Fig. 6. Fgf8b is required for the normal induction of Lefty2 and T, and for proper alignment between the AP axis and the shape of the embryo. (A-C) In situ hybridization with an RNA probe detecting both Lefty1 and Lefty2 in E6.5 murine embryos of the indicated genotypes. Expression of Lefty1 in the anterior visceral endoderm (AVE; marked by arrowhead) is unaffected in Fgf8^b/b (B) or Fgf8^-/- (C) embryos, whereas Lefty2 expression in the emerging primitive streak (arrow) is missing in the mutants. Insets show distal views of the embryos in A and C. Double-headed arrow in A indicates the height of the epiblast (see J). (D-F) Analysis of T expression in E6.5 embryos of the indicated genotypes. Arrowhead marks the T expression domain in the distal extraembryonic ectoderm, while the arrow marks T expression in the posterior epiblast. (G,H) Expression of Cer1 in the AVE of wild-type (G) and Fgf8^b/b (H) embryos at E5.75. Insets show distal views of the embryo. Broken double-headed arrow marks the long axis of the embryo. (I) Schematic summary of the AP polarity with respect to the shape of wild-type and Fgf8^b/b embryos at E5.75 and E6.5. Notice that the shift of AP axis fails to occur in the Fgf8^b/b embryo. (J) Distribution of the ratio of AP and LR dimensions relating to the height of the epiblast (indicated by double-headed arrow in A) between E6.0 and E6.75 from intercrosses of Fgf8^+/b mutants. Each dot represents one embryo. AP, anteroposterior; LR, left-right. Scale bars: 50 µm in F for A-H, including insets in G and H; 50 µm in inset in C for insets in A-C.