increase sensory surface area. While functionally similar, they are
structurally different. Microvilli are smaller than cilia and have
an internal cytoskeletal structure consisting largely of actin. Cilia
have an internal cytoskeleton organized around microtubules.The bulk of a gustatory receptor cell consists of a cell body looking
like most other cells, with a DNA-containing nucleus, mitochondria,
ribosomes, and so forth—the usual stuff of cells. At the base of the
receptor cell there is a contact point, a chemical synapse, with nerve
fibers that respond to neurotransmitter molecules released by the
taste receptor cells, initiating a signal that goes to the brain. The fibers
carrying signals from taste receptor cells to the brain are part of the
system of cranial nerves, notably cranial nerves 7,9, and 10.
Similar to the olfactory system, stem cells adjacent to the taste
receptor cells can differentiate into the various types of taste receptor
cells, characterized by their different taste receptor proteins. This
allows taste receptor cells to be regularly replaced, with a turnover
rate of approximately two weeks. As in the olfactory system, this reg-
ular replacement is presumably related to the fact that these cells are
constantly exposed to all kinds of toxic gunk from the environment
and thus subject to continual damage. In this way, gustatory and
olfactory receptor cells differ from visual and auditory receptor cells
(see Chapters 14 and 15); the latter exist within structures shielding
them from direct contact with potentially toxic chemicals from the
environment, and they are not known to be associated with stem
cells.