Fluorescence histochemistry has revealed that a specific formaldehyde-induced yellow fluorescence, suggesting the presence of serotonin-like monoamines, was regularly observed in a group of cells (monoamine-containing cells: MC cells) in the normal gustatory epithelia of the carp, catfish, frog and rabbit.
The present report was also concerned with the ultrastructural features of the MC cell in the rabbit's foliate bud. The MC cells were found to have synaptic contacts with the intragemmal nerves. The synaptic morphology showed that the neural transmission may take place from the cell to the nerve. Two types of vesicles were encountered in the cytoplasm of MC cells. One was large cored vesicles (100-150nm) suggesting storage of the serotonin-like monoamine. They were localized not only at the presynaptic area, but also in the infra- and supra-nuclear cytoplasm. The other was small vesicles (45-80nm) containing variable amounts of dark material. They were similar in distribution to the large cored vesicles.
The functional significance of the MC cells was discussed.