Development of the gut in Xenopus laevis.

The lining of the gut, together with the pancreas, liver, gall bladder, and respiratory system, is formed from the endoderm. The gut also contains smooth muscle and connective tissue of mesodermal origin. The amphibian Xenopus laevis is potentially an excellent model organism for studying how the cells of the endoderm and mesoderm become programmed to produce these internal organs. However, the anatomical complexity of the coiled gut presents a problem in studying its development. In order to overcome this problem we here present a comprehensive guide to the anatomy and histology of the developing Xenopus gut. We use a simple dissection to display its anatomy and the expression of four endodermal markers (alkaline phosphatase, IFABP, XlHbox8, and endodermin). We present schematic diagrams that show how the gut is arranged in three dimensions and how this organisation changes during development. We also present drawings of histological sections of the gut which allow any region to be identified and so represent an atlas for working with sections. Finally, we describe the histology of the cells of the various organs of the gut. This histological identification may be necessary for the identification of parts following experiments in which the normal pattern is disturbed.