Fig. 2. Conduction system maturation failure in Tbx5^del/+ mice. The atrioventricular canal conduction system in minK^lacZ/+ (A,B) and Tbx5^del/+/minK^lacZ/+ (C,D) hearts was studied in newborn (A,C) and adult (B,D) mice. The atria were removed and the atrioventricular canal was viewed from the superior/posterior, with the tricuspid annulus on the right and the mitral annulus on the left. Rings of specialized conduction cells observed in the atrioventricular canal of newborn minK^lacZ/+ mouse hearts (A) matured into a well-defined atrioventricular node (arrow) and atrioventricular bundle in adult minK^lacZ/+ mouse hearts (B). (A) Scale bar: 200 µm. (B) Scale bar: 800 µm. Rings of specialized conduction cells in the atrioventricular canal of newborn Tbx5^del/+/minK^lacZ/+ mouse hearts (C) failed to mature into a discrete atrioventricular node or atrioventricular bundle in adult Tbx5^del/+/minK^lacZ/+ mouse hearts (D). Instead the neonatal pattern (rings of specialized conduction tissue) was maintained. Arrow denotes expected location of the atrioventricular node.