Structure of taste buds in foliate papillae of the rhesus monkey, Macaca mulatta

Taste buds in foliate papillae of the rhesus monkey were examined by electron microscopy. Three distinct cell types were identified. Type I cells were narrow elongated cells containing an oval nucleus, bundles of intermediate filaments, several Golgi bodies, and characteristic apical membrane-bounded dense granules. These cells exhibited morphological variations: some had a moderately dense cytoplasm, perinuclear free ribosomes, and flattened sacs of rough endoplasmic reticulum; others had a more lucent cytoplasm, dilated irregular rough endoplasmic reticulum, lysosome-like dense bodies, and lipid droplets. Type II cells typically contained a spherical, pale nucleus, a prominent nucleolus, supranuclear and infranuclear Golgi bodies, mitochondria with tubular cristae, and one or two centrioles. This cell type, too, showed some variation in the relative amounts of ribosomes and smooth endoplasmic reticulum, which varied inversely with each other. Type III cells were characterized by a clear apical cytoplasm essentially devoid of ribosomes and containing microtubules. In a few type III cells, the peri- and infranuclear regions contained many ribosomes and some rough endoplasmic reticulum. In most Type III cells, there were large numbers of dense and clear vesicles in the peri- and infranuclear regions; some of the vesicles were grouped in synapse-like arrangements with adjacent nerves. The morphological variations exhibited by all three cell types could be accounted for by age differences in each of the cells. This would be consistent with the notion that cell renewal occurs in each of the three cell populations.     

RENEWAL OF TASTE BUD CELLS IN RAT CIRCUMVALLATE PAPILLAE

The life span of taste bud cells in rat circumvallate papillae was measured by autoradiography after labeling them with a pulse of [³H]thymidine. Specimens of circumvallate papillae were taken daily 1·5-18·5 days after the isotope was administered; thereafter, specimens were taken on alternate days until 25·5 days. For each time interval, the number of labeled cell nuclei was counted in 200-450 taste buds and plotted as the ratio of labeled cells/taste bud v. time after injection of [³H]TdR. In all, 6958 taste buds were counted. The total number of labeled cells (dark plus light) per taste bud reached peaks at 6·5, 13·5 and 20·5 days. The curve for the number of labeled dark cells/bud had essentially the same shape as that for total cells. The number of labeled light cells/bud reached a modest peak at 6·5 days and slowly declined to a plateau for the remainder of the experiment. The data show that an average of 2 days elapsed after injection before labeled dark cells entered the bud and they spent an average of 7 days in the non-proliferating taste bud compartment; thus, the life span of the dark cell was 9 days. The life span of the light cell was difficult to estimate quantitatively, but this cell type was labeled at a much slower rate than dark cells and is assumed to have a significantly longer tenure in the taste bud.     

The septal organ of the rat during postnatal development

The septal organ of Masera (SO) is a small, isolated patch of olfactory epithelium, located in the ventral part of the nasal septum. We investigated in this systematic study the postnatal development of the SO in histological sections of rats at various ages from the day of birth (P1) to P666. The SO-area increases to a maximum at P66-P105, just as the animals reach sexual maturity, and decreases thereafter, significantly however only in males, indicating a limited neurogenetic capacity for regeneration. In contrast, the main olfactory epithelium area continues to expand beyond P300. The modified respiratory epithelium ('zwischen epithelium') separating the SO and the main olfactory epithelium contains a few olfactory neurons up to age P66. Its length increases postnatally so that the SO becomes more ventral to the OE. Although the position of the SO relative to other anatomical landmarks changes with development it is consistently located just posterior to the opening of the nasopalatine duct (NPAL). Thus, a possible function of the SO is in sensing chemicals in fluids entering the mouth by licking and then delivered to the nasal cavity via the NPAL; therefore the SO may be involved in social/sexual behavior as is the vomeronasal organ (VNO). We suggest that the SO is a separate accessory olfactory organ with properties somewhat different from both OE and VNO and may exist only in species where the NPAL does not open into the VNO.     

