tract septum: The cardiac outflow tract include the ventricular outflow tract,
the semilunar valves and the aortopulmonary septum.
There has been much debate regarding this process, the following is
a summary of various theories [6, 36, 40].
that there are three embryological areas, the conus, the truncus and the
pulmonary arterial segments. Each
segment develop two opposing ridges of endocardial tissue, the opposing
pair of ridges and those from various segments meet to form the septum
separating the two outflow tracts and the aortopulmonary trunks.
The aortopulmonary septum is formed by ridges separating the fourth
(future aortic arch) and the sixth (future pulmonary arteries) aortic
arches. The truncus ridges
are formed at the area where the semilunar valves are destined to be
formed, therefore forming the septum between the ascending aorta and the
main pulmonary artery. The
conus ridges form just below the semilunar valves and from the septation
between the right and left ventricular outflow tracts.
agreed that there are three pairs of ridges forming in the aortopulmonary,
truncus and conus regions, however, he stated that the pairs of ridges
fuse independently and later on fuse with each other to complete the
septation. His theory indicate that the truncus ridges form
first, and as they fuse they form a truncal septum which then fuses
with the aortopulmonary septum which is formed by invagination of the
dorsal wall of the aortic sac between the fourth and the sixth aortic arch
arteries. Figure 11
Asami(1980), followed Van Mieropís theory, however, he stated that these
ridge fuse in the opposite direction of what Van Mierop has indicated,
i.e. from the outflow tract to the aortopulmonary region.
1984) and Orts Llorca et al (1982), stated that there are only tow septa, a conotruncal (or bulbar)
and an aortopulmonary septum.
et al (1989),
introduced a new theory. They
stated that the septation process of the ventricular outflow tracts,
pulmonary and aortic valves and the great vessels is mostly caused by a
single septation complex, which they termed aortopulmonary septum.
This septation complex develops at the junction of the muscular
ventricular outflow tract with the aortopulmonary vessel.
This junction has a saddle shape, i.e. not in one plane which would
allow the right ventricular outflow tract to be long with a short main
pulmonary artery, while the left ventricular outflow tract become short
with a long ascending aorta. (Figure
12) The ventricular outflow septation is formed by condensed mesenchyme,
embedded in the endocardial cushion tissue just proximal to the level of
the aorto-pulmonary valves. The
condensed mesenchyme will come in close contact with the outflow tract
myocardium, from the area just above the bulboventricular fold, and
participate in the septation of the outflow tract by providing an analogue
to muscle tissue. [6-9].
Myocardium in contact with the mesenchymal arch grow rapidly and
form the bulk of the outflow septum and is continuous with the primary
fold on the parietal wall of
the right ventricle and the myocardium on the right side of the primary