Fig. 2. Impact of loss of Dkk1 on mesoderm and endoderm differentiation and anterior morphogenesis. (A) Dkk1^-/- embryo shows an expanded domain (black asterisk) of BATGal (2/2 embryos) and Axin2 expression (3/3 embryos) in the head folds and anterior tissues, and loses head structures anterior to the upper hindbrain level (red arrow) with an abrupt truncation of Shh-expressing axial mesoderm and floor plate. Scale bar: 100 µm. (B,C) Marker analysis of the wild-type and Dkk1^-/- embryos at E7.0 (B: Foxa2, Chrd and Lhx1; C: Cer1 and Sox17), E7.5 (B: Mixl1, T and Tbx6), E8.5 (C: Foxa2) and E9.0 (C: Sox17). Scale bar: 50 µm. Markers of anterior visceral endoderm are expressed in the mutant embryos (black asterisk). The domain (indicated by lines) of Foxa2 and Chrd expression in the node area is broader, and T expression extends more anteriorly in the head process mesoderm. The endoderm is formed (Cer1), but fewer Sox17- expressing cells are present in the mutant embryo (black arrow). Sox17 expression is absent in the foregut of the E9.0 mutant embryo. At E8.5, the head truncation is clearly visible (Foxa2: whole-mount embryo, red arrow), and the histology (transverse section of the head) shows reduced Foxa2 expression in the foregut (red asterisk) and the neural plate (green bar).