-third of the FL. These results reveal a sharply defined somatotopical organization of the FL of Arius and support the hypothesis that the number and lengths of the barbel lobules within the FL of catfishes are directly related to the number and relative lengths of the barbels. An additional subcolumn, the intermediate nucleus of the FL (NIF), which develops in the medioventral region of the caudal two-thirds of the FL, receives projections in a diffuse somatotopical fashion from the barbels, lower lip, and palatine branches. Trigeminal fibers of the barbel and lower lip branches project in a somatotopic fashion to the FL. The present findings suggest that the FL of Arius is highly organized somatotopically to detect, by tropotaxis, precise spatial information concerning taste and tactile stimuli in the environment.

Animals↗
PubMed1971

Conductance change associated with receptor potentials of gustatory cells in rat.

The electrical properties of gustatory cells and cells which do not respond to chemical stimuli in the taste bud of fungiform papillae in rats were studied by means of intracellular microelectrodes. Neither of these cell types showed spike electrogenesis. Gustatory cells showed a depolarization, the receptor potential, associated with an increase in the membrane conductance in response to NaCl, sucrose, and HCl, whereas quinine produced a decrease in the conductance together with an increase in the receptor potential magnitude. The reversal point of the receptor potential in response to NaCl or KCl was close to zero membrane potential, but in the case of quinine it was at a more negative potential level than the resting potential. From these results two receptive processes are postulated in the gustatory cell membrane. When the gustatory cells were stimulated for a long duration by concentrated NaCl or sucrose, receptor potentials showed adaptation with decrease in magnitude, but adaptation of the responses to HCl and quinine were hardly detected. Adaptation of the receptor potential was not correlated with conductance change.

Adaptation, Biological↗
PubMed2004

Structural organization of subgemmal neurogenous plaques in foliate papillae of tongue.

Little is known about subgemmal neurogenous plaques in the foliate papillae of tongue, which prompted the present investigation. The plaques were immunohistochemically studied in biopsies from 16 adults with the use of neural, stromal, basement membrane, and cell-cycle markers. They displayed a zonal pattern of organization. The neurofibroma-like superficial zone expanded lamina propria and was contiguous to the epithelium covering the papillae. It consisted of tangled composites of S-100 protein-positive spindled cells and fibrils stained for protein gene product 9.5 and neurofilament protein. The composites also expressed the CD56 antigen, could be traced into overlying taste buds, were associated with abundant laminin, and were intermingled with scattered dendritic cells expressing factor XIIIalpha and with mast cells decorated on staining for CD117. Similar composites, but arranged in fascicles and invested by epithelial membrane antigen-positive and collagen IV-positive sheaths, characterized the deep zone of the plaques. Intrafascicular CD57-positive myelin-like annuli and CD34-positive spindled cells were also features of this zone. The sheaths and, most often, the CD57-positive annuli and CD34-positive cells were progressively spread apart toward the intermediate zone of the plaques and were lost superficially. Ki67 and Bcl-1 were not expressed in the plaques. The results suggest that composites of Schwann cells and unmyelinated axons make up the superficial bulk of the plaques, whereas perineurial cells, endoneurial fibroblasts, and myelinated axons are present more deeply. It is possible that the composites achieve neuroeffector relationships and are not neoplastic. Trophic influences from gustatory nerve fibers could play a role in the development of the plaques.

Adult↗
PubMed2003

Identification of beta and gamma subunits of laminins localized in the basement membrane of rat circumvallate papillae.

Taste bud cells have elongated shape and are anchored to the basement membrane. Here we analyzed the subunits of laminin, a major component of the basement membrane, in circumvallate papillae of rat tongue. Two beta subunits, beta1 and beta2, were detected by RT-PCR, of which the beta2 subunit was immunohistochemically identified as a major component of the basement membrane. The gamma1 subunit was also immunohistochemically identified as a major component. Profiles of these two laminin subunits were nearly the same as that obtained by an anti-laminin antibody, indicating that laminins of the basement membrane in the papillae contain beta2 and gamma1 as major beta and gamma subunits, respectively. As potential receptors for these laminin ligands, integrin subunits were analyzed by RT-PCR. Three integrin beta subunit species, beta1, beta4, and beta5, were shown to be expressed in the epithelium of the circumvallate papillae by RT-PCR among the five species examined.

