Specification of taste bud progenitor cells

Taste buds are multicelluar endorgans which are unique among receptor cells in several aspects. Developmentally, taste buds resemble epithelial cells in that they arise from proliferative basal cells in the oropharyngeal epithelium. Taste cells, despite their epithelial origin, also possess properties of neurons, and thus represent an intersection of epithelial and neural cell types. The oropharyngeal endoderm (OPE), which lines the mouth and pharynx of amphibians, is specified to produce taste cells during gastrulation, which occurs in axolotls during the second day of development, by signals from the axial mesoderm. Following gastrulation the OPE produces taste buds autonomously, without influence from other embryonic tissues. However, differentiated taste buds do not appear until hatching, at about day 10. During this long developmental period (day 3--10), taste bud progenitor cells must be specified, and subsequently divide to produce proliferative basal stem cells, which in turn generate fusiform taste cells. This dissertation addresses: (1) when during development does specification of taste bud progenitor cells occur (Chapter 2); (2) what are the cellular mechanisms responsible for this specification (Chapter 2); and (3) what are the molecular mechanisms of specification (Chapter 3). In addition, preliminary data are presented on the development of a new method of altering gene expression in axolotl embryos via a combination of microinjection and electroporation (Chapter 4).; Through a series of disaggregation/reaggregation assays, I determined that taste bud progenitor cells are specified during a 12-hour window during tailbud stages (shortly after neurulation is complete). In addition, these experiments ascertained that taste bud progenitors are specified by a cell-contact dependent mechanism(s) (Chapter 2). Finally, I demonstrate that members of the Shh and BMP signaling pathways are involved in this process of taste bud progenitor specification (Chapter 3). BMPs are inhibitory signals during specification of taste bud progenitors, and also likely act similarly during the subsequent development of taste cell lineages from these progenitors. The studies presented here indicate that not only do taste cells posses characteristics of both epithelial and neural cell types, but that their development reflects a unique intersection of developmental mechanisms of both epithelial and neural tissues.