An investigation of endoderm to mesoderm signalling in gut development

It has been suggested that the intestinal endoderm is responsive to signals received from underlying mesoderm, indicating cross-talk between the layers. Cdx2 mutant mice develop heterotopias of stomach-type epithelium within the paracaecal region of the intestine. They are therefore an ideal in-vivo model in which to study whether the loss of endodermal gene expression required for intestinal development leads to stomach-specific mesodermal gene expression. In this study, Sox2, a stomach endoderm-specific marker, was used to detect gastric heterotopias in Cdx2 mutant embryonic intestine using whole-mount in-situ hybridisation. The nature of the underlying mesoderm was investigated using a second marker, Barx1, which is known to be specifically expressed in the stomach mesoderm. RT-PCR was used to detect low levels of Barx1 expression in Cdx2+/- caecum samples. Further investigation using in-situ hybridisation techniques indicated regions of Barx1 expression in a similar distribution to the regions detected using the Sox2 probe. This finding confirms that the mesoderm underlying the Cdx2 mutant gastric heterotopias expresses a stomach-specific gene and therefore that the mesoderm is responsive to endodermal signals. It has been suggested that Cdx2+/- mice do not develop gastric-type intestinal heterotopias postnatally. If proven, this would indicate that intestinal stem cell potential becomes limited at some point during development and prevents the epithelium responding to a postnatal loss of Cdx2 protein. A conditional Cdx2 mouse model is required in order to investigate this hypothesis. The creation of this mouse model formed the second part of this project. Following creation of a targeting vector, transfection into ES cells and injection into blastocysts, chimeric mice were obtained. These mice were successfully bred for germline transmission.