Between 22 and 28 days after conception, the fetal heart is transformed from a pulsatile tube into the hollow single chambered pump pictured below. During the fourth week of development, the heart tube begins looping to form a shape that more closely resembles the adult heart. Around day 28 new structures begin to form which direct and divide the flow of blood inside the heart.

Endocardial cushions

The first division is accomplished by the endocardial cushions. The atrioventricular cushions are derived from endocardial cell proliferation and migration. The endocardial cushions begin the separation of the heart into right and left, upper and lower chambers. These chambers will become the atria and ventricles. The endocardial cushions are crucial to formation of the atrial and ventricular.

The Atrial Septum

Atrial division begins around day 33, about the same time nostrils and hand plates are developing. The atrial septum begins as a thin walled sagittal fold in the common atrium that grows toward the cardiac cushions. The common atrium is divided by the atrial septum, also known as the septum primum. The atrial septum reduces the flow of oxygenated blood from the right atrium to the left. It continues to grow until the foramen primum is closed. However, as the flow of oxygenated blood across the foramen primum almost ceases, a focal area of cellular death occurs in the atrial septum causing an opening to form called the foramen secundum. The foramen secundum allows for continued delivery of oxygenated blood to the left atrium. Septum secundum begins forming about day 38 and is complete by day 45. It is thicker than the atrial septum and overlaps the foramen secundum, creating the foramen ovale. The foramen ovale establishes a one-way passage for blood from the right atrium to the left. Oxygenated blood from the placenta is thus directed into two streams, one across the foramen ovale to the left atrium and onward to the aorta, the other to the pulmonary vasculature.

Interventricular Septum

The interventricular septum and the atrial septum begin to about the same time, near the end of the fourth week/beginning of the fifth week. The interventricular septum is composed of three components (Bahtia 2020):

• The thick muscular bands of the interventricular septum are derived from the ventricular walls and grow outward toward the endocardial cushions. When this muscular septum is complete, a large opening (the interventricular foramen) still remains between the two ventricles.
• Membranous interventricular septum is formed by the downward growth of the aorticopulmonary septum towards the muscular interventricular septum and posterior-inferior proliferation of tissue from the endocardial cushions.
• The infundibular septum is in continuation with the atrioventricular canal. (See septal zone graphic)

The interventricular foramen is usually completely closed by week seven.

References

Annabi M.R., Kerndt C.C., Makaryus A.N. [Updated 2022]. Embryology, Atrioventricular Septum. [Updated 2022 May In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK482372/

Bhatia, S., Munir, S., Aly, A. (n.d.).  Pediatric cardiology part 1. Embryology.
Retrieved July 9, 2022, from https://www.utmb.edu/pedi_ed/CoreV2/CardiologyPart1/CardiologyPart13.html

Haddad F., Hunt, S. A., Rosenthal, D. N., & Murphy, D. J. (2008). Right ventricular function in cardiovascular disease, part I. Circulation, 117(11), 1436–1448. https://doi.org/10.1161/circulationaha.107.653576

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