FGF-signalling in the differentiation of mouse ES cells towards ...

Figure 1-1: Early embryo development from blastocyst stage to development through the primitive streak. A)Schematic representation of mouse embryonic development and signal gradients of BMP4, WNT, and nodal fromblastocyst to late-streak stages. The red line in the late streak-stage embryo marks a cross-section, which is shown inB). Names of marker genes expressed in a tissue-type are noted in brackets. B) Cross-section of embryo illustratinghow epiblast cells (in blue) move through the PS and form the definitive endoderm (DE) germ layer (in yellow)displacing the visceral endoderm (VE; in green). In between the DE and epiblast/ ectoderm, the mesoderm germ layer(in red) forms. Ant, anterior; AVE, anterior visceral endoderm; DE, definitive endoderm; Dist, distal; ExE, extraembryonicectoderm; ICM, inner cell mass; Post, posterior; Prox, proximal; PS, primitive streak; VE, visceralendoderm. Modified from (Gadue et al. 2005; Wolpert 2002).Cells start gastrulation by undergoing an epithelial-to-mesenchymal transition, enabling them tomove through the PS (Figure 1-1B, cross-section of the PS). FGF3, 4 and 8 are present in the PSand FGF4 and 8 are required for gastrulating cells to leave the PS (Sun et al. 1999). From chickenstudies it is suggested that FGF4 and 8 act as a chemo-attractant and a chemo-repellent,respectively on the migrating cells, driving cell movements after ingression through the PS(Wilkinson et al. 1988; Hebert et al. 1991; Niswander and Martin 1992; Sun et al. 1999; Yang etal. 2002b; Bottcher and Niehrs 2005). When cells have moved through the PS, they become eitherdefinitive endoderm (DE), displacing the visceral endoderm, or mesoderm forming anintermediate germ layer between the DE and the epiblast (Lawson et al. 1991; Tam et al. 1993;Carey et al. 1995). The remaining cells of the former epiblast become the ectoderm germ layer.Cells moving through the mid- and posterior PS early become different types of mesoderm,including some extra-embryonic mesoderm, influenced by BMP4 and low concentrations ofnodal (Parameswaran and Tam 1995; Kinder et al. 1999). The mesoderm expresses Fetal-likekinase 1 (Flk1) along with Noggin and Chordin. DE is formed from the anterior regions of the PS,influenced mainly by high concentrations of nodal. Here, Forkhead box A2 (Foxa2) is expressedduring gastrulation and is later found in the DE (Tam and Beddington 1992; Robb and Tam2004). A specific marker for the DE germ layer at this stage is the Sry-related HMG box gene 17(Sox17), which is not expressed in the other two germ layers, but is found also in the VE (Kanai-Azuma et al. 2002). The VE expresses the Sry-related HMG box gene 7 (Sox7), which is notfound in DE (Futaki et al. 2004; Seguin et al. 2008). DE cells migrating through the PS earlyduring gastrulation populate the anterior parts of the endoderm, the later foregut, and cells leavingthe PS late in gastrulation populate the posterior endoderm, the later hindgut (Tam andBeddington 1992).Gut tube patterning and regionalizationBy the end of gastrulation around E7.5 in the mouse, the embryo contains two major signallingcentres, the anterior VE (AVE) and the node. The AVE is found at the anterior side of the embryo(Figure 1-1A) and acts to restrict the PS structure and is involved in the head fold (Wells andMelton 2000). The node is found in the distal-most part of the embryo cylinder, the anterior limitof the PS, from where it directs DE patterning among other activities.The DE is patterned along the A-P axis through interactions with the anterior ectoderm, cardiacmesoderm, and notochord to form the anterior gut tube and with the node, lateral plate mesodermand PS to form the posterior gut tube (Wells and Melton 1999; Wells and Melton 2000). At E7.5anterior and posterior gut tube can be distinguished from one another not only by theirlocalization in the embryo, but also by their expression of marker genes. Mouse-Cerberus 1 (m-Cer1), Orthodenticle homeobox 2 (Otx2), Hematopoietically expressed homeobox (Hex) andHomeobox expressed in ES cells 1 (Hesx1) are expressed anteriorly and Cdx2 and certainmembers of the Hox family are expressed posteriorly. Dividing the forming gut tube into fourregions can illustrate how they re-localize to form the proper gut tube (Figure 1-2A&B; E8.5):Region I, the most anterior part, seems to fold over region II and form the ventral foregut, givingrise to the lungs, stomach, liver and ventral pancreas. Region II gives rise to the oesophagus,stomach, duodenum and dorsal pancreas. Similarly, region IV seems to fold over region III,giving rise to posterior trunk endoderm and hindgut, forming the large intestine. Region III givesrise to the midgut and trunk endoderm, forming the small intestine (Wells and Melton 1999).Closure of the gut tube happens as the anterior intestinal portal (AIP) and caudal intestinal portal8