Development of the heart
Development of the heart begins to form angiogenic cell clusters formed in the splanchnic mesoderm and involves fusion of two endocardial tubes into a single primitive heart tube.
A. Primitive Heart Tube
- It is formed by fusion of two endocardial heart tubes of mesodermal origin in the cardiogenic region.
- It develops into the endocardium, and the splanchnic mesoderm surrounding the tube develops into the myocardium and epicardium.
- It forms fi ve dilations, including the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus.
- It undergoes a folding into a U-shape, bringing the arterial and venous ends of the heart together and moving the ventricle caudally and the atrium cranially.
B. Fate of Five Dilations of the Primitive Heart Tube
- Truncus arteriosus (ventral aorta) forms aorta and pulmonary trunk by formation of the aorticopulmonary (AP) septum.
- Bulbus cordis forms conus cordis or conus arteriosus (smooth part of right ventricle) and aortic vestibule (smooth part of left ventricle at the root of the aorta).
- Primitive ventricle forms trabeculated part of right and left ventricles.
- Primitive atrium forms trabeculated part of right and left atrium.
- Sinus venosus forms sinus venarum (smooth part of right atrium), coronary sinus, and oblique vein of left atrium.
C. Division of the Heart Into Four Chambers
Heart divides into its four chambers by formation of its septum and valves.
Four main septa involved in dividing the heart include the AP septum, the atrial septum, the AV septum, and the IV septum.
1. Partition of the Truncus Arteriosus and Bulbus Cordis
- The truncal ridges and the bulbar ridges derived from neural crest mesenchyme grow in a spiral fashion and fuse to form the AP septum.
- The AP septum divides the truncus arteriosus into the aorta and pulmonary trunk.
2. Partition of the Primitive Atrium
- Septum primum grows toward the AV endocardial cushions from the roof of the primitive atrium.
- Septum secundum forms to the right of the septum primum and fuses with the septum primum to form the atrial septum, which separates the right and left atria.