Endoderm patterning in Xenopus laevis

The endoderm is the inner germ layer of the vertebrate embryo from which the respiratory and digestive systems are derived. These include organs such as the liver, pancreas, stomach, lungs and intestine. Recent research has helped our understanding of early vertebrate endoderm specification and terminal differentiation of specific endodermal lineages. However, very little is known about the molecular mechanisms that control endoderm patterning and morphogenesis during vertebrate development. As a way to identify genes involved in these elusive steps of development, I performed a differential hybridisation screen in a macroarray tailbud ventral foregut cDNA library coupled with in situ hybridisation analysis. My aim was to identify and characterise new regionally expressed endodermal genes in Xenopus laevis, a classical embryologic model organism. Here, I report the identification and characterisation of a dozen novel regionally expressed endoderm genes. At tailbud stages their expression patterns fall into three re-occurring domains; anterior ventral midgut endoderm, posterior endoderm and dorsal endoderm. In addition, regional expression of some of these genes is observable at gastrula stages, endoderm specification. These are the first early stable endodermal markers for different regions of the gastrula endoderm. This suggests that the earliest steps in endoderm patterning are concurrent with endoderm specification. Furthermore I describe the identification of a mesodermal transcription factor, which appears to be expressed in ‘early embryonic macrophages’ - and a poorly characterised embryonic cell population. I present an overview of endoderm development together with the results from my screen. Overall, these results reveal an unexpected degree of early endodermal patterning and assist our understanding of the link between early and late events of vertebrate endoderm development. In addition, this work provides us with new and very useful markers for endodermal patterning, and potentially some key developmental regulators of endodermal formation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo