In the last two decades the molecular analysis of axis induction and embryonic patterning has provided a blueprint for body axis

INTRODUCTION In the last two decades the molecular analysis of axis induction and embryonic patterning has provided a blueprint for body axis formation and organizer function in several species, including amphibians, chicken, zebrafish and mouse (Arnold and Robertson, 2009; De Robertis et al., 2000; Niehrs, 2004; Nowotschin and Hadjantonakis, 2010). In the mouse, the anterior-posterior (A-P) axis is firmly established when the anterior visceral endoderm (AVE) is formed on the future anterior side of the embryo before gastrulation commences at embryonic day (E) 6.5. At this stage, the embryo resembles an egg cylinder-shaped epiblast that is surrounded by extraembryonic visceral endoderm (exVE), which constitutes the endodermal component of the yolk sac, and by extra-embryonic ectoderm (ExE) that gives rise to the placenta. The pluripotent epiblast cells give rise to all embryonic lineages, namely ectoderm, mesoderm and definitive endoderm (DE), during gastrulation at E6.5-7.5. At E5.5, Bmp4 from the ExE induces autoregulatory Nodal signalling that spreads through the epiblast in a proximal to distal direction to induce the Nodal inhibitor Lefty1 and the Wnt inhibitors Dkk1 and Cerl in the distal visceral endoderm (DVE) (Brennan et al., 2001). Migration of the DVE cells to the anterior side of the epiblast and the formation of the AVE converts the proximal-distal (P-D) axis into the A-P axis between E5.5 and E6.5 (Takaoka et al., 2011; Yamamoto et al., 2004). The AVE cells express genes such as Dkk1 (Mukhopadhyay et al., 2001), Hex (Martinez-Barbera et al., 2000), Hesx1 (Hermesz et al., 1996; Thomas and Beddington, 1996), Lim1 (Lhx1) (Barnes et al., 1994; Shawlot and Behringer, 1995) and Otx2 (Ang et al., 1994; Simeone et al., 1992; Simeone et al., 1993), which are involved in anterior neuroectoderm induction. Mutations in these genes specifically result in anterior head truncations, indicating that the AVE displays head organizer function (Beddington and Robertson, 1998). Besides this function, the AVE also expresses inhibitors of the Wnt/β-catenin and Nodal/TGFβ signalling pathways, which restricts the activity of these pathways to the posterior side of the embryo. Here, Wnt3 is first expressed in the posterior visceral endoderm (PVE) beginning at E5.5 (Rivera-Pérez and Magnuson, 2005) and is subsequently induced in the proximal and posterior epiblast via a Nodal-dependent Bmp4 activation at E5.5-6.5 (Ben-Haim et al., 2006). The restriction of Nodal/TGFβ and Wnt/β-catenin signalling to the posterior epiblast allows the formation of the gastrula organizer, the formation of the primitive streak (PS), and specifies the mesoderm and DE in a dose-dependent manner at E6.5-7.5 (Arnold and Robertson, 2009). The pre-gastrula stage embryo is patterned before organizer genes, such as Gsc, Foxa2, Chrd and Nog, are induced in the posterior epiblast region during gastrulation (Beddington and Robertson, 1998; Kinder et al., 2001; Takaoka et al., 2011; Thomas and Beddington, 1996). It has long been known that the PVE covers the posterior PS region where the gastrula organizer is induced (Tam and Beddington, 1992); however, whether it is essential for gastrula organizer gene induction remains a long-standing question (Beddington and Robertson, 1999; Liu et al., 1999; Rivera-Pérez and Magnuson, 2005; Tam and Behringer, 1997). 1Institute of Stem Cell Research, Helmholtz Zentrum München, 85764 Neuherberg, Germany. 2Institute of Diabetes and Regeneration Research, Helmholtz Zentrum München, 85764 Neuherberg, Germany. 3Adolf-Budenandt Institute and Munich Center for Integrated Protein Science (CiPSM), Ludwig-Maximillian University, Schillerstrasse 44, 80336 Munich, Germany.