The unique organization of the vertebrae head is primarily due to the origin of ectodermal placodes and neutral crest, two embryonic tissues that form much of the peripheal nervous system. Identifying the neutral structures that arise from these tissues, and understanding their interactions with adjacent epithelial tissues, constitutes long-term goals of the laboratory. The epibranchial neurogenic placodes, a series of ectodermal thickenings located dorsal to each embryonic pharyngeal pouch, give rise to sensory neurons of the ganglia of the facial, glossospharyngeal, and vagal nerves, which innervate taste buds. These placodes are known to be induced by pharyngeal endoderm, but the time frame of their induction is not known. In order to determine the temporal window for the induction of epibranchial placodes (aim 1), presumptive placodal ectoderm of embryonic axolotls will be cultured in isolation or heterotopically transplanted onto the trunk of host embryos. Although the taste buds of embryonic axolotls develop autonomously from pharyngeal endoderm, the predominant model for the development of taste buds has been a neural induction model. In many vertebrate groups, however, pharyngeal taste buds arise from both endoderm and ectoderm. It is possible that endodermal and ectodermal taste buds are induced in different ways. In order to test the hypothesis that ectodermally derived taste buds, such as occur on the trunk of catfish, are induced by sensory fibers of the facial recurrent ramus (aim 2), these fibers will be prevented from entering the trunk, or trunk ectoderm will be cultured prior to its innervation. There is growing evidence that retinoic acid acts as a signal to establish the anteroposterior axis of many embryonic structures. To test the hypothesis that retinoic acid acts as a signal to pattern endoderm directly during gastrulation (aim 3), presumptive pharyngeal endoderm will be excised, exposed to retinoic acid, and cultured.
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