Septation of ventricles (embryology)

By the start of the fifth week of gestation, there are two primitive ventricles in continuity: the left one is derived from the true primitive ventricle; the right is derived from the bulbus cordis. Both differentiate to become trabeculated - rough-surfaced - internally. The muscular ridge between them on the caudal part of the inner wall starts to elongate cephalically by the growth of cells on the outside of the chamber and the regression of cells internally. This sheet is termed the muscular interventricular septum.

The growth of the muscular septum does not extend as far as the endocardial cushions. The space between the two provides a means of communication between the ventricles; it is termed the interventricular foramen. Eventually, the foramen is filled by:

• the inferior elongation of the conus septum, the dividing sheet across the conus cordis
• growth of cells from the inferior endocardial cushion along the superior margin of the intermuscular septum;

The interventricular foramen forms the membranous part of the interventricular wall in the mature, normal heart.