The outflow tract of the heart--embryologic and morphologic correlations

Abstract

The separation process of the heart by which two great arteries and two outflow tracts are formed, was studied microscopically in 20 human embryos, ranging from 6 to 28 mm crown-rump length and macroscopically in eight hearts, ranging from 28 weeks of gestation to 80 years of age. The proximal (primary fold) and distal (ventriculo-arterial junction) borderlines of the outlet segment of the embryonic heart are important landmarks in this process. The remarkable, curved and twisted configuration of the ventriculo-arterial junction implies that the position of the arterial orifices, as well as the relative dimensions of the corresponding outflow tracts, are, already in a very early stage, similar to those in the fully developed heart. It furthermore implies that the separation by the aorto-pulmonary septum starts at this level and immediately involves the outlet segment where the two columns of the aorto-pulmonary septum mobilize the myocardium to form the posterior wall of the right ventricular outflow tract, rather than a septum between both outflow tracts. These findings make the morphology of the outflow tract of the normal heart comprehensible from a developmental point of view and throw a new light upon the morphogenesis of outflow tract malformations.