Fig. 2. Smad4^N/N embryos fail to gastrulate. (A,B) Wild-type (WT) and Smad4^N/N embryos (N/N) at E6.5. The wild-type embryo displays normal egg cylinder morphology with prominent epiblast (epi) epithelium. By contrast, the mutant embryo (B) shows retarded growth, with circumferential thickening of the visceral endoderm (ve). (C,D) Sagittal sections of E6.5 wild-type and Smad4^N/N embryos within the deciduum. The wild-type embryo has distinct embryonic and extra-embryonic regions and has begun gastrulation, as evidenced by the formation of posterior mesoderm (m). By contrast, Smad4^N/N embryos show no mesoderm formation, the visceral endoderm accumulates distally and the epiblast is significantly reduced, with intermingling of extra-embryonic ectoderm (eee). epc, ectoplacental cone. (E-J) Whole-mount in situ hybridization at E6.5 of (E,G,I) wild-type and (F,H,J) Smad4^N/N mutant embryos. (E,F) Oct4 is robustly expressed in the epiblast of both mutant and wild-type embryos. (G,I) Bmp4 and eomesodermin (Eomes) expression identifies extra-embryonic ectoderm atop the epiblast. In Smad4^N/N embryos, this expression domain is displaced distally into the embryonic region (H,J). (I) Eomes transcripts also mark the nascent primitive streak in wild-type embryos.