Animals↗
PubMed2004

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.

Animals↗
PubMed1998

PROP taster status is related to fat perception and preference.

Individuals who are sensitive to the bitter compound 6-n-propylthiouracil (PROP) are also more sensitive to selected bitter and sweet substances, to sharp foods and to the trigeminal irritant capsaicin. PROP tasters have a greater density of fungiform papillae, and it is speculated that they also have more trigeminal innervation. Since oral texture perception is also mediated, in part, by trigeminal fibers, it has been proposed that individual differences in fat perception might also be linked to PROP taster status and taste bud density. This work tests the hypothesis that individuals who are PROP tasters: (i) have a higher density of fungiform papillae; (ii) are more sensitive to capsaicin; and (iii) have increased ability to discriminate differences in fat content in salad dressing. Hedonic ratings for the salad dressing were also collected and related to the perceptual judgments. Individual subjects were classified as PROP nontasters, medium tasters or supertasters (n = 25/group) by comparing their psychophysical function for PROP to that of NaCl. Papillae densities (papillae/cm2) were significantly different among the three taster groups (p < 0.0001), and were highest among the supertasters. Both medium tasters and supertasters perceived more oral burn from capsaicin than did nontasters at concentrations of 50, 70 and 100 ppm (p < 0.0001). Medium tasters and supertasters could also discriminate differences in fat content between 40% fat and 10% fat salad dressings (p < 0.005), but the nontasters could not. Although medium and supertasters showed no preference for either dressing, the nontasters preferred the 40% fat sample. The reasons for these latter findings are unclear at present. These data support the hypothesis that fat perception and preference can be linked to genetic and anatomical differences between individuals.

Dietary Fats↗
PubMed1971

Peripheral interactions among single papilla inputs to gustatory nerve fibers.

Recordings were made from single fibers of the rat chorda tympani nerve while the peripheral receptor fields were mapped using a stimulator developed to stimulate single fungiform papillae which in the rat contain a solitary taste bud. The results indicate that several fungiform papillae may supply input to a single fiber, and the most sensitive papilla of these provided, on the average, about one-half of the response of that fiber to stimulation of the entire tongue. The magnitude of the response to each concentration of stimulus and the shape of the concentration-response curves differ among papillae innervated by the same fiber. If one of the papillae supplying input to the fiber was stimulated individually with NaCl solution, application of this stimulus to the tongue surface surrounding the isolated papilla resulted in enhancement of the fiber response. If the papilla was stimulated with NaCl and potassium benzoate solution was applied to the surround, a depression of the response occurred. The excitatory input of the cationic stimuli and the depressing influence of the anionic stimuli interacted to determine the resultant steady-state impulse frequency of the single afferent fiber. A hypothetical model involving the summation of generator currents along the unmyelinated terminals of the single afferent neuron is presented as a speculative explanation of the integration of inputs from several receptors innervated by the same single fiber.

Action Potentials↗
PubMed1996

Experimental trigeminal nerve injury.

