| Summary: | The definitive endoderm is one of the three germ layers of the embryo that are generated during gastrulation and gives rise to the lungs, liver, pancreas and the gut. Understanding the development of the definitive endoderm is crucial for future in vitro based approaches to regenerative medicine for diabetes or liver regeneration.
However, progress in the understanding of definitive endoderm development has been limited by the lack of genetic markers specific to this tissue. To address this, our lab previously performed gene expression profiling of the definitive endoderm using Serial Analysis of Gene Expression. From this study, a number of genes expressed specifically in this tissue, including Nephrocan and Peptide YY, were uncovered. In an attempt to uncover novel genes expressed in the definitive endoderm, an extended study of this gene expression profiling was done, and two ESTs, Endy and NAPS, were further identified. Through whole mount in situ hybridization analysis of the early mouse embryo, expression of Endy and NAPS was seen in different cell populations of the developing definitive endoderm.
In addition to the SAGE analysis, an ES cell differentiation system for definitive endoderm was set up, as a tool to study some of the early signaling pathways leading to the development of the definitive endoderm. Differentiation of mES cells using Activin induced expression of the novel definitive endoderm markers, Nepn and Pyy, in a temporal manner. Inhibition of TGFβ signaling during differentiation resulted in a significant down regulation of these genes. Furthermore, differentiation of mES cells mutant for TGFβ signaling factor Foxh1 revealed an expression pattern for Nepn and Pyy that was inconsistent with what is observed in vivo. Lastly, the ES cell differentiation system was also used to test expression of Endy and NAPS as little molecular information existed for these ESTs.
Collectively, the characterization of these markers in vivo, and manipulations of the ES cell differentiation system to definitive endoderm will facilitate the creation of more accurate fate maps of the definitive endoderm, and address some of the questions regarding early lineage decisions during specification and patterning of this tissue.
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