Development of the Primary Mouth

The extreme anterior of the deuterostome embryo is unique in that ectoderm and endoderm are directly juxtaposed, without intervening mesoderm. From this region the initial opening between the gut and outside of the embryo breaks through. We have termed this the primary mouth, which is essential for eating and therefore life. In vertebrates, the neural crest grows around the primary mouth to form the face and a secondary mouth opening must therefore form. The primary mouth then becomes the pharyngeal opening. In order to establish a molecular understanding of this important process we have examined primary mouth formation during Xenopus development. We find that multiple steps are involved. An early step involves dissolution of the basal lamina separating ectoderm and endoderm. A subsequent step requires cell death in the ectoderm. Later, the ectodermal and endodermal layers overcome their normally separate identities and intercalate to generate a single cell layer. The final step is perforation, where the primary mouth breaks through. From fate mapping, we have defined the ectodermal and endodermal regions that will form the primary mouth. Extirpations and transplants indicate that, surprisingly, ectoderm from any region of the embryo can be used to generate the primary mouth. In contrast, endoderm specifically in the presumptive primary mouth region provides essential signals. We have performed expression microarray analysis to define genes expressed during primary mouth formation, and have defined at least two that are required for this process.