Advanced - Outflow Tract

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Cross-sections of the outflow tract illustrating the truncal and bulbar ridges

Cardiac neural crest originates between somites 1 and 3 of the neural tube and migrates through the pharyngeal arches to contribute to the conotruncal septum. Active proliferation of pharyngeal mesenchymal cells in the bulbus cordis during the fifth week creates bulbar ridges which are continuous in the truncus arteriosus (see image to the right). The cardiac neural crest migrates into these ridges, condensing as cellular columns to support the outflow tract septum. The ridges form a 180° spiral to create the helical aorticopulmonary septum. Myocardialisation of the ridges gives a zippering effect resulting in fusion. Fusion occurs in a distal to proximal direction during the sixth week, allowing for cleavage of the aorta and pulmonary trunk. The spiralling nature of the ridges causes the pulmonary trunk to twist around the aorta. The bulbus cordis accounts for the smooth conus arteriosus (or infundibulum) in the right ventricle and the aortic vestibule in the left ventricle. The animation below depicts the septation of the outflow tract. (Click image to play on current page or Play video on new page).

<html5media height="720" width="560">File:Outflow tract 001.mp4</html5media>

The outflow tract is one of the most common sites of cardiac abnormalities, as it requires normal development and proliferation of multiple cell types. Abnormalities commonly occur via defects in the following areas:

  • Anterior/secondary heart field - abnormal contribution/proliferation leads to an elongation defect
  • Neural crest cells - abnormal migration/proliferation leads to a septation defect
  • Myocardium - abnormal rotation/laterality leads to an alignment defect
  • Endocardium - abnormal EMT/proliferation leads to a cushion defect

These abnormalities are consequently expressed as a host of disorders involving the conotruncal region such as common arterial trunk, double outlet right ventricle, interrupted aortic arch, transposition of the great arteries, tetralogy of fallot and ventricular septal defect.

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