Electroporation of Nucleic Acids into Chick Endoderm Both In Vitro and In Ovo

During gastrulation, vertebrate embryos generate three different germ layers, the ectoderm, mesoderm and endoderm. The endoderm layer is situated in the most ventral part of the embryo and differentiates into various tissues including the gut, respiratory and endocrine epithelium. In amniotes, the endoderm spreads out in a sheet-like manner and forms the most ventral layer in the early embryo. Subsequently, the anterior-most endoderm folds ventrally and forms a sack-like structure, the foregut. As the foregut extends posteriorly, the endoderm at the most posterior portion of the embryo forms another sack-like structure, the hindgut, and this grows rostrally. Finally, the foregut and hindgut meet at the level of the small intestine and form a simple tube. After the formation of tube, the endoderm becomes the lining epithelium of the gut and differentiates into various organs according to their position along the anterior7#x2014;posterior and the dorsal-ventral axes. These include the esophagus, lung, stomach, duodenum, pancreas, liver, small intestine and large intestine. These organs show specific morphologies and express particular factors depending on their function (Wells and Melton, 2000).

It has been well established that the differentiation of the endoderm is controlled by the surrounding tissues, principally the mesodermal mesenchyme (Fukuda and Yasugi, 2005; Kim et al., 1997; Yasugi and Fukuda, 2000). Hence, endodermal differentiation is regarded as a model example of tissue interaction during devel opment. As an example of the types of processes that occur during endodermal differentiation, the presumptive dorsal pancreas precursor cells, which reside at the midline of somite level 4–7 in the endodermal layer of the stage 10 chick embryo (Matsushita, 1999), facilitate dorsal pancreatic morphogenesis. In addition, these cells maintain the expression of early pancreas genes via the activity of activin and bFGF secreted from the notochord at around embryonic stage 12 through the repression of Shh (Hebrok et al., 1998). At a later stage, the pre- pancreas endoderm is separated from the notochord by the dorsal aorta. It has also been reported that vascular endothelial growth factor (VEGF) secreted from the dorsal aorta is a key molecule that upregulates insulin expression within the pancreatic endoderm (Lammert et al., 2003). After pancreas bud formation, activin is secreted from the surrounding mesenchyme whereby it induces exocrine cells and prevents an endo crine cell fate arising from the pancreas endoderm (Kumar and Melton, 2003). To further understand the processes of endodermal differentiation, it will be necessary to elucidate the chronological order of the multi-step tissue interactions that take place between endoderm and the surrounding tissues.