Ultrastructure of mouse vallate taste buds. I. Taste cells and their associated synapses.

The ultrastructural features of murine vallate taste bud cells and their associated synapses have been examined in thin and thick sections with conventional transmission electron microscopy and high-voltage electron microscopy. Computer-assisted reconstructions from serial sections were utilized to aid in visualization of taste bud cell-nerve fiber synapses. We have classified taste bud cells on the basis of previously established criteria-namely, size of the nucleus, shape and density of chromatin, density of cytoplasm, and presence or absence of dense-cored or clear vesicles, other cytoplasmic organelles, and synaptic foci. Both dark cells and light cells are present, as well as cells with intermediate morphological characteristics. Synapses were observed from taste bud cells onto nerve fiber processes. In virtually all instances, synapses are associated with the nuclear region of the taste cell. These synapses are characterized by the presence of 40-70 nm clear vesicles embedded in a thickened presynaptic membrane separated from the postsynaptic membrane by a 16-30 nm cleft. Synapses are not unique to any particular cell type. Dark, intermediate, and light cells all synapse onto nerve fibers. Two general types of synapses exist: spot (or macular) and fingerlike. In the latter, the postsynaptic region of the neuronal process protrudes into an invagination of the taste cell membrane. Differences in synaptic morphology are not correlated with taste cell type. In some cases a single taste cell was observed to possess both macular and fingerlike synapses adjacent to one another, forming a synaptic complex onto a single neuronal process. On the basis of the presence of synaptic contacts, we conclude that both "dark" and "light" cells are gustatory receptors.