Fig. 3. Altered specification of midline properties in the absence of retinoids. Dorsal views and transverse sections of normal and VAD whole-mount quail embryos hybridized with the markers indicated. (A-B') Compared with normal embryos where Raldh2 expression is localized in prechordal mesendoderm (pme) and at lower levels in anterior endoderm (ae), VAD embryos have patches of ectopic expression along the whole length of anterior axial mesoderm (B, arrowheads in inset and in B'). (C-D') Ectopic Gsc expression is detected in presumptive notochord (no) cells (arrowhead in D'). (E-F'') Axial mesodermal Crs expression is found more posteriorly in VAD embryos than in normal embryos (compare arrows in E,F). Transverse sections shown (E'-F'') indicate absence of Crs expression in anterior (en) (arrowhead in F') and lateral embryonic endoderm (le) (arrows in F'') of VAD embryos. (G-H') Similar to the previous markers, Bmp2 expression in prechordal mesoderm is posteriorly expanded in VAD embryos (arrow in H). In addition, ectopic expression is detected in ventral neuroectodermal cells (asterisk in H'). (I-J') The notochord marker chordin shows a posteriorly displaced anterior expression boundary in VAD embryos when compared with normal embryos (arrow in inset in J). Transverse sections at the level of presumptive notochord indicate diminished expression of chordin in VAD embryos (arrowhead in J'). (K-L') The axial mesodermal marker Shh is expressed at lower levels in the most anterior region (arrow in inset in L and arrowhead in L'). Coloured lines designate the AP position of the transverse sections shown laterally. Arrowheads in C,D,I,J indicate the position of the somites. Note that the embryo in H is slightly older than the embryo in G. Scale bar in A: 100 µm in A,B,E,F,G,H; 125 µm in C,D; 150 µm in I-L; 50 µm in A'-L'.