Development of Blood Vessels

Blood vessels begin to appear in the embryo in the fourth week post fertilization and develop very rapidly. The requirements of developing tissues for oxygen and nutrients make early development of a functional cardiovascular system necessary. The developing blood vasculature initially displays bilateral symmetry, but this quickly changes during the second month of the embryonic period. Abnormalities in the division of the cardiac outflow into the aorta and pulmonary trunk may result in severe congenital heart disease. Development of venous patterns shows marked individual variation, but this is rarely clinically significant.

Formation of Blood Vessels

The first blood vessels, consisting only of endothelial cells, form from mesenchymal precursors (angioblasts), a process known as vasculogenesis. Blood vessels appear first in the 3rd week in extra-embryonic tissues, and begin to be formed within the embryo in the 4th week.
Subsequently, new blood vessels form by extension of existing blood vessels, a process known as angiogenesis.

The cardiovascular system in the trilaminar embryonic disc prior to embryo folding (4th week). Red: The dorsal aortae, first couple of aortic arches and umbilical arteries. Gray's Anatomy (1918) figure 458 (originally from Eternod). Source: Wikimedia Commons

Development of Arteries

Fourth week

The right and left aortae and the umbilical arteries form in the 4th week. Aortic arches 1 and 2 appear at about 4 weeks, but within a week they are minor vessels. The aortic arches are arteries through each of the pharyngeal arches that connect the aortic sac (ventral aorta) to the dorsal aortae.

Fifth week

In the 5th week, the dorsal aortae begin to fuse, and aortic arches 3, 4 and 6 appear.

Sixth week

In the 6th week, formation of the conotruncal septum divides the cardiac outflow into two circulations: aortic (systemic) and pulmonary. The septum spirals so that the outflow of the right ventricle goes to both 6th aortic arches, and that of the left ventricle goes to the other aortic arches.

Later changes in arterial vasculature

The unfused part of the right dorsal aorta disappears. The 3rd aortic arches become the common and internal carotid arteries; the left 4th aortic arch, the arch of the aorta; and right 4th aortic arch, the proximal right subclavian artery. The proximal portions of the 6th aortic arches become the pulmonary arteries, and the distal portion of the left 6th aortic arch, the ductus arteriosus.

Lateral view of embryo in the 5th week showing the cardiovascular system. Gray's Anatomy (1918) figure 472 (originally from His). Source: Wikimedia Commons

Transverse sections through the aortic bulb (ventral aorta) and truncus arteriosus to show the development of the aortico-pulmonary (conotruncal) septum from cranial (right in the figure) to caudal (left in the figure). The septum is formed by growth and fusion of conotruncal ridges. Gray's Anatomy (1918) figure 471 (originally from His). Source: Wikimedia Commons

Scheme of the aortic arches of vertebrates showing the embryonic arterial segments that form the most important arteries. Gray's Anatomy (1918) figure 473 (originally from Kollmann). Source: Wikimedia Commons

Diagram of arteries derived from the left aortic arches, and left ventral and dorsal aortae. Gray's Anatomy (1918) figure 474 (originally from Tandler). Source: Wikimedia Commons

Development of Veins

The first embryonic veins

Pairs of vitelline (omphalomesenteric), umbilical (umbilicoallantoic) and cardinal veins form in the 4th week. At 4 weeks, they connect to the sinus venosus.

Beginning of asymmetry in venous vasculature

In the 5th week, the right horn of the sinus venosus becomes dominant. This is evident even in adults as the venae cavae are on the right.
Anastomoses form between the omphalomesenteric (vitelline) veins. The liver develops around the proximal ends of these veins creating sinusoidal channels through the liver parenchyma. The left omphalomesenteric vein disappears.
The right umbilical vein and the proximal part of the left umbilical vein disappear, leaving the (left) umbilical vein connected to hepatic sinusoids. A venous shunt (ductus venosus) through the liver develops, connecting the umbilical vein with the inferior vena cava (the proximal right omphalomesenteric vein as it enters the sinus venosus).

Pulmonary veins

A single pulmonary vein enters the left atrium in the 5th week. By the 7th week, progressive incorporation of the pulmonary vein and its four main tributaries into the wall of the left atrium results in creation of the sinus of the left atrium (the smooth part) and the presence of (usually) four pulmonary veins.

Derivatives of the sinus venosus

The right horn of the sinus venosus becomes the sinus venarum (the smooth part of the right atrium). The left horn becomes the coronary (venous) sinus.

Dorsal view of the heart in the 6th week showing dominance of the right horn of the sinus venosus . Gray's Anatomy (1918) figure 464 (from model by His). Source: Wikimedia Commons

Derivatives of the omphalomesenteric veins

The right omphalomesenteric (vitelline) vein persists only as superior mesenteric, portal & hepatic veins, and hepatic part of inferior vena cava.

Derivatives of the umbilical veins

The (left) umbilical vein and ductus venosus form an important part of the circulation of the placental stage carrying oxygenated blood from the placenta to the heart. After birth, the umbilical vein becomes the ligamentum teres of the liver and the ductus venosus becomes the ligamentum venosum.

Principal veins in the vicinity of the liver at about 5 weeks (ventral view). The figure shows early changes in the omphalomesenteric and umbilical veins. The cardinal veins and their tributaries are not shown. Gray's Anatomy (1918) figure 475 (originally from His). Source: Wikimedia Commons

Principal veins in the vicinity of the liver at about 5 weeks. The embryo illustrated here is only slightly older than the previous one. Gray's Anatomy (1918) figure 476 (originally from His). Source: Wikimedia Commons

Venae cavae

The superior vena cava is derived from the right common and anterior cardinal veins.
The inferior vena cava forms chiefly from the right subcardinal vein (a branch of the right posterior cardinal vein). The superior (hepatic) segment of the inferior vena cava is derived from the right omphalomesenteric vein.

The veins at about 4 weeks. Gray's Anatomy (1918) figure 477. Source: Wikimedia Commons

Early stages of development of the inferior vena cava. Gray's Anatomy (1918) figure 478. Source: Wikimedia Commons

Development of transverse veins. Gray's Anatomy (1918) figure 479. Source: Wikimedia Commons

Schematic diagram of the main veins of the trunk upon completion of their development. Gray's Anatomy (1918) figure 480. Source: Wikimedia Commons

External Links

Human Developmental Anatomy Center (HDAC) of the National Museum of Health & Medicine

• O'Rahilly R & Müller F. Developmental Stages in Humans. Carnegie Institution of Washington, 1987.
  

Embryology.ch - an online course in embryology for medical students

Other Resources

O'Rahilly R and Müller F. Human Embryology & Teratology, 3rd Ed. New York: Wiley-Liss, 2001.