The Secondary Heart Field: Understanding Conotruncal Defects from a Developmental Perspective

The development of a septated outflow is complicated, and insight into this process has been limited. However, our lab and others have recently described a secondary source of myocardium that adds to the lengthening outflow tract at later stages of development. Without the addition of this secondary heart field (SHF), the outflow is shortened and can not undergo the rotation necessary to make appropriate ventriculoarterial connections. Defects of outflow alignment such as tetralogy of Fallot and double outlet right ventricle may therefore be the result of problems with the addition of the secondary heart field. Moreover, the SHF adds a smooth muscle component to the outflow above the semilunar valves which may be necessary for proper insertion of the coronary stems into the aorta. This review will use the newly discovered SHF as well as other recent advances in the understanding of outflow development to shed light on the developmental pathology behind conotruncal defects.