Taste buds consist of a diverse collection of sensory cells embedded in the oropharyngeal epithelium. The taste receptor cells arise embryologically from local epithelial elements rather than being a neural cell lineage like other receptors of the nervous system. This project will utilize molecular and cellular methods to determine: 1) What molecular signals are involved in the differentiation of taste buds and taste papillae form the surrounding epithelium, and 2) whether the diverse phenotypes of taste receptor cells found in adult taste buds can be attributed to their origins from different progenitor cells. In the first aim, in situ hybridization will be utilized to characterize the timing and tissue distribution of mRNA's of developmental patterning genes related to the sonic hedgehog (Shh) signaling protein which is present in the epithelium at the outset of gustatory morphogenesis. The role of Shh in determination of taste buds will be tested by exogenous application of Shh protein in explant cultures of the developing tongue from mice and rats. The presence of the specific gustatory neurotrophin BDNF, will be used as an indicator of early taste papillary morphogenesis. In addition, we will survey the explants for altered distribution and levels of mRNA's downstream from Shh in other developing systems. These experiments will reveal whether the presence of the Shh signaling protein is sufficient to initiate gustatory morphogenesis. The final series of experiments in this first aim will test the respective roles of epithelium and mesenchyme during formation of taste buds and taste papillae. For these experiments, epithelial and mesenchymal co-grafts, or epithelium- only grafts from early pre-papillary embryos, will be placed in oculo. Early taste bud development will be assessed by examining the tissue for the presence of molecular markers of early gustatory differentiation such as BDNF or Shh. These experiments will demonstrate whether the epithelium by itself is capable of initiating gustatogenesis.
The second aim of the proposed studies will test whether the different cytochemical phenotypes of taste receptor cells correlate to origins from separate progenitor cell populations. In these experiments, mosaic analysis of hemizygous female mice carrying an X-linked marker, will be utilized to determine whether serotonin-immunoreactive or gustducin- immunoreactive taste cells tend to be related to one another, i.e. share a common lineage. Conversely, other experiments under this aim will test whether some taste cell phenotypes correlate with a cell's post-mitotic age, that is to test whether some cytochemical phenotypes may arise only after a certain period of post-mitotic maturation.
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