Question

Taste receptors in the tongue are replenished periodically, presumably from a stem-cell niche residing in the tongue itself. For an image of the organization of taste receptors in the tongue. Both FGF and Wnt signaling are known to be important for the pattern of where some types of taste receptors are positioned in the tongue. Based on your answer to part (a), develop a hypothesis that explains how this might occur and how a niche for taste receptors might be regulated. Then, propose experimental approaches to test your hypothesis

Answer 1

FGF and Wnt signalling, both are important and associated in patterning the taste papillae. How?

Taste buds are clusters of neuroepithelial cell receptors, that are housed in highly organized structures known as the taste papillae. Considering a rodent tongue, three papillae are known: anterior fungiform, posterior foliate and single midline circumvallate. The number of taste buds in each of these regions vary. It is hypothesized that the taste bud formation and patterning of papillae occurs at an early tongue development stage during embryogenesis. The steps of patterning possibly includes formation of epithelial thickenings (taste placodes), which evaginates to form raised structures (papillae). It is within the mature papillae that the taste bud cells then differentiate.

To understand the effect of Wnt signalling, mutation of the Wnt 10 could be carried out. It is most likely to have results such as reduction of fungiform papillae in humans as well as null mice (mutated or knocked out), but no complete elimination will be noticed. Infact, imaging studies have proven that addition of a Wnt activator increases the papillae in number and size and subjecting to treatment with Wnt antagonist results in decreased or eliminated papillae. Thus, Wnt mainly regulates the tongue papillae pattern. However, FGF may bring about post-transcriptional modifications, which may affect Wnt’s regulation of the papillae size.

Experimental studies on SHH is the best to learn effects of Wnt and FGF on tongue taste patterning. Shh is a marker of the fungiform papillae. Size and general pattern of papillary field can be assessed. Also, inter-papillary space should be quantified between genotypes (different mutants created). Mature papillae can be checked also by immunofluorescence staining using antibody against cytokeratin 8, that marks all cell types and also by staining for the neuronal ?-III tubulin marker. In-situ hybridization detection, coupled with complementation studies using Wnt knockout and FGF mutated systems could help in establishing the relationship between the two signalling pathways.