Last updated 30 March 2006

• There are 2 processes of formation:

1. In the yolk sac, connecting stalk and chorion, blood vessels develop from aggregations of extraembryonic mesoderm called blood islands, which interconnect to form a vascular network from which larger vessels will be derived:
• Peripheral cells of the blood islands form endothelial cells
• Central cells form haemoblasts (primitive blood cells)
2. In the embryo, vessels form from splanchnopleuric mesoderm cells called angioblasts, which flatten and join together to form angiocysts, which in turn link together to form tubular angioblastic cords.
• It is this process that is known as in situ vesicle formation and fusion, or vasculogenesis.
• The angioblastic cords form angioblastic plexuses that continue to grow in 3 ways:
1. Continued vasculogenesis
2. Sprouting of new vessels from existing cords, called angiogenesis
3. Intercalation of mesodermal cells into the walls of existing vessels.

• Haematopoiesis begins firstly in the yolk sac, and takes place in a number of organs in the embryo proper, including the liver, spleen, thymus and bone marrow.

• The pattern of blood vessel formation in the embryo is regulated by inductive signals from the underlying endoderm.

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The Aortic Arches

• The pharyngeal arches in embryonic humans are derived from the branchial arches (gill bars) of fish, and each has an aortic arch arising ventrally from the aortic sac and dorsally from the right or left dorsal aorta.
• The truncus arteriosus and aortic sac develop into the ascending aorta, with the aortic sac also contributing to the arch of the aorta, whilst the dorsal aortae fuse in the 4^th week to form the descending aorta.

• There are 5 pairs of pharyngeal arches, named 1, 2, 3, 4 and 6 for their homology to the gill arches of fish; and the aortic arches within them develop craniocaudally:
• The 1^st pair of aortic arches is formed on days 22-24, with embryonic folding pulling the cranial ends of the dorsal aortae into a dorsoventral loop, whilst the other pairs develop by vasculogenesis and angiogenesis.
• The 2^nd pair of aortic arches forms on day 26, whilst the 1^st pair undergoes partial regression, the remnants of which becomes part of the maxillary arteries.
• The 3^rd and 4^th pairs form on day 28
• The 6^th pair develops on day 29, whilst the 2^nd pair undergoes partial regression, the remnants of which form the embryonic stapedial arteries.

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Development of the great arteries

• The 3^rd, 4^th and 6^th aortic arches give rise to major vessels in the upper body:

• The 3^rd aortic arch takes over the cranial portions of the dorsal aortae’s role in blood supply to the head, forming the common carotid arteries, and roots of the internal carotid arteries; with cranial portions of the dorsal aortae forming the rest of the internal carotids, and the external carotids in turn sprouting from them.

• The 4^th aortic arch arteries develop asymmetrically:
• The left 4^th aortic arch becomes the middle region of the arch of the aorta, with the left 7^th cervical intersegmental artery (initially supplying the left upper limb) becoming the left subclavian artery.
• The right 4^th aortic arch forms the root of the right subclavian artery, whilst the right subclavian artery develops from:
1. The right 4^th aortic arch proximally
2. The right dorsal aorta forming an intermediate segment
3. The right 7^th cervical intersegmental artery (initially supplying the right upper limb) distally

• The brachiocephalic artery develops from a portion of the aortic sac.

• The left 6^th aortic arch becomes the distal part of the pulmonary trunk and the ductus arteriosus, whilst the right 6^th aortic arch degenerates;
• The pulmonary arteries originate from the 4^th arch arteries, but shift their origin to the 6^th arch position of the pulmonary trunk
• The ductus arteriosus, the second foetal shunt, closes at birth to become the ligamentum arteriosum.

• The asymmetric development of the 4^th and 6^th aortic arches is reflected in the asymmetry of the recurrent laryngeal nerves that form a loop around the most caudal persisting aortic arch on each side, when the arches descend relative to the larynx.
  • The left recurrent laryngeal nerve loops under the right subclavian artery (the persisting right 4^th aortic arch);
  • The right recurrent laryngeal nerve loops around the ductus arteriosus/ligamentum arteriosum (the persisting left 6^th aortic arch)

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• The dorsal aorta develops ventral, lateral and dorsolateral branches:
• Ventral branches, called vitelline arteries, supply blood to the gut tube and its derivatives, condensing from the vitelline arterial plexus over the yolk sac and gut tube and its connections to the dorsal aortae.

• Lateral branches supply the suprarenal (adrenal) glands, gonads and kidneys:

• Dorsolateral branches, called intersegmental arteries, grow out between the somites to supply the body wall, vertebral column, spinal cord, and limbs.

• The limbs receive blood through intersegmental artery branches, initially via a central axis artery.

• The umbilical arteries develop in the connecting stalk, initially joining the dorsal aortae but shift their origin to the internal iliac arteries.

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Development of the veins

• The embryonic venous system consists of:
  1. The vitelline system, draining the yolk sac and gut tube
  2. The umbilical system, bringing oxygenated blood from the placenta
  3. The cardinal system, draining the body wall, neck and head.

• Initially, these systems are bilaterally symmetric, draining into the horns of the sinus venosus via the umbilical, vitelline and common cardinal veins.
• Remodelling:
1. Makes the right side dominant for the vitelline and cardinal systems, producing the superior and inferior venae cavae (SVC & IVC)
2. Makes the left side dominant for the umbilical system, although it still drains into the right side of the heart.

• The vitelline system produces the liver sinusoids, ductus venosus, the portal venous system and the cranial part of the IVC.

• The right umbilical vein degenerates, whilst the left umbilical vein loses its connection to the left sinus horn, draining into the IVC via the ductus venosus

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• The cardinal system initially consists of symmetric, paired anterior and posterior cardinal veins draining into the sinus horns via short common cardinal veins; the anterior cardinals draining the head and neck and the posterior cardinals draining the body wall.

• The posterior cardinals mostly degenerate after being superseded by 2 pairs of longitudinal veins:

• The right subcardinal vein forms the next most caudal part of the IVC after the right vitelline vein, into which the left cardinal vein will then drain

• The supracardinal system, has 2 portions:

• Thus, the inferior vena cava (IVC), from cranial to caudal, is composed of:

1. The right vitelline vein
2. The right subcardinal vein
3. The right supracardinal vein
4. The posterior cardinal veins

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• The anterior cardinal system drains the head, neck and upper limbs, and forms the SVC.

The coronary circulation

• The coronary arteries develop from blood islands deep to the epicardium, receiving their blood from coronary arteries that sprout from the ascending aorta, and draining via coronary veins into the coronary sinus.

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Lymphatic system development

• In the 5^th week, the lymphatic system begins forming, similarly to the blood vessels.
• It initially consists of several lymph sacs and paired bilateral thoracic ducts that drain into the junctions between the internal jugular and subclavian veins.

• The thoracic duct forms from the caudal part of the right thoracic duct and the cranial part of the left thoracic duct.

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Changes in the circulatory system at birth

• The single circuit foetal circulation routes blood through the aorta, bypassing the pulmonary circulation, via 2 shunts:
1. The interatrial shunt (R.Atrium --> F. Ovale --> Ostium secundum --> L. Atrium)
2. The ductus arteriosus (pulmonary trunk --> descending aorta)