The fine structure of substance-P-like immunoreactive [SPI] fibers in the taste buds of the circumvallate papillae of the rat tongue was investigated by means of electron microscopy using the unlabeled antibody-enzyme method.
Outside the epithelium, SPI and non-SPI fibers are surrounded by the cytoplasm of Schwann cells. When the SPI fibers enter the epithelium, they immediately lose this cytoplasmic sheath and begin to traverse the taste buds. Though passing through the taste buds, no profiles suggesting clear synaptic contact between SPI fibers and underlying cells are identified. SPI terminals are filled with small synaptic vesicles and contain a few mitochondria. No SPI-positive structures are found in nerve endings that make synaptic contact with type III cells, the gustatory receptor cells.