SEPTATION
Atrial septation is initiated when the second heart field derived dorsal mesenchymal protrusion and the myocardial
primary atrial septum (or septum primum) extend ventrally into the, yet
undivided, common atrium. In the mouse, this process takes place between
embryonic day (ED) 9.5 to 10.5; in humans the process occurs around day 30. The
space between the leading edge of the atrial septum and the fusing
atrioventricular cushions in the atrioventricular canal is the primary atrial
foramen. As the primary atrial septum grows toward the mesenchymal
atrioventricular cushions, thereby closing the primary interatrial foramen,
perforations appear in the upper part of the primary atrial septum. These
perforations will eventually coalesce and form the secondary interatrial
foramen. As this part of atrial septation process nears completion, the
secondary atrial septum (or septum secundum) appears in the space between the primary atrial septum and the left venous valve in the roof of the right atrium.
Eventually, the upper part of the primary atrial septum will fuse with the
secondary atrial septum, thereby closing off the secondary atrial foramen and
completing the atrial septation process. Failure of fusion of the two atrial
septa will lead to the congenital defect patent foramen ovale.
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| FIG 1.5 Nervous tissue of embryo at 24 days and 4 weeks. |
Compared with atrial septation, the creation of the ventricular septum is a rather straightforward
process. As the tubular heart expands, undergoes looping, and remodels,
distinctive left and right ventricular components appear. During this process,
a myocardial ridge, the interventricular septum, emerges between the left and
right ventricle. Subsequent outward expansion of the ventricles, a process
sometimes referred to as “ballooning,” in combination with upward growth of the
interventricular septum and eventual fusion of crest of the septum with the
atrioventricular cushions, completes the process of ventricular septation. Cell
lineage tracing experiments in the mouse demonstrated that, like the right
ventricle, the interventricular septum is largely derived from the second heart
field.
The third septal structure that is required for separating the respective
blood flows in the heart is found in the outflow tract. After completion of
cardiac looping, a single outflow tract can be found connected to the right
ventricular component of the yet unseptated heart. Septation of this outflow
tract is required for the formation of an aorta, which eventually connects to
the left ventricle, and a pulmonary trunk that comes from the right ventricle.
Two sets of endocardial ridges are located within the outflow tract, and as a
result of their fusion, these will separate the common outflow tract into an
aorta and a pulmonary trunk. Failure of fusion can lead to congenital defects,
including a double outlet right ventricle.
The cardiac neural crest is also important in the septation process that
separates aorta and pulmonary trunk. Abnormal development of the cardiac neural
crest specifically affects the formation of the aorticopulmonary septum
downstream of the semilunar valves (Fig. 1.5). This can result in the
congenital defect common arterial trunk (or
truncus arteriosus) or in aorticopulmonary window.
