The septation of the outflow tract
Due to the various crease directions at various locations, each of the septa in the outflow tract receives its typical localization. Here also hemodynamic factors play an important role in that the blood moves to the outflow tract in a spiral stream. In addition, the outflow tract lengthens. In stage 11, the conus, which lies within the pericardium, still borders directly on the aortic sac that lies in the mesenchyma of the neck. The boundary between the aortic sac and the conus part can be recognized in that the myocardial mantle still only surrounds the conus part.
In the course of further development of the heart the segment between the conus part and the aortic sac lengthens within the pericardium. Among other places, the material for this segment comes fromthat migrate over the pharyngeal arches III, IV and VI and are responsible for the development of the truncus arteriosus as well as the divisions of the outflow tract. Based on this fact, one always speaks in the followin. the conotruncus
The lengthening of the truncus part causes a further sulcus to occur between the conus and truncus that in the later development marks the formation of the endocardial cushions for the aorta and pulmonary valves (see semilunar valves). Very important for the division of the outflow tracts is the fact that the two streams of blood thus flow around each other spirally, influencing the growth of both the conus and truncus septa.
Three components are responsible for forming the septa in the outflow tract (according to how the blood flows):
- Conus septum
- Truncus septum
- Aorto-pulmonary septum
For the conus and truncus septa initially only the endocardial cushions, formed through the influence of immigrated neural crest cells, are involved.They grow into the lumen and join up in the middle.
The aorto-pulmonary septum consists of the mesenchyma between the pharyngeal arches IV and VI. (see also: development of the arteries). It grows as an unpaired relief in a direction that is against that of the blood stream.
Through the spiral-shaped blood flow its right process grows onto the lower and its left process towards the upper truncus septum and unites with it. The upper truncus septum grows against the flow onto the right conus septum and the lower towards the left conus septum.Thus the outflow tract septum has completed a rotation of 180 degrees.
Finally, the left conus septum joins with the free border of the septum interventriculare after the crista prima has regressed. The right conus septum joins with the dorsal av-septum.