Fine structure of the canine taste bud with special reference to gustatory cell functions.

Canine taste buds were observed by transmission electron microscopy with special reference to the gustatory cell function. The cells forming the taste bud were divided into five types: four of these corresponded to the previously reported Types I, II, III and IV (basal cell); another type, a slender, immature-looking cell located at the outermost layer, was identified as the peripheral cell known in the rat. The Type I cell was supportive in nature, located between other cell types and enveloping nerve fibers. This cell apically secretes dense mucous substances. The Type II cell was in broad contact with nerve fibers and constantly contained a subsurface cistern beneath them. In the dog, this cell was characterized by a large supranuclear Golgi apparatus. The Type III cell was, as in other animals, regarded as the gustatory cell since it made synaptic contacts with nerves and contained synaptic vesicles. Numerous large corded vesicles were intermingled with some small clear vesicles. In addition to accumulating to the synaptic areas, the vesicles often filled the base of the cell, which was irregularly thickened, showing a more or less extensive contact with the basement membrane. In this area, numerous large cored vesicles approached the cell base. Blood capillaries close to the base of the taste bud were fenestrated in the endothelium. These findings supported a hypothesis that the transmitters may be released from the base of the gustatory cell and possibly exert paracrine and endocrine (hemocrine) effects. The salivary gland of Ebner was suggested as one such target. The nerve terminals on the Type III cell also contained synaptic vesicles, suggesting a reciprocal nature of the synapses. The Type IV cell was the immature basal cell which has been established in other species; mitosis could be seen in this cell.