Formation of the Ventricles

In the newly formed bulboventricular loop the primitive right and left ventricles appear as expansions in the heart tube. Externally the interventricular sulcus separates the right and left ventricles and internally they are separated by the bulboventricular flange. Remember that the right ventricle arises from the proximal bulbus cordis.

Formation of two-parts of the ventricular chamber

During the shifting of the atrioventricular canal the proximal bulbus cordis expands forming the right ventricle. Both ventricles will continue to expand until the late 7th/early 8th week. The growth of the ventricles is due to the centrifugal growth of the myocardium and the diverticulation of the internal walls. (This is what gives the ventricle its trabeculated appearance).The muscular interventricular septum forms as a result of the expanding ventricles. The walls of the right and left ventricules grow in opposition to each other to form the muscluar septum. Thus,the septum will cease to grow when the ventriclar walls are no longer expanding.

Partitioning of the Outflow Tract

The final morphological change in the heart is the partitioning of the outflow tract—the truncus arteriosus and the conus cordis—into the aorta and the pulmonary trunk. This is accomplished by the development of a septum that forms in the outflow tract and the emergence of the two great vessels.

Stages of Development of Ventricular Septum

The septum forms from two pairs of swellings which grow from the walls of the outflow tract.These are the truncus swellings and the conus swellings.

Truncal Swellings: Right superior which grows distally and to the left. Left inferior which grows distally and to the right. Both develop at the proximal part of the truncus and proceed to grow in two directions; 1) distally towards the aortic sac and 2) into the lumen of the outflow tract where they will eventually fuse together.

Conus Swellings: Right dorsal which is continuous with the right superior Left ventral which is continuous with the left inferior Like the truncal swellings, the conal swellings grow distally and towards each other, however they appear after the first pair. These conus swellings eventually fuse with the truncal swellings.