Heart Development

• The formation of the heart and blood vessels starts in the middle of the 3rd week.
• Formation begins due to demand for oxygen and nutrients by the embryo.
• The cardiovascular system develops from mesoderm and neural crest cells.
• Two heart tubes develop separately.
• The heart tubes join to form a single heart tube due to lateral folding of the embryo.
• The heart begins to beat around day 22-23.
• Blood flow begins during the 4th week.
• Doppler ultrasonography can visualize the blood flow.

Later Development of the Heart

• The external layer of the embryonic heart, the primordial myocardium, forms from mesoderm as the heart tubes fuse.
• The endothelial tube becomes the endocardium.
• The primordial myocardium becomes the myocardium.
• The visceral pericardium forms the epicardium.
• The heart tube elongates and develops dilatations and constrictions.
• The heart tube develops into the bulbus cordis, ventricle, atrium, and sinus venosus.
• The bulbus cordis is composed of the truncus arteriosus, conus arteriosus, and conus cordis.
• The sinus venosus receives the umbilical vein, vitelline veins, and common cardinal vein.

Partitioning of the Atrioventricular (AV) Canal

• Endocardial tissue develops on the dorsal and ventral walls of the AV canal in the end of the 4th week.
• The endocardial tissue divides the canal into right and left AV canals.

Partitioning of the Primordium Atrium

• Partitioning involves the formation of the septum primum and septum secondum.
• The septum primum and septum secondum leaves a small opening, the foramen ovale.
• Before birth, the foramen ovale allows most of the oxygenated blood from the IVC to pass to the left atrium.
• The right sinual horn receives blood from the head and neck through the SVC.
• The right sinual horn receives blood from the placenta and lower body through the IVC.
• The sinus venosus forms the smooth part of the wall of the right atrium.

Partitioning of the Primordial Ventricle

• Partitioning is formed by muscular and membranous interventricular septa.
• The septum divides the ventricle into right and left ventricles.

Partitioning of the Bulbus Cordis and Truncus Arteriosus

• Partitioning occurs via formation of the aorticopulmonary septum.
• This septum divides the bulbus cordis and truncus arteriosus into the pulmonary trunk and ascending aorta.

Development and Fate of Veins

• Three paired veins drain into the heart of a 4-week embryo.
• The vitelline veins returns poorly oxygenated blood from umbilical vesicle (yolk sac).
• The umbilical veins carries well-oxygenated blood from the placenta.
• The common cardinal veins return poorly oxygenated blood from the body of the embryo.

Summary Points

• The heart and blood vessels develop from mesoderm at the 3rd week of fertilization.
• Partitioning of the AV canal is done by endocardial cushion.
• Partitioning of the primitive atrium is done by septum primum and septum secondum.
• Partitioning of the ventricle is by the interventricular septum.