Question

define Partitioning of the Conotruncus

Answer 1

During the fifth week of devel­opment, proliferation of the endocardial tissue in the wall of the cbnus cordis results in the formation of ridges (swell­ings) in the right dorsal and left ventral wall of the conus cordis. These ridges, called bulbar ridges (also called conus swellings), grow toward each other to form a septum, called conal septum, which divides the conus cordis into two parts: (1) an anterolateral part, which forms the outflow tract of the right ven­tricle (called conus arteriosus or infundibulum), and (2) a posteromedial part which forms the outflow tract of the right ventricle (called aortic vestibule). Distally the conal septum joins the sep-
tum developing in the truncus arteriosus, whereas proximally the conal septum contributes tissue to the de­veloping interventricular septum (vide infra).
When bulbar ridges are being formed in the conus cordis, similar ridges, called truncal ridges (also called truncus swellings) appear in the truncus arteriosus. These ridges pur­sue a spiral course in the truncus arte­riosus. Fusion of the truncal ridges with each other produces a spiral aorticopulmonary septum, which divides the truncus arteriosus into two large arterial channels: ascending aorta and pulmonary trunk.
As mentioned earlier, the distal end of the conal septum joins the cau­dal end of the aorticopulmonary sep­tum, so that the two septa behave as one continuous septum. This arrange­ment also ensures that the blood from the infundibulum passes into the pulmonary trunk, whereas the blood from the aortic vestibule passes into the as­cending aorta.
Initially the ascending aorta and pulmonary trunk are fused to each other because the aorticopulmonary septum forms a common partition be­tween these two subdivisions of the truncus arteriosus. However, as a re­sult of programmed cell death, a split occurs in the aorticopulmonary septum. Consequently, these two great arteries become separate and independent vessels. It is to be noted that, due to the spiral course of the aorticopulmon­ary septum, the separated ascending

aorta and pulmonary trunk slightly in­tertwine, the latter crossing ventral to the ascending aorta.
Neural crest cells migrate through the pharyngeal arches to reach the developing bulbar ridges and truncus swellings. These cells contribute greatly to the formation of septa in the conus and truncus. Abnormalities in migration, proliferation, or differentia­tion of neural crest cells result in the production of many congenital heart defects (described later).