The unique organization of the vertebrate head is primarily due to the origin of ectodermal placodes and neural crest, two embryonic tissues that form much of the peripheral nervous system. Identifying the neural structures that arise from these tissues, and understanding their interactions with adjacent epithelial tissues, constitute long-term goals of this laboratory. Although neurons of the sensory ganglia of the facial, glossopharyngeal, and vagal nerves are known to arise from epibranchial placodes, a placodal series closely associated with the pharyngeal pouches, and from neural crest, the exact origin of the neurons that innervate taste buds is unknown. In order to determine whether crest-derived and/or placodally derived sensory neurons innervate taste buds, axolotl embryos at the one- or two-cell stage will be injected with biotinylated dextran amines and the labeled neural crest or placodal tissues homotopically transplanted to unlabeled donors. By following the labeled peripheral processes of the sensory neurons that develop from these tissues, and identifying their targets, it will be possible to determine the origin of the neurons that innervate taste buds (aim 1). The progenitor cells that form taste buds have been experimentally demonstrated to arise locally within pharyngeal epithelium but may be induced by pioneering sensory fibers of the gustatory nerves and/or the earlier arrival of neural crest cells that form the branchial skeleton. In order to identify the role of pioneering fibers and branchial neural crest in the induction and maintenance of taste buds (aim 2), these tissues and pharyngeal endoderm will be manipulated in a series of in vivo and in vitro experiments using embryonic axolotls. The induction of epibranchial placodes is a third aspect of the development of the gustatory system that is poorly understood. Chordamesoderm, neural crest and pharyngeal endoderm are suspected inducers. The roles of these tissues and the possible involvement of Hox genes in the specification of epibranchial placodes will be examined (aim 3) in a series of in vivo transplantation and in situ hybridization experiments. These experiments should begin to unravel the complex interactions of the tissues that form the vertebrate head, and they are a necessary preamble to identifying the molecular signals underlying the development of the gustatory system.
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