Type II Taste Cells
Taste buds are the sensory end organs of the gustatory system. They contain three basic types of cells—Type I, Type II, and Type III cells. These networks of cells allow for the detection of five distinct taste qualities: sweet, bitter, umami, salty, and sour. Type II cells are sensitive to sweet, bitter, and umami taste qualities.
Type II cells are wide and spindle shaped, extending from the base of the taste bud to the apical pore. Here, an opening in the tongue epithelium allows the finger-like tips of taste cells to access chemicals present in the oral cavity. Type II cells are more abundant in taste buds than Type III cells, but less abundant than Type I cells.
Though they communicate to nerve fibers innervating the taste bud, Type II cells do not form traditional synapse structures with adjacent nerve fibers. Instead, the nerve fibers wrap around Type II cells. At points of contact with these surrounding nerve fibers, Type II cells have atypical mitochondria. Mitochondria are the energy factories of cells—they produce ATP (adenosine triphosphate), which is used to fuel many cellular processes. Electron microscopy images show mitochondria to be generally ovoid, with layers of membrane stacked inside. The atypical mitochondria in Type II cells, in contrast, are much larger and oddly shaped, with inner membranes that appear more twisted and less orderly than typical mitochondria. Type II cells use ATP as a neurotransmitter, as well as an energy source. It is thought that these atypical mitochondria are located at contact points with nerve fibers to ensure that there is a large, available source of ATP for signaling to afferent nerves.
Taste bud cells are continually turning over during the life of an organism. As taste cells are damaged and die, a population of renewing basal cells repopulates the taste bud with new taste receptor cells. Type II cells survive for approximately two weeks before being replaced by new, maturing taste cells.
Each Type II cell is sensitive to bitter, sweet, or umami stimuli. While not neurons, Type II cells behave similarly to neurons. They too fire action potentials—the electrical signal that travels from one end of a neuron to the other—and release ATP as a neurotransmitter when stimulated with bitter, sweet, and umami substances.
Any one Type II cell expresses apically located receptors for one of three stimuli: bitter, sweet, or umami. Bitter receptors belong to the Taste 2 Receptor (T2R) family and are more varied than either sweet or umami receptors. Bitter taste reception is thought to warn organisms of possible poisonous substances before ingestion. This helps explain why they are the most varied receptors—poisonous substances can come in a wide variety of molecular structures, and the receptors evolved to match them. Sweet receptors are made up of two subunits, called Taste 1 Receptor 2 (T1R2) and Taste 1 Receptor 3 (T1R3). Both of the subunits are necessary to make the sweet receptor. Some animals (cats, for example) lack functional T1R2 genes, and thus are unable to taste sweet substances. Umami can be thought of as the “savory” quality—indicating the protein content of the food. Mushrooms and meats are high on the “umami” scale. Umami receptors also involve two subunits: Taste 1 Receptor 1 (T1R1) and T1R3. T1R3 is a subunit of both the umami and sweet receptors. Like the sweet receptor, both T1R1 and T1R3 are necessary to form the umami receptor.
Regardless of whether a Type II cell expresses bitter, sweet, or umami receptors, it processes this signal via the same signaling pathway. T2Rs and T1Rs are all G protein—coupled receptors, meaning they activate G proteins when they detect the proper substrate. These activated G proteins then start a set of processes called a signaling cascade inside the cell. For taste receptor—associated G proteins in Type II cells, this cascade causes an increase in the calcium concentration inside the cell. The increase of calcium, in turn, causes calcium-activated ion channels to open. Ion channels are pores in the cell membrane that open when activated, allowing ions to pass through. There are many kinds of ion channels that pass different kinds of ions and are activated by different conditions. These particular ion channels allow sodium ions into the cell. Since sodium ions are charged, they disrupt the delicate electrical balance of the cell, ultimately causing an action potential. By an unknown mechanism, this action potential causes Type II cells to release ATP through wide pores in the membrane called hemichannels. This ATP then stimulates the afferent nerve fiber, which carries the signal to the central nervous system. Eventually, this signal reaches the insular cortex, which processes taste information.
Courtney E. Wilson
See also: Bitter Sensation; G Proteins; Sweet Sensation; Taste Bud; Taste System; Type I Taste Cells; Type III Taste Cells; Umami
Chaudhari, Nirupa, & Stephen D. Roper. (2010). The cell biology of taste. Journal of Cell Biology, 190(3), 285—296.