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Fig. 7. Several Bmp-dependent developmental processes occur in the absence of embryonic Smad4. (A-G) Heart formation in Smad4 mutant embryos. Specification of the heart field in control (A) and mutant (B) E7.5 embryos as demonstrated by the cardiac marker Nkx2.5 (A,B, lateral view; A',B', anterior view). (C) Higher magnification view of the anterior region of a mutant embryo at E8.5. (D) Coronal section shows heart tube formation with cardiomyocyte differentiation, which is further confirmed by expression of cardiac alpha actin at E8.5 (E,F; higher magnification of mutant shown in G). (H,I) The chorion (ch) forms a stalk-like structure often associated with epiblast-derived allantoic mesoderm (al), as shown by lacZ expression in Sox2Cre/+;Smad4^CA/N;ROSA26^R/+ embryos. (J-M) Alkaline phosphatase (AP) staining for identification of primordial germ cells (PGCs) viewed in flattened tissue preparations. (J) At E7.5, PGCs cluster at the base of the allantois in wild-type embryos. (K) In Smad4 mutants, PGC formation is not observed. (L) In wild type at E8.5, PGCs begin their anteriorwards migration and intercalate into the hindgut endoderm. (M) Despite formation of allantois, most Smad4 mutants lack recognizable PGCs at this stage.

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