Three-Dimensional Reconstructions of Mouse Circumvallate Taste Buds Using Serial Blockface Scanning Electron Microscopy: I. Cell Types and the Apical Region of the Taste Bud

ABSTRACT Taste buds comprise four types of taste cells: 3 mature, elongate types: Type I, Type II, Type III; and basally-situated, immature post-mitotic Type IV cells. We employed serial blockface scanning electron microscopy to delineate the characteristics and interrelationships of the taste cells in the circumvallate papillae of adult mice. Type I cells have an indented, elongate nucleus with invaginations, folded plasma membrane, and multiple apical microvilli in the taste pore. Type I microvilli may be either restricted to the bottom of the pore or extend outward reaching midway up into the taste pore. Type II cells (aka receptor cells) are characterized by a large round or oval nucleus, a single apical microvillus extending through the taste pore, and specialized “atypical” mitochondria at functional points of contact with nerve fibers. Type III cells (aka “synaptic cells”) are elongate with an indented nucleus, possess a single, apical microvillus extending through the taste pore and are characterized by a small accumulation of synaptic vesicles at points of contact with nerve fibers. About one-quarter of Type III cells also exhibit an atypical mitochondrion amidst the presynaptic vesicle clusters at the synapse. Type IV cells (non-proliferative “basal cells”) have a nucleus in the lower quarter of the taste bud but have a foot process extending to the basement membrane often contacting nerve processes along the way. Type I cells represent just over 50% of the population, whereas Type II, Type III, and Type IV (basal cells) represent 19%, 15%, and 14% respectively.