The Xenopus tadpole gut: fate maps and morphogenetic movements

We have produced a comprehensive fate map showing where the organs of the g ut and respiratory system are derived from in the early Xenopus laevis endo derm. We also show the origin of the associated smooth muscle layer on a se parate fate map. Comparison of the two maps shows that for most organs of t he gut the prospective epithelium and smooth muscle do not overlie each oth er 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 t he single layered epithelium, and whether or not the archenteron cavity act ually 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 lin ing does not become the definitive gut lumen. Instead the archenteron cavit y almost closes at tailbud stages before providing a nucleus for the defini tive gut cavity, which opens up during elongation. Based on this work we pr esent a model explaining the morphogenesis of the gut.