Type I 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 I cells are thought to play a glial-like support role in the taste bud and may participate in salt detection.
Type I cells are the most abundant of the three cell types—they make up approximately 50 percent of the cells in each taste bud. Unlike Types II and III cells, which are spindle-shaped, Type I cells have extensive membrane protrusions that allow them to wrap around neighboring cells in the taste bud. They extend from the base of the taste bud to the apical pore, where an opening in the tongue epithelium allows taste cells to access chemical stimuli in the oral cavity. Near the apical pore, Type I cells form “bushy” membrane structures targeted toward the pore. Type I cells are often adjacent to nerve fibers innervating the taste bud, but do not form synapse-like structures that would suggest direct chemical communication between the two cell types.
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 cells. Type I cells renew more quickly than either Type II or Type III cells in the taste bud—they have been reported to turn over every seven days or so.
In comparison with other cell types in the taste bud, relatively little is known about Type I cell function. Since Type I cells wrap around other taste cells in the taste bud, this suggests a glial-like role. Glial cells in the brain generally function as support cells for neurons, but in some cases also participate actively in signaling from one neuron to the next. They can perform many roles in the brain: some insulate axons (the wire-like parts of neurons along which electrical signals travel), others regulate blood flow in certain brain areas, still others regulate activity at synapses (the communication points between two neurons) and may provide metabolic support for adjacent neurons.
Type I cells perform another glial-like role in the taste bud. Glial cells in the brain often remove or process excess neurotransmitters released by signaling cells. Type II cells in taste buds release a small molecule, adenosine triphosphate (ATP), as a neurotransmitter, allowing them to communicate taste information to innervating nerve fibers and, ultimately, the taste centers of the brain. Type I cells express a protein on their membranes called NTPDase (ecto-nucleoside triphosphate diphosphohydrolase), which “chews up” extra ATP in the taste bud so nerve fibers receive the appropriate amount of neurotransmitter. They also express proteins that can transport neurotransmitter molecules from the extracellular space into Type I cells. Regulating neurotransmitter concentration in taste buds ensures the efficient and precise communication of taste information from the taste bud to the central nervous system.
There is also some evidence to suggest that Type I cells might play a role in taste reception itself, rather than just supporting other taste cells. They express a membrane ion channel (a protein that forms a pore in the cell membrane and lets certain charged particles pass through) that can detect salt (Vandenbeuch et al., 2008). How Type I cells might communicate this taste information to nerve fibers is still unknown.
Courtney E. Wilson
See also: Salty Sensation; Taste Bud; Taste System; Type II Taste Cells; Type III Taste Cells
Chaudhari, Nirupa, & Stephen D. Roper. (2010). The cell biology of taste. Journal of Cell Biology, 190(3), 285—296.
Kandel, Eric R., James H. Schwartz, Thomas M. Jessell, Steven A. Siegelbaum, & A. J. Hudspeth (Eds.). (2012). Principles of neural science (5th ed., Ch. 32). New York, NY: McGraw-Hill.
Vandenbeuch, Aurelie, Tod R. Clapp, & Sue C. Kinnamon. (2008). Amiloride-sensitive channels in type I fungiform taste cells in mouse. BMC Neuroscience, 9, 1. http://dx.doi.org/10.1186/1471-2202-9-1.