The Xenopus tadpole gut: fate maps and morphogenetic movements.

We have produced a comprehensive fate map showing where the organs of the gut and respiratory system are derived from in the early Xenopus laevis endoderm. We also show the origin of the associated smooth muscle layer on a separate fate map. Comparison of the two maps shows that for most organs of the gut the prospective epithelium and smooth muscle do not overlie each other in the early embryo but come together at a later stage. These fate maps should be useful for future studies into endoderm specification. It was not previously known how the elongation of the endoderm occurs, how the single-layered dorsal and many-layered ventral endoderm gives rise to the single layered epithelium, and whether or not the archenteron cavity actually gives rise to the gut lumen. Using a variety of labelling procedures we show firstly, that radial intercalation occurs in the gut transforming a short thick tube into a long thin tube; secondly, that the archenteron lining does not become the definitive gut lumen. Instead the archenteron cavity almost closes at tailbud stages before providing a nucleus for the definitive gut cavity, which opens up during elongation. Based on this work we present a model explaining the morphogenesis of the gut.