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

SOX2 and OCT4. Later, the PE and epiblast transiently express Fgf5 (Haub and Goldfarb 1991;Hebert et al. 1991).Gastrulation and germ-layer formationAt E5.5 the epiblast is formed as a cup-like structure surrounded by the visceral endoderm (VE)which patterns the epiblast and initiates gastrulation (Figure 1-1A; (Rossant 2004)). It is at theposterior, proximal part of this cup that the primitive streak (PS) forms around E6.5 from where itstarts to migrate distally (Figure 1-1A). There is an anterior-posterior (A-P) patterning of theepiblast prior to gastrulation. Onset of PS formation is initiated by a gradient of the transforminggrowth factor β (TGFβ)-family member nodal and wingless-type MMTV integration site 3(WNT3) signalling from the posterior epiblast. Nodal generates a proximal-to-distal gradient andWNT3a forms a posterior-to-anterior gradient (Liu et al. 1999; Ben-Haim et al. 2006; Arnold andRobertson 2009). The nodal gradient moves distally, forming the PS along the way and finallyends at the distal-most part of the embryo, the node (Gadue et al. 2005). At the late streak stage,nodal expression is only found in the node where it forms a distal-to-proximal signal gradient. TheWNT3 expression pattern is restricted to the posterior epiblast during PS-formation. At the sametime, the TGFβ family member bone morphogenetic protein 4 (BMP4) from the extra-embryonicectoderm (ExE) signals to the adjacent epiblast. Thereby a proximal-to-distal signal gradientopposing that of nodal is formed (Lawson et al. 1999). The shape of these morphogeneticgradients is modulated by the reciprocal expression of antagonists or inhibitors, these being leftyand cerberus-like inhibiting nodal-signalling, dickkopf-related protein 1 (DKK1) inhibitingWNT3-signalling, and chordin and noggin inhibiting BMP4-signalling (Gadue et al. 2005).Thereby, the extension of the PS is restricted to the posterior side of the embryo.The primitive streak expresses the T-box transcription factor Brachyury (T) in migrating cells ofthe PS and Mix1 homeobox-like (Mixl1), along with Even-skipped homeobox homolog 1 (Evx1)(Figure 1-1A; (Bastian and Gruss 1990; Dush and Martin 1992; Kispert and Herrmann 1994; Nget al. 2005)). Goosecoid (Gsc) is expressed in the progressing PS and localises to the anteriorstreak (Blum et al. 1992).7