Fig. 7. Colonization of Smad1 mutant embryos by wild-type ES cells fails to rescue gastrulation defects. Chimeras were generated by injecting lacZ-positive wild-type ES cells into host blastocysts collected from Smad1^+/- intercrosses. (A,B) Two examples of Smad1^-/- embryos significantly colonized by wild-type ES cells. (A,A') The posterior visceral endoderm defect is apparent in whole-mount and transverse section. (B,B') A distal section from a second strongly chimeric embryo illustrates that the twisting of the epiblast is not rescued by the presence of wild-type cells. The arrowhead points to the VE ruffles. Anterior is towards the left in all panels. ep, epiblast; me, mesoderm; ve, visceral endoderm.