Morphogenesis of bulboventricular malformations. I: Consideration of embryogenesis in the normal heart

Abstract

The results are described of a microscopical study of 17 embryos between the 14th and 22nd horizons of Streeter. The investigation concentrated on the development of the ventricular outflow tracts, and the establishment of the crista supraventricularis of the right ventricle. During the 14th horizon the atrioventricular orifices both open into the primitive ventricle, which in turn communicates with the bulbus via the bulboventricular foramen. The truncus is directly continuous with the bulbus and is unseptated. Thus, both presumptive outflow tracts originate above the bulbus. The bulbotruncal ridges which fuse to septate the truncus are arranged in spiral fashion at first, but they fuse in straight fashion after detorsion of the truncus. After fusion of these ridges the aorta is still present above the bulbus, but during the 16th and 17th horizons it migrates to lie above the primitive ventricle, after the absorption of posterior distal bulbus. This process also brings the proximal edge of the bulbotruncal septum into the cavity of the newly formed right ventricle, and reorientates the primary bulboventricular foramen as the aortic outflow tract. The secondary interventricular foramen is restricted anteriorly by the proximal margin of the bulbotruncal septum, and is finally closed by growth from the inferior cushion which forms the membraneous septum. The crista supraventricularis is formed in part by the right hand margin of the bulbotruncal septum, but this tissue underlies the right hand portion of the bulboatrioventricular ledge, which therefore constitutes an important part of the crista. The significance of these processes of differential bulbar absorption and truncal rotation are discussed in relation to previous theories of cardiac embryogenesis, and the structure of the crista supraventricularis is discussed in relation to cardiac malformations.