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Most mammalian taste buds are located in fungiform, foliate
and circumvallate papillae on dorsal surface of the tongue. Two
large, well-defined fields of vallate papillae are situated on
each side of the posterior tongue in guineapigs, while in other
rodents only a single vallate papilla is present on the midline
of the tongue root. Taking the advantage of this special
arrangement of guinea pig vallate papillae, we studied the fine
structure, immunocytochemistry and nerve distribution of vallate
taste buds. The present study includes fiveparts: 1) Fine
structure of vallate taste buds, 2) Immunohistochemical studies
of neuropeptides and neurotransmitters in vallate papillae, 3)
Immunoelectron microscopic studies of PGP9.5- and CGRP-
immunoreactive elements in vallate taste buds, 4) Unilateral
innervation of vallate papillae and 5) Apoptosis in vallate
papillae after neurectomy. Ultrastructural studies revealed
four distinct cell types (type I, II, III and basal) in guinea
pig vallate taste buds. Type I cells were narrow elongated
cells containing an oval nucleus and possessed large, electron-
dense granules(about 300nm) apically. Type II cells were
characterized by a large and roundnucleus and a conspicuous
stack of smooth endoplasmic reticulum in the supra- and infra-
nuclear regions. Type III cells made synaptic contacts with
nerveterminals and contained dense-cored vesicles (about 90nm)
accumulating in the presynaptic areas. Basal cells were situated
at the base of buds and considered to be precursors of the other
types of cells. Indirect immunohistochemical detection of
protein gene product 9.5 (PGP9.5), calcitonin gene-related
peptide (CGRP), substance P (SP), vasoactive intestinal
polypeptide (VIP), galanin and 5-hydroxytryptamine (5HT) showed
that numerous PGP9.5- and CGRP-immunoreactive nerve fibers were
found to form plexuses in the lingual epithelium both
intragemmally and extragemmally. In the subgemmal connective
tissue, PGP9.5-, CGRP-and SP-immunoreactive nerve fibers were
also observed. Besides, some taste cells were PGP9.5 and 5HT
immunoreactive. Neither VIP nor galanin immunoreactivity was
detected in nerve fibers and taste cells. The results from
immuno-electron microscopy on the location of PGP9.5 and CGRP
showed that immunoreactivity for PGP9.5 was localized in all the
neural elementsand the cytoplasm of the type III cells. An
intensive PGP9.5-immunoreactivenerve fibers formed a
subepithelial plexus which issued branches upwards throughthe
basal lamina then formed intragemmal and extragemmal networks.
CGRP-immunoreactivity was found in nerve terminals making
synaptic contacts with type III cells. HRP tracing and
unilateral glossopharyngeal neurectomy were
performedsimutaneously to determine the innervation pattern by
primary afferent nerve fibers and the neurotrophic effect on
taste cells in guinea pig vallate taste buds. After the
injection of HRP into the proximal ends of the right
glossopharyngeal nerve, light microscopy revealed that the HRP-
labelled nerve fibers innervated the vallate taste buds of the
injected side. Most of the cells in tastebuds were labelled with
HRP. At the ultrastructural level, the reaction products were
identified in type I, II and III cells, but not basal cells. Our
neurectomical results showed that, on the denervated side, the
taste buds decreasedsignificantly in number during the first 2
wk, and disappeared completely by week 3; no mature taste buds
were present even 24 wk afterneurectomy. Finally,based upon the
speculation that gustatory nerve sectioning may enhance
apoptosis of taste bud cells with taste buds decreasing in
number andultimately disappearing, we further investigated the
issue of taste bud cell apoptosis using terminal
deoxynucleotidyl transferase mediated nick-endlabelling (TUNEL)
method after unilateral glossopharyngeal neurectomy. There
results showed that although only very few positively stained
nuclei indicating apoptosis were observed innormal taste buds,
in the surgically denervated vallate taste buds, the numberof
apoptotic cells increased from 6 h, reached at peak on day 1 and
gradually decreased from day 3 and day 7 postneurectomy.
Electron microscopy revealed that apoptotic cells in both normal
and neurectomized taste buds showed characteristic fine
structural morphology of apoptosis. From the above
observations, we conclude that, in guinea pig vallate tastebuds,
1) type III cells are the bona fide gustatory receptor cells
and PGP9.5is a marker of type III cells, 2) CGRP-containing
nerve fibers and 5HT-containing taste cells may be primarily
involved the neural transmission and its modulation of the taste
sensation, respectively, 3) right and left vallate papillae are
unilaterally innervated by the glossopharyngeal nerve without
cross-innervation, 4) glossopharyngeal neurectomy enhances the
apoptosis of taste bud cells and 5) an apporpriate innervation
of glossopharyngeal nerve is an essential component in
maintaining the normal function of taste buds.
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