Fig. 1. Retinoids are synthesized in tissues involved in early anterior patterning. (A-G') Dorsal views and sections of stages 3-7+ whole-mount chick embryos hybridized with the markers indicated laterally. (A) In anterior regions, Raldh2 is expressed in the hypoblast (hy), and it remains confined (B,C) to the lower layer in anterior endoderm (ae in mid-sagittal section in inset). Asterisk in C indicates anterior endoderm. (D) At head process stages, anterior Raldh2 expression is mainly localized in prechordal mesendoderm (pme), as seen in transverse sections (D'). Lines (D) indicate the position of the transverse sections (D',D''). (E) At two-somite stages, Raldh2 is maintained in prechordal mesoderm and foregut endoderm with a sharp posterior boundary at the level of the notochord (blue arrow in mid-sagittal section in E', dorsal is left). (F,F') Expression of Raldh3 in the node (no) and in ingressing mesodermal cells (red arrowhead). Strong expression is also visible in the marginal zone surrounding the embryo. (G,G') Stage 4+ embryos hybridized with RXRg have a restricted expression in mesodermal cells in the node region (G', red arrowhead). Red arrows indicate the position of the node. Scale bar in A: 260 µm in A; 400 µm in B,C,D,F,G; 220 µm in E; 200 µm in D'-D''',E',F',G'; 270 µm in inset in C.