Figure 4. Insulation by collagen in the atrioventricular junction and near the atrioventricular bundle.

(A) Sagittal section of the ventricular base of a 1-year alligator, showing complete insulation by connective tissue ventrally (blue arrowhead) and myocardial atrioventricular continuity dorsally (insert, enlarged in A’). (B-B’) The dorsal pathway expresses Tbx3. (C) Dorsally, there is broad atrioventricular continuity, this section is 800 µm to the right of the section of A and there is still atrioventricular continuity (dashed circle). (D) In this region of the heart, Tbx3 is confined to the atrioventricular canal (avc) and is not expressed in the ventricle as in B.