A glimpse into the molecular entrails of endoderm formation
- Department of Biochemistry and Biophysics, Programs in Developmental Biology, Genetics, and Human Genetics, University of California, San Francisco, San Francisco, California 94143-0448, USA
This extract was created in the absence of an abstract.
During organogenesis, the endoderm forms the epithelial lining of the primitive gut tube from which the alimentary canal and associated organs, such as the liver and pancreas, develop. Despite the physiological importance of these organs, our knowledge of the genes regulating endoderm development has been limited. In the past few years, we have witnessed a rapid pace of discoveries regarding the initial formation of this germ layer. Because the insights have come from studies in several model systems, I have chosen to discuss endoderm formation not only in vertebrate model systems but also inCaenorhabditis elegans, Drosophila, sea urchins, and ascidians. These studies reveal a high degree of conservation in some of the transcriptional regulators of endoderm differentiation, but seemingly more divergence in the intercellular signaling events leading to formation of this tissue. For example, transcription factors of the Gata and Forkhead families are implicated in endoderm development across the phyla. On the other hand, a role for TGF-β, and more specifically Nodal, signaling, which is critical for endoderm formation in zebrafish, mouse, and Xenopus, appears to be vertebrate specific. The studies on vertebrate endoderm formation also suggest that we have now gathered sufficient information to attempt to coax mammalian stem cells towards the endodermal lineage.
Definition of the endoderm
The endoderm is classically defined as the innermost layer of the three Metazoan “germ layers” and as such it gives rise to the inner lining of the gut and its associated organs. However, the term “endoderm” has also been coopted for other developing structures such as some of the extraembryonic tissues in mammalian embryos. Until the time when the nomenclature is adequately revised, the classically defined embryonic endoderm is often referred to as “definitive endoderm” and these are the cells whose formation, or initial differentiation, I will cover in this review. In …
