Generation of the body axis in Xenopus: Characterization of germ layer and organizer formation

Axis formation in Xenopus laevis requires generation of the three primary germ layers, endoderm, mesoderm and ectoderm, as well as establishment of Spemann\u27s organizer. Spemann\u27s organizer patterns the primary germ layers to establish the anteroposterior and dorsoventral axes. To better understand vertebrate axis formation, I have characterized molecular events that generate the primary germ layers and Spemann\u27s organizer. I have characterized the role of the maternal transcription factor VegT and the TGFβ family member Nodal in the regulation of early endodermal and mesodermal gene expression. Also, I have demonstrated the ability of the endoderm-specific transcription factor Sox17α to prevent the expression of mesodermal genes in response to Nodal signals. My results suggest that direct regulation of Sox17α by VegT establishes the endoderm, distinct from the mesoderm, by preventing the expression of mesodermal genes in response to Nodal signals. Finally, I have characterized the cooperation between vegetal TGFβ signals, known to regulate endoderm and mesoderm formation and Siamois, a Wnt target gene, in generation of the organizer. These data support a model in which the organizer forms in the region of overlap between maternal Wnt signals, localized to the dorsal side of the embryo, and TGFβ signals present in the vegetal hemisphere. While the distinct responses of the three germ layers to the influence of Spemann\u27s organizer are critical for proper axis formation, my results indicate that establishment of Spemann\u27s organizer is dependent on the signals that generate the endoderm and mesoderm