This Daily Doodle features the embryology of the heart, one of my favorite organs!

The cardiovascular system is the first major system to function in the developing embryo. In the third week of development, the primodial heart and vascular system starts to appear and take form. The heart starts off as two paired endothelial tubes called the angioblastic cords. As the embryo begins to fold, these two tubes come together and fuse to form the heart tube [1]. The heart tube begins to elongate and dilate in multiple areas to form four distinct regions: the sinus venous, the atrium, the ventricle and the truncus areteriosus (see doodle notes below) [2].

In a complex manner, the heart beings to fold, forming an S-shaped structure. The atria are pushed up as the ventricles dilate and fold under. Eventually, the heart takes on its anatomical heart-shaped form. Inside the heart, septums develop to form the chambers. The four main structures involved in this process are: the endocardial cushions, interventricular septum, aorticopulmonary (spiral) septum and the interarterial septum [2].

The interventricular septum is formed from two parts:

1. Muscular interventricular septum
2. Membranous interventricular septum

The spiral septum spirals down the truncus arteriosus, dividing it in half, creating two twisting vessels – the aorta and the pulmary arteries.

The interatrial septum is formed from 2 septa: the septum primum and the septum secundum. Holes in these septums allow oxygenated blood from the placenta to pass to the left side of the heart through the right atrium, by-passing the lungs that are still developing.

By day 22 to 23, the heart begins to beat and blood begins to flow during the fourth week [1]. The process of folding becomes quite complicated, but here is video by Drs. Robert Rushmer and Richard Blandau, University of Washington, that describes the process so well [3]. Amazing!

Pop Quiz

1. What is the first major system to function in the embryo?
2. What week does the heart begin to develop?
3. When does the heart begin to beat?
4. When does blood begin to flow?

References

1. K. Moore and T. Persaud. “The Developing Human”. 8th Edition. Saunders 2008.
2. B. Crawford. “Embryology of the Heart”. UBC MD Lecture Notes. 2015.
3. Youtube video “Normal Heart Development” by Drs. Robert Rushmer and Richard Blandau, University of Washington. https://www.youtube.com/watch?v=5DIUk9IXUaI

“I Heart Embryology” Daily Doodle by Michiko Maruyama