Stem Cells and Sensory Function
Stem cells encompass multiple cell types but they all have several properties in common. They exhibit self-renewal and self-differentiating properties. They exist in many different tissues and play a role in self-generation of each tissue. However, all stem cells exhibit several similar properties – one of the most critical is the requirement that specific localized chemical properties are present and are secreted in a continuous manner to maintain the integrity both of the stem cell itself, and to induce the stem cell to initiate growth and development of the sensory structures in which the stem cell is found. The stem cell itself is relatively long-lived whereas its secretions to activate its sensory components may be readily amenable to multiple pathologies which interfere with these secretions.
For the taste and smell systems, stem cells reside in the receptors by which sensory information is received and processed. This indicates that taste buds and olfactory epithelial cells each contain stem cells to allow sensory perception of taste and smell function to occur. Further, taste and smell receptor cells (taste buds and olfactory epithelial cells) exhibit unique anatomical characteristics – they do not exhibit blood vessels or lymphatics and they do not exhibit mitosis, i.e. they themselves do not appear to divide metabolically. Yet, these sensory receptors turn over on a 24-hour basis and are thus metabolically very active. How does this occur if these sensory cells themselves do not exhibit mitosis?
They depend upon the action of the stem cells to initiate tissue growth and development under the influence of specific growth factors which initiate this activity in these stem cells. These growth and transcription factors are secreted in the protein-containing glands whose secretions bathe these stem cells on a regular basis – parotid glands for taste buds and nasal serous glands for olfactory epithelial cells. These growth factors are similar for both taste buds and olfactory epithelial cells and include cAMP, cGMP, and sonic hedgehog. They also include moieties such as hormones, vitamins, trace metals and other metabolic moieties which comprise the complex milieu of these fluids necessary for the activation of these stem cells to initiate growth and development of these receptors.