Changes in number and morphology of fungiform taste buds in rat after transection of the chorda tympani or chordalingual nerve

In normal rats there is one taste bud on the apical surfaceof each fungiform papilla. These taste buds are innervated bythe chorda tympani proper nerve (CT). According to general consensus,after cutting the nerve the taste buds should disappear. Inthis study, performed on 24 rats divided in six groups, theCT nerve on the left side (singly denervated) and the combinedchorda-lingual (CT-L) nerve on the other side (doubly denervatedwere permanently interrupted. The animals were sacrificed after5, 10, 20, 35,60 and 100 days and their tongues serially sectionedfor light microscope examiation. Some papillae were examinedunder an electron microscope. The papillae were categorizedinto three groups: papillae with a normal looking taste bud,with an abnormal looking taste bud and without a taste bud.The results showed a substantial number of papillae with a normallooking taste bud present at all time intervals in all animals.More specifically, on the singly denervated side the proportionof normal looking taste buds stayed below 10% until day 60,when it increased to 15% and to 23% on day 100. The proportionof abnormal looking taste buds decreased from above 92% by day5 to 49% on day 100. The percentage of fungiform papillae withoutsigns of a taste bud was relatively low on the singly denervatedside at times (1, 5, 16, 29, 34 and 28%). On the doubly denervatedside fewer than than 4% normal looking taste buds were foundthroughout the time period. The proportion of abnormal lookingtaste buds decreased from {small tilde} 96% by day 5 to 35%on day 100. A significantly higher proportion of papillae withno taste bud was observed on this side from day 10 onwards.(1, 29, 32, 52, 60 and 63%). The reasons for the differencein tast bud number between the doubly and singly denervatedsides are unknown, but it is possible that collaterals fromother (non-gustatory) nerves have an ability, although limited,to induce and maintain fungiform taste buds. In other words,the capacity to induce taste bud formation is not limited exclusivelyto gustatory nerves.     

Amitriptyline reduces olfactory neuron adenylyl cyclase activity

Adenylyl cyclase plays an important role in olfactory signaltransduction. Recently, a novel type III adenylyl cyclase hasbeen localized in olfactory neurons (Pfeuffer et al., 1989;Bakalyar and Reed, 1990). Because amitriptyline (AMI), a tricyclicantidepressant, appears to have an inhibitory effect on adenylylcyclase activity in other in other neuronal tissue (Yamaokaet al., 1988; Wong et al., 1991), we measured the effect ofAMI on forskolin-stimulated adenylyl cyclase activity in membranepreparations of olfactory mucosa from adult rats. In the presenceof 5'-guanylyl-imidodiphosphate, AMI (0.5–8.0 µM)inhibited forskolin-stimulated adenylyl cyclase activity ina dose-dependent manner. To determine whether this effect wasspecific for olfactory neurons, as opposed to other cells inthe olfactory epithelium, rats were unilaterally bulbectomizedin order to reduce selectively the number of olfactory neuronson the side ipsilateral to the bulbectomy. In membrane preparationsfrom unilaterally bulbectomized animals we saw significantlylower adenylyl cyclase activity in ipsilateral olfactory mucosa,compared with adenylyl cyclase activity from non-bulbectomizedmucosa. These results indicate that AMI inhibition of adenylylcyclase activity is primariy localized in olfactory neurons.     

Regeneration ability of fungiform papillae and taste-buds in rats

We have earlier shown that the taste-bud-bearing fungiform papillaeform a stable pattern on the tongue of rats. In this study theeffect of removal of the fungiform papillae in rats was investigated.The fungiform papillae on a 10 x 5-mm area on one side of thetongue were removed after mapping of both sides under an operatingmicroscope. Serial sections of five rat tongues within 1 dayof biopsy showed that all but one papilla were gone. After 4,6 and 12 months an average of seven papillae with taste-budswere found in the operated area, compared to 20, 26 and 23 inthe controls. Comparison of tongue maps before and after theseperiods showed that papillae had not migrated from areas outsidethe area of the biopsies. To test the assumption that the extentof biopsy determined the amount of regeneration, only the upperpart of the papillae with their taste buds were removed in 15rats. Complete regeneration of papillae and taste buds was obtainedwithin 14 days. The function of the regenerated taste buds wastested by bilateral recording from the chorda tympani propernerves. No difference in response amplitudes was observed betweenthe sides. When, however, the whole papilla including its basewas removed, neither the papilla nor the taste-bud regenerated.The results show that the ability of the fungiform papilla andthe taste-bud to regenerate after removal of the papilla isrelated to the extent of the biopsy. If the entire papilla includingits base is removed, it will not regenerate. If only the upperpart is removed, complete regeneration of both papilla and itstaste-bud will occur.