Fig. 3. Assessment of cardiac defects in MLC2a-null embryos. Left (A,D,G,J,M,P,S,V), front (B,E,H,K,N,Q,T,W) and right (C,F,I,L,O,R,U,X) views of wild-type (+/+) and MLC2a-null (–/–) embryos at somite pairs 10 (A-F), 13 (G-L), 16 (M-R) and 22 (S-X). Relative to the linear heart tubes of wild-type littermates (A-C), mutant heart tubes appeared to be enlarged and amorphous, with no clear distinction between the bulbus arteriosus and future left ventricle of the heart (D-F). With progression through looping morphogenesis, mutant hearts exhibited aberrant morphologies in each cardiac chamber, accompanied by overall abnormalities in looping architecture (J-L,P-R,V-X).