Trends in Genetics
ReviewEndoderm development: from patterning to organogenesis
Section snippets
Formation of the gut tube
Although ectoderm and endoderm share the common feature of forming a tube from a flat sheet of cells, the way in which the tubes form is different. The neural tube arises from the apposition of two lateral ridges of ectoderm that close in an anterior to posterior direction, whereas the gut tube, in mouse and chicken embryos, forms from a crescent-shaped fold, the so-called anterior intestinal portal (AIP), which appears in the endoderm at the anterior tip of the embryo when somitogenesis
Gut organs have overlapping presumptive territories
Soon after the formation of the gut tube, the onset of organogenesis is revealed by the swelling, budding and coiling of specific regions. How organ formation correlates with specific regions of the flat endodermal sheet has been a recurrent question, asked as early as 1874 when His17 published a map of the presumptive digestive and respiratory organs of the chick blastoderm. More recently, fate-mapping experiments have traced the fate of cells in the definitive digestive organs in the chick7,
Budding organs off the main gut tube
The endoderm not only gives rise to the digestive tract but also to organs that branch from the main tube. These organs include, from anterior to posterior, the thyroid, parathyroids, thymus, ultimobranchial body, respiratory system, liver, gallbladder, pancreas and caecum (Fig. 4). Gland formation generally begins as a thickening of the epithelium. Epithelial cells then either remain connected by a duct, as for the liver, gallbladder and pancreas, or epithelial buds migrate away from the gut
Morphogenesis
After the position of each organ is defined, a complex cell choreography that involves variations in cell adhesion, mobility, attractive and repulsive cues as well as proliferation and cell death, shapes the organs.
Differentiation of cell types within the different regions
Markers of terminal differentiation are expressed prior to endoderm organ morphogenesis64 . For instance, expression of intestinal fatty acid binding protein (IFABP) is already observed at E7.5 in the mouse3 and glucagon-producing cells appear in the gut epithelium prior to the formation of the pancreatic bud3. Cells at this stage do not express the entire panel of differentiation genes required for their function in the adult and the meaning of this early differentiation is still obscure.
To
Conclusions
One can expect that the endoderm will receive increased attention in the next few years as researchers turn their attention to organogenesis and the genetic control of physiological function. Targeted mutations in mice will probably reveal many genes that specify cytodifferentiation in endodermal derivatives, as well as important information on cell lineages. From our embryological perspective, there are two key issues: (1) understanding how the gut tube is patterned to make different organs,
Acknowledgements
We regret citing reviews in many instances rather than the original publications owing to space constraints. We are grateful to J. Wells, M. Hebrok and M. Rhamalho Santos for helpful comments on the manuscript and to D. Procupez for her help with Fig. 1. A.G-B. was supported by fellowships from the Association Française pour la Recherche contre le Cancer and Human Frontier Science Program. Work in the authors’ laboratory is supported by the Howard Hughes Medical Institute and National
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