Fig. 9. Activity of the Gata4 lateral mesoderm enhancer requires BMP4. (A,B) The G2-lacZ transgene was crossed into either a Bmp4^+/– or a Bmp4^–/– background, and embryos were collected at 7.5 dpc and stained with X-gal. Heterozygous Bmp4^+/–; G2-lacZ Tg/0 embryos displayed strong expression of lacZ in the lateral mesoderm (LM) and allantois (al; A). By contrast, expression directed by the Gata4 lateral mesoderm enhancer in Bmp4^–/–; G2-lacZ Tg/0 embryos was dramatically attenuated in all embryos examined (B), indicating that transgene activity is dependent on BMP4. (C-F) Explanted tissue containing the heart (hrt) and septum transversum (ST) of embryos from a single G2-lacZ stable transgenic line (C,D) or a single SMaa-lacZ stable transgenic line (E,F) was collected at 9.5 dpc and cultured for 48 hours in the presence of BSA (C,E) or recombinant Noggin (D,F). Following incubation, explants were X-gal stained. Gata4 lateral mesoderm enhancer activity was significantly reduced in all 15 viable embryo explants treated with Noggin when compared with the 15 viable embryo explants treated with BSA. By contrast, no differences were observed in X-gal staining in the heart or somites in SMaa-lacZ explants treated with BSA or Noggin (compare E and F), indicating that embryo explants were not in general crisis in the presence of Noggin. Viability of embryo explants was assessed by the observation of a continually beating heart.