Fig. 5. Foxf1 is required for Bmp4 expression in lateral and extraembryonic mesoderm. X-Gal staining was used to visualize expression from the Bmp4^lacZ allele in E8.5 Foxf1^+/+ (A-E) and Foxf1^-/- (F-J) mouse embryos. (A) Intact wt conceptus with embryo that has just initiated turning. High Bmp4 expression can be seen in both the allantois and yolk sac. (B) Unturned wt embryo with yolk sac removed showing Bmp4 expression in the amnion (speckled pattern covering embryo), allantois, heart, and extending from the posterior primitive streak into the splanchnopleure and somatopleure. (C) Transverse section showing Bmp4 expression in yolk sac mesoderm (at the top of the image), throughout the allantois and in the amnion (bottom). (D) The inner (mesodermal) surface of the yolk sac shows Bmp4 expression associated with the developing vasculature. (E) Section showing Bmp4 expression in yolk sac mesoderm and amnion. (F,G) Lateral (F) and dorsal (G) views of an Foxf1^-/- embryo showing lack of Bmp4 expression in the yolk sac and allantois. Only the primitive streak and heart retain high expression. (H) Section showing scattered and very weak Bmp4 expression in Foxf1^-/- allantois. (I) The inner (mesodermal) surface of a Foxf1^-/- yolk sac shows only traces of Bmp4 expression. (J) Section of Foxf1^-/- yolk sac with characteristic separation between the endodermal and mesodermal layers. Occasional mesodermal cells express low levels of Bmp4. al, allantois; am, amnion; ps, primitive streak; ys, yolk sac; yse, yolk sac endoderm; ysm, yolk sac mesoderm.