Study of the development of the atrioventricular conduction system as a consequence of observing an extra atrioventricular node in the normal heart of a human fetus.

We have observed an extra atrioventricular node in the normal heart of a human fetus. It is located in the septal wall of the right atrium, subendocardially, and just where Todaro's tendon leaves this wall to go toward the inferior vena cava valve. In its trajectory, this tendon gives way to a remarkable prominence in the cavity of the right atrium: the sinus band. In order to explain the embryogenesis of this extra atrioventricular node, we have studied the normal development of the atrioventricular specific system and have concluded that the atrioventricular node is formed from a growth and displacement toward the atrium of the primitive atrioventricular specific material, which originates from the myocardium of the posterior wall of the atrioventricular canal. Likewise, during its development, the atrioventricular node keeps in close proximity with the Todaro's tendon. In our view, this accounts for the embryogenesis of the extra atrioventricular node, since a fragment of the atrioventricular node can remain cranial to Todaro's tendon and be displaced by it in a craniodorsal direction. This fragment would then lead to the formation of an extra atrioventricular node like the one present in the heart of the fetus we have examined.