ABSTRACT
Two body systems, the gastrointestinal (GI) and respiratory tracts, can be thought of as elaborate tubes that deal with these inputs and, with the exception of the role played by the kidneys, also handle most of the waste issues. The definitive endoderm is one of the three embryonic germ layers, and, among other organs, gives rise to many of the component tissues of the GI and respiratory tracts. The stem cells associated with the GI and respiratory tracts serve well to illustrate the demands that need to be satisfied by tissue-restricted stem cells in organs whose cell turnover can range from low, as for the respiratory system, to very high in the GI tract. The endoderm forms the innermost layer of the embryo and as gastrulation proceeds, morphogenetic movements transform it into the primitive gut tube, which is surrounded by the mesodermal layer.
