Fig. 2. Hedgehog signaling controls vasculogenesis and Foxf1 expression. (A-D) Morphology and histology of wt and Ihh^-/-; Shh^-/- E9.5 mouse embryos. (A) E9.5 wt embryo with well-developed yolk sac vasculature. The embryo has completed turning and reached the fetal position. (B) E9.5 Ihh^-/-; Shh^-/- embryo with an almost completely avascular yolk sac. The embryo is still in the primitive, unturned position and the amnion adheres to the inner surface of the yolk sac (arrowheads in B). (C) Section of a wt E9.5 yolk sac with large blood vessels containing embryonic erythrocytes. (D) Section of an Ihh^-/-; Shh^-/- E9.5 yolk sac. The yolk sac mesoderm consists of a thin lining of the inner surface of the yolk sac which in some areas adheres to the outer, mesodermal layer of the amnion (arrowhead). (E,F) Whole-mount in situ hybridization of E9.5 embryos with a Foxf1 probe. (E) Wt embryo showing high-level expression of Foxf1 in the lateral plate and extraembryonic mesoderm of allantois and yolk sac. (F) Lateral (top) and posterior-dorsal (bottom) views of an Ihh^-/-; Shh^-/- embryo. Foxf1 expression is absent in lateral mesoderm and yolk sac, but present in allantois and posterior primitive streak (arrowheads). al, allantois; am, amnion; lp, lateral plate mesoderm; ys, yolk sac; yse, yolk sac endoderm; ysm, yolk sac mesoderm.