The taste bud in the rat vallate papillae was observed by immunohistochemistry for PGP 9.5 at the light and electron microscope levels; routine transmission electron microscopy was also performed. Immunoreactivity for PGP 9.5, a marker protein of paraneurons, was localized in the cytoplasm of the Type III or gustatory cells. More intensive immunoreactivity occurred in the nerve fibers, though a part of the nerve fibers remained unstained.
The nerve fibers as detected by the immunostaining and by routine electron microscopy formed a coarse subepithelial plexus which issued branches upwards through the basal lamina. In the basal portion of the taste bud, these fibers formed a hitherto unknown intragemmal plexus of dense and delicate meshwork. This plexus, in turn, extended beaded fibers between the taste bud cells, forming synaptic contacts with Type III cells. Some of the immunoreactive nerve fibers were confirmed to reach the taste pore by light microscopy; electron-microscopic examination could not demonstrate their directly being exposed to the space of the taste pore.
This study also deals with the fine structure of the rat taste bud with special reference to the Type III cells and their synapses with nerve fibers, as the rat has recently been rather seldom used in this field of study.