The aortico-pulmonary (AP) septum arises within the truncus arteriosus. The septum results from the downwards growth and fusion of bulbar ridges, induced by invasion of neural crest cells. The AP septum serves to divide the ventricular outflow between the pulmonary artery and the ascending aorta.
In addition to dividing cardiac outflow, the AP septum contributes to the formation of the semilunar valves. Anomalous formation of the AP septum can lead to several anomalies. Eisenmenger's syndrome occurs as a result of equal division of the truncus with an incomplete fusion of bulbar ridges distally. When the division of the AP septum occurs asymmetrically, the tetralogy of Fallot results.
Once fusion has occurred, the AP septum has the following appearance (the tube has been made transparent to show the fused bulbar ridges). Note the symmetric division of the truncus arteriosus, and the 180-degree twist given to the septum. The process of septal formation can be seen as a 3D animation (avi file, 767kb).