The successful reinnervation of peripheral targets after injury varies with the axonal population of the nerve that is injured and the extent of the dislocation of its central component from the peripheral endoneurial tube. Larger-diameter axons such as those supplying mechanoreceptors recover more readily than narrower axons such as those supplying taste. A complex, bi-directional interaction between lingual epithelium and sprouting nerve results in the redifferentiation of taste buds after denervation. Dentin and the dental pulp provide a strong attraction to sprouting nerves and will become reinnervated from collateral sources if recovery of the original innervation is blocked. The most effective repair technique for transected lingual nerves is one which brings the cut ends together rather than one that provides a temporary bridge. Injuries can result in cell death in the trigeminal ganglion but only if the injury is severe and recovery is prevented. Lesser damage results in chromatolysis and the increased expression of neuropeptides. All nerve injuries bring about changes in the trigeminal nucleus. These occur as changes in receptive field and the incidence of spontaneously active neurons, effects which are consistent with the unmasking of existing afferents. These functional changes are short-lived and reversible. Morphologically, nerve injury results in terminal degeneration in the nuclei and an increased expression of the c-Fos gene and some neuropeptides. Only a chronic constriction injury induces behavioral changes. The adult trigeminal system retains considerable plasticity that permits it to respond successfully to nerve injury. Much remains to be learned about this response, particularly of the trophic factors that control peripheral recovery and the central response to more severe injuries.

Adult↗
PubMed1986

Central projections of the hamster superior laryngeal nerve.

The superior laryngeal nerve (SLN) is known to innervate taste buds on the epiglottis of several mammalian species. Because of an increasing interest in the physiology of the gustatory system of hamsters, the brainstem projections of the SLN were investigated in this species. Crystallized HRP was applied to the proximal portion of the cut SLN or to one of its five distal branches. Anterograde transganglionic transport of HRP revealed afferent fibers of the SLN projecting into the ipsilateral solitary tract (ST) from 0.3 to 3.0 mm caudal to the dorsal cochlear nucleus (DCN), with the major area of termination in the nucleus of the solitary tract (NST) between 0.6 and 1.6 mm caudal to DCN. Some afferent fibers crossed the midline approximately 2.0 mm caudal to DCN to terminate contralaterally within the NST. Efferent cell bodies were retrogradely labeled within the nucleus ambiguus (NA) and in and around the more rostral portions of NST. There were five identifiable distal branches of SLN, termed A1, A2, M1, M2 and P, from anterior to posterior. Afferent fibers were carried in A2 and P, whereas efferent fibers were evident in all five branches. The heaviest projection from the NA occurred in the two middle branches (M1 and M2) and that from the NST in the posterior branch (P). Afferent projections of the Xth cranial nerve, along with those from the VIIth and IXth, into the NST provide a neural substrate for the integration of sensory inputs related to a number of oral and respiratory reflexes.

Animals↗
PubMed1997

Parallel medullary gustatospinal pathways in a catfish: possible neural substrates for taste-mediated food search.

Taste and tactile fibers in the facial nerve of catfish innervate extraoral taste buds and terminate somatotopically in the facial lobe (FL)-a medullary structure crucial for gustatory-mediated food search. The present study was performed to determine the neural linkages between the gustatory input and the spinal motor output. Spinal injections of horseradish peroxidase (HRP) label spinopetal cells in the octaval nuclei, the nucleus of the medial longitudinal fasciculus, and reticulospinal neurons (Rsps) in the brainstem medial reticular formation (RF), including the Mauthner cell. A somatotopically organized, direct faciospinal system originating from superficial cells scattered in the lateral lobule of the facial lobe (ll) is also labeled. The brainstem reticulospinal cells are segmentally organized into 14 clusters within eight segments of the reticular formation and includes one cluster (RS5) directly ventral to the FL. Injections of HRP or fluorescent tracers into the medial lobule of the FL label a facioreticular projection terminating around the Rsps of RS5. DiI injections into this area of the RF retrogradely label deeply situated bipolar neurons, especially in the medial and intermediate lobules of the FL. Electrophysiological recordings in and around RS5 show units with large receptive fields and with responses to chemical and tactile stimulation. The FL projects to the spinal cord via two pathways: (1) a topographically organized direct faciospinal pathway, and (2) an indirect facioreticulospinal pathway in which reticular neurons process and integrate gustatory information before influencing spinal circuitry for motor control during food search.

Animals↗
PubMed1980

The cytologic composition of primate laryngeal chemosensory corpuscles.

