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Fig. 5. Conditional removal of Smad2 in the context of Smad3 deficiency leads to gastrulation and streak patterning defects. Whole-mount views or sections as indicated of (A) Sox2Cre;Smad2^Robm1/CA (S2 ca/-), (B-D) Sox2Cre;Smad2^Robm1/CA;Smad3^null/+ (S2 ca/-;S3^+/–), (E-H',K-L'',O,P) Sox2Cre; Smad2^Robm1/CA;Smad3^null/null (S2 ca/-;S3^–/–) and (I-J',M-N) wild-type (WT) embryos at (I-L'') E7.5 and (A-H',M-P) E8.5. (A) Sox2Cre;Smad2^Robm1/CA mutants display anterior truncations similar to Smad2^+/–;Smad3^–/– embryos and normal bilateral somites (s) (A'). (B) Combined reduction of Smad3 gene dose to one wild-type allele and loss of Smad2 in the epiblast leads to consistent anterior truncations and elimination of notochord (nc), node and definitive endoderm, which results in somite fusions across the midline (B'). (C,D) Failure to detect Shh transcripts by whole-mount in situ hybridization confirms absence of midline structures in Sox2Cre;Smad2^Robm1/CA;Smad3^null/+ mutants. (E-H') Combined loss of Smad2 and Smad3 in the epiblast significantly impacts mesoderm formation and patterning. Mutant embryos are mainly composed of neuroectoderm (ne). (I-L'') T expression in the presumptive posterior marks nascent mesoderm forming in the primitive streak (ps). However, production of embryonic mesoderm is greatly diminished, as evidenced by restricted T expression and absence of the paraxial mesoderm marker Meox1 (M-P). Small pockets of presumptive heart mesoderm are occasionally observed. By contrast, extra-embryonic mesoderm (xm) is formed, lining the visceral yolk sac and forming a compact structure resembling an allantois (F-H').

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