Taste buds are the peripheral sensory organs of the gustatory system. They are garlic-shaped clusters of cells that sense five basic tastes: sweet, bitter, umami, salty, and sour.
Taste buds contain 50—100 elongate cells that sense chemical tastants and communicate this information to taste nerve fibers. Each taste bud has an apical pore, where a small opening in the epithelial tissue allows the apical ends of taste receptor cells to access chemicals in the oral cavity. At its base, a taste bud is innervated by taste nerves that contact the taste receptor cells inside.
Taste buds are found in specialized regions both on the tongue and in separate tissues in the oral cavity. On the tongue, taste buds are housed in the fungiform, circumvallate, and foliate papillae. Fungiform papillae are small, rounded protrusions scattered across the top of the anterior tongue. These bumps are more concentrated at the very tip of the tongue, and each contains one to five taste buds. The circumvallate papillae are located at the back of the tongue—these large bumps mark semicircular troughs in the tongue epithelium, which are lined with taste buds. Taste buds in the circumvallate papillae are much more concentrated than in the fungiform papillae. The foliate papillae house taste buds in several pits lining each side of the posterior tongue—taste buds here are highly concentrated, as in the circumvallate papillae. Taste buds even exist outside the tongue—humans have a few taste buds on the soft palate, the tissue posterior to the hard palate on the roof of the mouth, as well as in the larynx.
Taste bud tissue patterning and development occurs during gestation, such that we are born with our first functioning taste buds. Unlike sensory cells in the auditory and visual systems (hair cells and retinal neurons, respectively), which are quite limited in their ability to regenerate, taste buds are constantly turning over and renewing throughout the life of an organism. Taste buds are repopulated by a population of basal progenitor cells that are capable of differentiating into either taste cells or epithelial cells. Some of these cells then migrate into the base of the taste bud, becoming taste-specific progenitor cells, which eventually differentiate into one of the three types of mature taste cells: Type I, Type II, or Type III.
While taste bud turnover tends to remain stable throughout a lifetime, the ability of taste buds to regenerate can be perturbed—chemotherapeutic drugs, for example, can disrupt taste in head and neck cancer patients. It is thought that these drugs disrupt or eliminate the sense of taste by disabling proper taste cell renewal (Barlow, 2015).
Taste buds throughout the oral cavity are responsible for sensing at least five basic tastes: sweet, bitter, umami, salty, and sour. Each of these tastes indicates important information about ingested foods. Sweet foods are high in sugars, suggesting nutritional content. Umami, a Japanese term for “savory,” generally indicates the protein content of food, which is also of nutritional importance. Salt is important for many processes in the body—with taste buds that sense salt content, the body can regulate salt intake. Bitter and sour are generally thought to be aversive qualities—taste buds help us to avoid ingesting poisonous and perhaps rotten foods by recognizing bitter and sour compounds. Many plants produce toxic chemicals. To avoid these, organisms evolved a sense of bitter taste, which recognizes a wide array of toxic molecules we might find in our diets. Foods that are too high in acid may be rotten or rancid and will taste sour, allowing us to avoid ingesting potentially damaging foods. All of these taste qualities are sensed by specialized taste receptor cells in taste buds. Bitter, sweet, and umami qualities are sensed by Type II cells, sour and some salty substances are sensed by Type III cells, and some salt sensation may occur through Type I cells, though these cells are primarily support cells. Taste buds may also sense fatty acids and calcium, but whether these are primary taste qualities have not yet been confirmed.
Contrary to the popularized “taste map” of the tongue, the five tastes are not strictly segregated to specific regions of the tongue. Some taste buds have more of one particular cell type than another—for example, there are more Type III cells per bud in the circumvallate papillae than in fungiform papillae. But most, if not all, taste buds contain cells to detect all five taste qualities.
Courtney E. Wilson
See also: Taste System; Type I Taste Cells; Type II Taste Cells; Type III Taste Cells
Barlow, Linda A. (2015). Progress and renewal in gustation: New insights into taste bud development. Development, 142(21), 3620—3629. http://dx.doi.org/10.1242/dev.120394.
Kandel, Eric R., James H. Schwartz, Thomas M. Jessell, Steven A. Siegelbaum, & A. J. Hudspeth (Eds.). (2012). Principles of neural science (5th ed., Chap. 32). New York, NY: McGraw-Hill.