The specialized sense organ for taste (gustation) consists of approximately 10,000 taste buds, which are ovoid bodies measuring 50–70 μm. There are four morphologically distinct types of cells within each taste bud: basal cells, dark cells, light cells, and intermediate cells (Figure 1). The latter three cell types are also referred to as Type I, II, and III taste cells. They are the sensory neurons that respond to taste stimuli or tastants. Each taste bud has between 50 and 100 taste cells. The three cell types may represent various stages of differentiation of developing taste cells, with the light cells being the most mature. Alter natively, each cell type may represent different cell lineages. The apical ends of taste cells have microvilli that project into the taste pore, a small opening on the dorsal surface of the tongue where tastes cells are exposed to the oral contents. Each taste bud is innervated by about 50 nerve fibers, and conversely, each nerve fiber receives input from an average of five taste buds. The basal cells arise from the epithelial cells surrounding the taste bud. They differentiate into new taste cells, and the old cells are continuously replaced with a half-time of about 10 days. If the sensory nerve is cut, the taste buds it innervates degenerate and eventually disappear.
Fig1. Taste buds located in papillae of the human tongue. A) Taste buds on the anterior two-thirds of the tongue are innervated by the chorda tympani branch of the facial nerve; those on the posterior one-third of the tongue are innervated by the lingual branch of the glossopharyngeal nerve. B) The three major types of papillae (circumvallate, foliate, and fungiform) are located on specific parts of the tongue. C) Taste buds are composed of basal stem cells and three types of taste cells (dark, light, and intermediate). Taste cells extend from the base of the taste bud to the taste pore, where microvilli contact tastants dissolved in saliva and mucus. (Modified with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. New York, NY: McGraw-Hill; 2000.)
In humans, the taste buds are located in the mucosa of the epiglottis, palate, and pharynx in the walls of papillae of the tongue (Figure 1). The fungiform papillae are rounded structures most numerous near the tip of the tongue; the circumvallate papillae are prominent structures arranged in a V on the back of the tongue; the foliate papillae are on the posterior edge of the tongue. Each fungiform papilla has up to five taste buds, mostly located at the top of the papilla, while each vallate and foliate papilla contain up to 100 taste buds, mostly located along the sides of the papillae. The von Ebner glands (also called gustatory glands or serous glands) secrete saliva into the cleft around the circumvallate and foliate papillae. Secretions from these glands may function to cleanse the mouth to prepare the taste receptors for a new stimulant. Recent work also suggests that circumvallate papilla and von Ebner glands form a functional complex that is important in actual taste detection because of the enzymes secreted by the gland.
