Figure 11.

The images show histological sections stained with hematoxylin and eosin, in four-chamber orientation, revealing the arrangement of the primary ring at its inferior transition from the ventricular to the atrial components of the developing heart. (A) Panel A shows the arrangement at Carnegie stage 17, equal to around 6 weeks of gestation. At this stage, the vestibular spine and mesenchymal cap have still to muscularise. (B) Panel B shows the situation at the end of the embryonic period of development. The spine and cap have muscularised and provide the septal inputs to the compact node, which is derived from the primary ring. The ring extends between the horns of the inferior atrioventricular cushion as the ring itself transitions to become the compact atrioventricular node. At this stage, the insulation between the base of the node and the crest of the muscular septum has yet to be formed.