Fig. 5. Extra-embryonic defects in Smad1^-/- embryos. (A) Afp expression in wild-type (left) and mutant (right) 7.5 dpc embryos. Compared with the wild-type littermate, Afp is excluded from the anterior extra-embryonic region of the mutant embryo. (A') A transverse section through the proximal portion of the mutant embryo demonstrates that Afp expression is localized to the ruffled VE (arrowhead). (B) Eomesodermin (eom) expression demarcates the extra-embryonic ectoderm and anterior streak. In Smad1^-/- embryos, eom-positive cells in the extra-embryonic region are greatly expanded. (C) BMP4 expression delineates the extra-embryonic ectoderm and mesoderm. In the mutant embryo, BMP4 expression is expanded throughout the extra-embryonic part of the embryo. (D) Sox2 is expressed throughout the embryonic and extra-embryonic ectoderm of 8.0 dpc embryos. Note the canal of tissue that normally extends from the anterior portion of the chorion towards the head of the embryo (arrow). (E) In 8.0 dpc Smad1^-/- embryos, Sox2-positive tissue emanates from the posterior portion of the embryo towards the chorion (*). (E') A transverse section through the embryo in (E) demonstrates that the posterior Sox2 projection (*) is a ring of cells. (E'') A section near the EPC shows the erratic folding of the chorion. am, amnion; ec, ectoderm; me, mesoderm; ve, visceral endoderm; ys, yolk sac.