Chapter 120. Septation of Ventricles, Truncus Arteriosus, and Conus Cordis - Review of Medical Embryology Book

A MUSCULAR CREST (ridge or fold) appears on the anterior ventricular wall near its floor and almost in a median plane near the apex, at the same time that the interatrial septum is forming, at about week 5. This is the interventricular septum primordium

The interventricular septum is incomplete and has an upper free concave edge
Most of its increase in length, at first, is the result of dilatation of the ventricles on each side of the septum, which produces an external interventricular groove
With active growth, the septum forms the muscular portion of the interventricular septum
A crescenteric interventricular foramen is seen between the free edge of the septum and the fused endocardial cushions, allowing for communication between the right and left ventricles, until about the end of week 7

During week 5, aortic arch VI appears and contributes to the formation of the pulmonary arteries. Just cephalic to arch VI, the subendocardial tissue in the bulbus cordis thickens into 2 opposing ridges called the truncoconal or bulbar ridges. Semilunar ridges also form in the truncus arteriosus and are continuous with those in the bulbus cordis
The bulbar ridges soon fuse with those of the truncus arteriosus

The fusion takes a spiral orientation, possibly due to the streaming of blood from the ventricles, and forms the aorticopulmonary septum, which definitively separates the aorta and the pulmonary artery

As a result of the spiral orientation of the septum, the aorta and pulmonary artery twist around each other

Blood from the aorta now passes into the third and fourth parts of the aortic arches
Blood from the pulmonary trunk flows into the sixth pair of aortic arches

In the adult right ventricle, the bulbus cordis is seen as the conus arteriosus or infundibulum; and in the left ventricle, it is seen as the aortic vestibule

CLOSURE OF THE INTERVENTRICULAR FORAMEN and completion of the interventricular septum

Ventricular septation is completed by closure of the interventricular communication (foramen) around the end of week 7, as the bulbar ridges fus Closure from fusion of subendocardial tissue is from 3 sources

Fusion of the two ridges and the endocardial cushion outgrowth forms the membranous portion of the interventricular septum and is completed toward the end of month

This tissue fuses with the aorticopulmonary septum and muscular portion of the interventricular septum, thus, the pulmonary trunk communicates with the right ventricle and the aorta with the left ventricle