Fig. 4. Analysis of potential downstream effector targets of Bmp signaling in Isl1Cre/+;Bmpr1a mutants and wild-type littermate controls. (A-O,T,U,V) Whole-mount and section RNA in situ analysis of transcription factors required for cardiac development. In Isl1Cre/+;Bmpr1a mutants, expression of Tbx20 is down (A,G), Isl1 is upregulated in outflow tract (B,C,H,I); Tbx2 is downregulated in the heart and Tbx3 is downregulated both in heart and hindlimb bud (D-F,J-L,M-O,T-V), relative to somite-matched littermate controls. (P-S,W-Z) X-gal staining of a conduction system marker, CCS-lacZ in wild-type and Isl1Cre/+;Bmpr1a mutant backgrounds. Conduction system cells are formed and present in Isl1Cre/+;Bmpr1a mutants, suggesting that severe reduction of Tbx3 staining does not reflect absence of Tbx3-expressing cells. OFT, outflow tract; RV, right ventricle; LV, left ventricle; A/V, atrioventricular canal; LA, left atrium; RA, right atrium. Arrows in A,G indicate the heart; in B,C,E,H,I,K indicate the OFT; in D,F,J,L,M,O,T,V indicate A/V; and in N,U indicate the hind limb bud. In S and Z, thin arrows indicate A/V, thick arrows indicate ventricular septum.