(A,B) α-phospho Smad 1/5/8 and (C,D) α-phospho Smad 2/3 antibody immunohistochemistry in E12.5 Wnt1cre;TgEngloxP (B,D) compared to wild type (A,C) embryos. Note that there is a similar percentage of pSmad 1/5/8-expressing cells in the wall of the vessel (red boxes in A,B) and no ectopic expression of pSmad 2/3 in the neural crest-derived vessels (arrows, C,D), as revealed by . (E-H) H&E analysis of sections through the heart region of E11 embryos which are either (E) wild type, (F) Wnt1cre;TgEngloxP, (G) Smad4 conditional allele on the heterozygous null background (Wnt1cre;Sm4c/n) alone, or (H) in compound mutant mouse embryos in which endoglin is over expressed and Smad4 is simultaneously inactivated in neural crest cells (Wnt1cre;Sm4c/n;TgEngloxP). Asterisks indicate hemorrhaging (F). Arrow indicates region of persistent truncus arteriosus (PTA) in panel H which is absent in panels E-G. (I) Immunoblot analysis of total α-SMA (vascular smooth muscle) and Flk-1 (endothelium) in E11.5 Wnt1cre;TgEngloxP embryos compared to wild type. (J) Endoglin immunoprecipitation followed by immunoblot analysis of endoglin expression upon cre-recombination in wild type versus SM22αcre;TgEngloxP transgenic embryos. (K) Immunoblot analysis of total α-SMA in E11.5 SM22αcre;TgEngloxP whole embryos compared to wild type embryos.