Specific effect of 5,6-dihydroxytryptamine on the monoamine fluorophore of the frog's gustatory cells

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.

A histochemical study of APUD ability in the taste buds of experimentally induced zinc-deficient mice

To explore the relationship between taste acuity and zinc deficiency, a histochemical investigation was made into the taste buds of mice fed a zinc-deficient diet. Nine weeks after the start of the diet, the average serum zinc level of the mice was 45% lower than that of a control group of mice. Moreover, growth was arrested significantly. Two-bottle preference tests revealed that the intake ratio of 10(-5) M quinine hydrochloride solutions had increased markedly in the zinc-deficient mice compared with the controls. The circumvallate taste buds showed no morphological changes. Fluorescent histochemical examination showed an uptake of a monoamine precursor (5-HTP) by the gustatory cells in the zinc-deficient mice after the 5-HTP treatment. Upon immunohistological examination, however, no serotonin immunoreactivity appeared in the gustatory cells of the zinc-deficient mice after the 5-HTP treatment. These results suggest that zinc-deficiency may induce hypogeusia and decrease the ability to transform a monoamine precursor to monoamine in the gustatory cells, albeit the monoamine precursor uptake ability is not affected.

A fluorescence histochemical study of the monoamine-containing cell in the developing frog taste organ

Sequential changes in the monoamine-containing cell (MC cell) of the developing frog tongue has been studied by fluorescence histochemistry using uptake of 5,6-dihydroxytryptamine. At st. 16, a few yellow fluorescent cells, here called MC cells, appeared in random order at the uppermost layer of the dorsal epithelium. They were round or elliptical in shape. At st. 18 the MC cells, greatly transformed, were found at the periphery of the sensory disc primordium which first appears during this stage. The MC cell was made up of three parts: perikaryon, process and terminal portion. The perikaryon was located at the upper half of the epithelium and from it a single process stretched vertically toward the basal lamina, above which the dilated terminal portion was found. Thereafter the perikaryon gradually moved toward the basal layer while remaining at the periphery of the disc primordium. Meanwhile the terminal portion moved over the basal lamina toward the center of the disc primordium. At st. 22, the whole of the MC cell lay flat above the basal lamina. The perikaryon was localized at the periphery of the sensory disc and from there the process stretched toward the center. Thus, the morphology of MC cells resembled the adult state, except for smaller size. MC cells were never observed in the subepithelial connective tissue in the present study. This seems to suggest that the MC cell of the frog fungiform papilla is of epithelial origin.