The present study defines the cellular composition of chemosensory corpuscles (taste buds) present in the mucosa overlying the arytenoid cartilages of the larynx in serial sections studied by electron microscopy. Three cell types can be defined in such chemosensory corpuscles. Basal cells are relatively undifferentiated in terms of cytologic characteristics and have been identified presumptively as the stem cells for differentiation of the other two cell types. Sustentacular or supporting cells are characterized by the presence of apical electron-opaque granules that apparently are extruded into the lumen of the pore, and are, thus, the source of the electron-opaque extracellular material present between the microvilli protruding from the apex of the cells into the chemosensory corpuscle pore. The chemosensory cells are characterized by the presence of synaptic specializations with appended nerve fibers at the base of the cell, the presence of specific cytoplasmic secretory granules, as well as numerous clusters of typical synaptic vesicles, and apical microvilli that extend into the pore of the corpuscle. Other cell profiles encountered in chemosensory corpuscles are cells in various stages of degeneration or transition, to one of the other cell types, from undifferentiated basal cells. Two types of synaptic specializations have been identified as being associated with chemosensory cells. The first is characterized by an increased electron-opacity of the respective plasma membranes of nerve fiber and chemosensory cell. In this situation numerous synaptic vesicles usually are present in the cytoplasm of the chemosensory cell subjacent to the membrane densities. A second type of synaptic association is characterized by the presence of subsurface cisterns of agranular endoplasmic reticulum in the cytoplasm of the chemosensory cell, and numerous synaptic vesicles often are present in the axoplasm of the subjacent neurite, suggesting a reciprocal synapse. These findings are interpreted to indicate that only one cell type is specialized for transducing chemical signals into neural activity and that this cell is modulated by activity of the nervous system.

Animals↗
PubMed1989

Gustatory innervation in the rabbit: central distribution of sensory and motor components of the chorda tympani, glossopharyngeal, and superior laryngeal nerves.

Although rabbits have been used extensively in neurophysiological studies of the gustatory system, there is little information about the anatomical organization of taste in this species. Afferent and efferent central connections of three nerves innervating oral or laryngeal taste buds in the rabbit, including the chorda tympani (CT), the lingual-tonsillar branch of the glossopharyngeal (IX), and the superior laryngeal nerve (SLN), were traced by means of horseradish peroxidase neurohistochemistry. After entering the brainstem, most afferent fibers of CT, IX, and SLN turned caudally in the solitary tract, with fibers of the CT terminating in the nucleus of the solitary tract from 1.0 mm rostral to 3.8 mm caudal to the caudal border of the dorsal cochlear nucleus. There was terminal label from the CT also in the principal trigeminal nucleus. There was terminal label from the CT also in the principal trigeminal nucleus and the oral and intermediate divisions of the spinal trigeminal nucleus. Preganglionic parasympathetic cell bodies of the superior salivatory nucleus were labeled retrogradely in the reticular formation ventral to the rostral pole of the solitary nucleus. Afferent fibers of the IXth nerve terminated in the solitary nucleus from 0.6 mm rostral to 5.0 mm caudal to the caudal border of the dorsal cochlear nucleus. There were also labeled terminals in the principal trigeminal nucleus and in all three divisions of the spinal trigeminal nucleus. Cell bodies composing the inferior salivatory nucleus were labeled in and around the solitary nucleus and subadjacent reticular formation just rostral to the caudal border of the dorsal cochlear nucleus. There were also a few lightly labeled cells within the nucleus ambiguus at its most rostral extent. Afferent fibers of the SLN terminated in the solitary nucleus from 1.2 to 6.8 mm caudal to the dorsal cochlear nucleus. There was also some terminal label in the intermediate and caudal divisions of the spinal trigeminal nucleus. Many cells were retrogradely labeled in the nucleus ambiguus following application of HRP to the SLN and a few cells were labeled in and around the solitary nucleus just caudal to the dorsal cochlear nucleus. These three nerves show an overlapping rostral to caudal distribution of afferent input within the nucleus of the solitary tract that may be related to their gustatory and visceral functions.

Animals↗
PubMed2001

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.

Animals↗