Class III mutant hearts had defects in AV node morphology. Hearts were sectioned and stained with Masson's trichrome. B and D, Higher magnifications of boxed areas in A and C, respectively. Note the fibrosis (yellow arrowheads in B) within 1 AV node and the twisted Y shape of the other AV node (D) compared with the ovoid compact control node (Figure 3A and 3B). Ao indicates aorta; AVN, AV node; His, His bundle; LV, left ventricle; and RV, right ventricle. Scale bar 500 μm(A and C) and 200 μm (B and D).