DEVELOPMENT OF THE CARDIOVASCULAR SYSTEM
During the first 20 days of development, the human embryo has no cardiovascular structures. However, very rapid and intricate development takes place over the next month to the point that the morphology of the heart and great vessels is nearly that of a term infant. This process transforms isolated angiogenic cell islets into a complex, 4-chambered heart. The single heart tube begins to beat at 23 days of gestation and blood circulates through the embryo by 30 days.1
Prior to embryonic folding, there is coalescence of angiogenic cell clusters on either side of the neural crest to form "cardiac cords." These soon canalize to form endothelial-lined capillaries in the mesoderm of the germ disk at about day 20. These capillaries then join to form pairs of blood vessels, with 1 pair on each side of the neural crest. As the embryo folds in its lateral dimension and acquires a tubular form, 2 outer primitive vessels meet in the median of the embryo (ventral to the primitive gut) and start fusing cranially to caudally. This forms the primitive heart tube, which is complete by about day 23.
The single tubular heart develops constrictions that outline future structures: the bulbus cordis, the primitive ventricle, and the primitive atrium. The cranial-most area is the bulbus cordis, which extends cranially into the truncus arteriosus. The truncus arteriosus connects cranially to the aortic sac and through the aortic arches to the dorsal aorta. Caudal to the bulbus cordis is the primitive ventricle, which connects to the primitive atrium. The atrium communicates with the sinus venosus, which receives the vitelline veins, cardinal veins, and umbilical veins.
At approximately day 23 of gestation, the heart tube begins to form a loop, such that the cephalic end bends ventrally, caudally, and slightly to the right. This process results in a U-shaped heart, with the bulbus cordis on the right and the ventricle (ventricularis) on the left. At the junction of the bulbus cordis and the ventricle, the bulboventricular fold forms. The atrium and sinus venosus are dorsal to the heart loop. If the primitive heart tube loops to the left rather than the right, L-transposition results (see page 349).
As the cardiac looping progresses, the paired atria form a common chamber and move into a more dorsal and cranial position. The common atrioventricular junction becomes the atrioventricular canal, connecting the left side of the common atrium to the primitive ventricle. The primitive ventricle will eventually become the left ventricle and the proximal portion of the bulbus cordis will develop into the right ventricle. The distal part of the bulbus cordis will form the outflow tracts of the ventricles. The truncus arteriosus will form the roots of the great vessels. The bulbus cordis gradually migrates medially until it is located in the sulcus between and ...