Ultrastructural and Immunohistochemical Study in Taste Buds of Circumvallate and Fungiform Papillae of the Mouse : A Computer-assisted Three-dimensional Analysis of Taste Buds

Taste buds in circumvallate and fungiform papillae of the mouse were examined with immunohistochemistry, electron microscopy and computer-assisted, three-dimensional reconstruction from serial ultrathin sections. In accord with observations by Murray (1973), four distinct cell types, type I, II, III and basal cells, were identified in taste buds of both papillae. After intraperitoneal injection of 5-HTP, a precursor of serotonin, some taste bud cells showed serotonin-immunoreactivity. The number and location of the serotonin-immunoreactive cells coincided with those of typeIII cells. The taste bud of the circumvallate papilla was made up of 70 to 100 cells. Of these, only the typeIII cells, about 5 to 10 percent of total number of cells, made synaptic contacts with nerve fibers. The typeIII cell made 3 to 5 synapses with 5 to 8 nerve terminals. These synapses were characterized by symmetric membrane thickenings of both the typeIII cell and apposed axon plasma membrane, and by accumulation of numerous small, clear vesicles and dense-cored vesicles in the typeIII cell cytoplasm. These findings clearly indicated a gustatory transducer function for the typeIII cell. The taste bud of the fungiform papilla contained about 40 cells. Similar to the circumvallate taste bud, only typeIII cells made synaptic contacts with nerve terminals and contained both small, clear vesicles and dense-cored vesicles. Synapses in fungiform typeIII cells contained many more synaptic vesicles than those of circumvallate taste buds. One to three typeIII cells, about 5 percent of total number of cells, could be identified within a fungiform taste bud. The typeIII cell made 6 to 8 synapses with 6 to 12 nerve terminals. There were no transitional cells showing intermediate morphological characteristics between cell types. The present results thus contrast with the studies by Kinnamon and his group. Although the exact nature and function of typeI and typeII cells in the taste buds still remains enigmatic, the present study strongly supports the view that there are separate taste cell lines that retain their unique identities, never transforming into one another in the mouse taste buds.