Figure 18.

This frontal section, from a human embryo at Carnegie stage 16, shows the distinction between the proximal and distal parts of the outflow tract. The distal part has been separated by the distal cushions into the intrapericardial portions of the aorta and the pulmonary trunk, the walls of which now possess an arterial phenotype. Note the continuation of the pulmonary trunk through the arterial duct, derived from the sixth arch, to the descending aorta. The proximal outflow tract, in contrast, is still enclosed within muscular walls. The arrows show the site of the developing sinutubular junction, formed at the initial dog-leg bend. Note that the proximal cushions are fused distally, with the site of fusion marked by condensed mesenchyme. The more proximal parts of the cushions, however, penetrated by prongs from the condensed mesenchymal mass, remain unfused. Completion of fusion, followed by muscularisation, will convert this area into the supraventricular crest of the right ventricle.