Life and Death of Taste and Smell Receptors: The Role of Stem Cells
Taste and smell receptors commonly turnover on a daily basis. This turnover may be physiologically necessary since taste receptors are exposed to many substances in food or beverages, hot or cold, spicy, noxious or similar substances which could either alter their function or outright kill them. Similarly, olfactory receptors are exposed to the outside environment which can be quite toxic to these exposed receptors. While there is some protection for taste buds in saliva and nasal mucus for olfactory receptors, the toxic effect of incoming substances put these receptors at risk. How many of us have experienced a burning mouth sensation from hot soup or pizza? How many of us have experienced a noxious response to smoke or a pungent, strong odor? These physical sensations are usually transient but in order to survive taste and smell receptors have adapted a unique pattern of reproducing in order to withstand these toxic events.
How does protection from these toxic events occur? One method is that taste and smell receptors turnover rapidly, not uncommonly, on a daily basis. This turnover depends upon the activation of stem cells in each organ by specific growth factors which maintain these systems.
However, if we view receptors themselves, in their normal state, we recognize that some cells of the receptors exhibit a normal, anatomical appearance whereas others appear to be degenerating. Thus, these tissues are experiencing an active growth and degeneration of their cellular receptors on a daily basis.
Anatomical examination of receptors for both taste and smell indicate newly forming cells and dying cells. Dying cells exhibit death by programmed apoptosis, a physiological mechanism by which cells die exhibiting large vacuoles and elements of cellular detritus. This phenomenon is associated with a process called necrosis, by which cells die due to an inflammatory process. There are multiple inflammatory cells, both acute and chronic, in the tissues associated with taste and smell receptors. The cellular components of these receptors die off at a rapid rate exhibiting apoptosis and are rapidly replaced by the persistent secretion of taste bud and olfactory epithelial growth factors which initiate rapid turnover of these organs in order to allow taste and smell perception to occur.