In the human embryo, the mesodermal germ layer gives rise to the entire cardiovascular system (heart, blood vessels and blood cells). The heart develops from two simple epithelial tubes which fuse to form a single chambered heart that is efficiently pumping blood by the fourth week of embryonic development.
Twenty-three days following conception, the single, simple epithelial heart tube lies within the embryo's pericardial cavity. At this time there are three cell layers present within the heart tube. The inner layer, known as cardiac jelly, is a structureless mass of cells which contain very few nuclei. The second and third layers are known as the cardiac mantle and will eventually give rise to the epicardium and myocardium.
The heart tube contains three specific areas: the cranial portion, the caudal portion and the bulbus cordis. As development progresses, the cranial one-third of the tube dilates to form the aortic sac which will give rise to the aortic arches.
The caudal one-third to one half also dilates to form the early embryonic ventricle.
The remaining mid-portion forms the bulbus cordis which has three distinct areas of development. The proximal one-third gives rise to the body of the right ventricle, which is initially referred to as the primitive ventricle. The remaining two thirds of the bulbus cordis is divided into two sections. The distal-most section is called the truncus arteriosus which develops into the aortic root and the parts of the ascending aorta. The remaining mid-portion is called the conus cordis and connects the primitive right ventricle to the truncus arteriosus.
The conus cordis partitions to form the outflow tracts of the right and left ventricles.
An increase in blood volume occurs following the 23rd day of development. With this increase in blood, blood circulation changes to a parallel flow. As the heart tube grows and becomes longer it usually bends to the right, which is known as d-looping. Rightward bending is responsible for the initial positioning of the primitive ventricle.
The cardiac jelly now acts as a valve for movement of blood from the atrial end of the heart tube to the distal end.
By the 24th day of gestation, the primitive ventricles have expanded and the cardiac jelly contains trabecula or supporting structures. Distal to the primitive right ventricle is an area known as the conotruncal region containing the conus cordis and truncus arteriosus. The conotruncal region and the primitive right ventricle are collectively known as the bulbus cordis.
As growth continues, the conotruncal region moves centrally with torsion and twisting, giving rise to the anatomical curve of the aorta and the pulmonary artery.