Vascular System & Blood Vessel Development

• The vascular system develops from the mesoderm, creating the heart, arteries, veins, lymph vessels, and all blood and lymph cells.

Blood Vessel Development

• The cardiovascular system is the 1st major system to function in the embryo.
• The primordial heart and vascular system appear mid-3rd week of embryonic development.
• Earliest blood vessels develop from mesodermal mesenchyme condensations, known as blood islands, around the yolk sac in the 3-week embryo.
• Initially, vessels consist of endothelium only.
• Adjacent mesenchyme cells soon surround the tubes, forming the muscular and fibrous tunics of the vessel walls.
• Blood islands lead to the development of blood cells, vessels, and the heart.

Heart Development

• Blood islands destined to become the heart form in the splanchnic mesoderm around the future head and neck of the embryonic disc in a horseshoe-shaped cardiogenic region.
• The cardiogenic area is located anterior to the neural plate initially.
• As the flat trilaminar disc folds due to neural tube closure and rapid CNS growth, the heart (paired tubular structures initially) moves into the cervical and then the thoracic area.
• By ~Day 20, the 2 endothelial heart tubes fuse to form a single tube.
• By ~Day 22, the single heart tube can pump and develops bulges in a caudal to cranial direction, forming the earliest unpaired heart chambers.
• The chambers include the sinus venosus, atrium, ventricle, and bulbus cordis.
• The sinus venosus initially receives all blood from embryonic veins and becomes the smooth-walled part of the right atrium, coronary sinus, and SA node, also contributing to the left atrium's posterior wall.
• The atrium forms the pectinate muscled walls of both the right and left atria.
• The ventricle forms the left ventricle, which is the strongest pumping chamber of the early heart.
• The bulbus cordis, along with its extension, the truncus arteriosus, leads to the formation of the right ventricle, pulmonary trunk, and the 1st part of the aorta.
• From ~Day 23-28, the heart tube begins to bend into an S-shape (cardiac looping), eventually positioning the ventricles and atria in their respective adult positions.
• During month 2, the embryonic heart undergoes changes resulting in the formation of septa and valves, which establish the 4 definitive heart chambers.
• Endocardial cushions, thickenings of the endocardium, aid in forming the heart’s midline septum and valves.
• The interatrial septum mainly forms by growing caudally from the heart’s roof.
• The interventricular septum forms by growing cranially from the heart’s apex.
• Neural crest cells migrate to where the atrium meets the ventricle, contributing to the developing heart valves and the bases of the pulmonary trunk and ascending aorta (the great arteries that form from the splitting of the bulbus cordis).

Circulatory System Development

• All major vessels are in place by the 3rd month, and blood flows in the same direction as in the adult.
• Two significant differences exist between fetal and postnatal circulation.
• During pregnancy, the placenta provides oxygen and nutrients to the fetus.
• The fetus supplies blood to the placenta.
• Fetal circulation bypasses the lungs and most of the liver.
• During fetal development, 3 early vascular systems are established: intra-embryonic, vitelline, and placental.
• The embryonic system's vessels supply blood to and drain blood from developing embryonic tissues.
• The vitelline system's vessels supply blood to and drain blood from the yolk sac and contributes to the blood supply of the gastrointestinal tract.
• The placental (umbilical) system includes umbilical arteries that deliver deoxygenated blood and fetal wastes to the placenta, and the umbilical vein which transports oxygenated, nutrient-rich blood from the placenta.

Adult Remnants of Fetal Structures

• Several adult structures are remnants of functional fetal circulatory components.
• The umbilical vein, which returns oxygenated blood to the fetus from the placenta, becomes the ligamentum teres (round ligament of the liver).
• The ductus venosus, which shunts oxygenated blood away from the liver to the IVC in the fetus, becomes the ligamentum venosum, passing through the liver from the left branch of the portal vein to the IVC.
• The umbilical arteries, which carry oxygen-poor blood from the fetus to the placenta, become the medial umbilical ligaments.
• The ductus arteriosus, which shunts blood from the pulmonary trunk into the ascending aorta in the fetus, bypassing the lungs, becomes the ligamentum arteriosum. It connects the left pulmonary artery to the aortic arch.
• The foramen ovale, an opening in the interatrial wall that shunts blood away from the non-functional lungs, becomes the fossa ovalis.