Development and septation of the ventricles and outflow tract

Abstract

As mentioned in chapter 2, the ventricles balloon out from the greater curvature of the tubular heart at the end of the looping stage by means of expansive apical growth [Harh & Paul, 1975; Steding & Seidl, 1980; Lamers et al., 1992]. As will be discussed in the next chapter, this ballooning is accompanied by the formation of myocardial trabeculation. In between the ventricles a gradually elongating ridge is formed, which is dubbed the (muscular part of the) interventricular septum. Rather than being passively formed by expansion of the trabeculed ventricles, the interventricular septum is an initially fenestrated structure that is formed by aggregation and compaction of apically enlarging trabeculae, as has been demonstrated in chicken embryos [Harh & Paul, 1975]. It should be stressed again that the inner curvature is not involved in the ballooning process but instead retains its primordial tubular phenotype, as do the outer curvature parts of the interjacent segments, being the sinus venosus, the atrioventricular canal and the outflow tract. By now, the primitive heart is on the eve of intricate developmental events which will transform the single heart tube into a double circuited structure that selectively passes the blood from the systemic veins to the pulmonary arteries and the blood from the pulmonary veins to the aorta. To this end, two separate pathways must be formed, which demands that the atria and ventricles will be connected to the proper up- and downstream segments.