The cardiovascular system develops 15–16 days in gestation.
In the extraembryonic yolk sac, mesodermal precursor cells aggregate to form blood islands, the sites of development of endothelial and primitive blood cells.
It arises from mesodermal cells that differentiate into:
Angioblasts → precursors of endothelial cells (lining of blood vessels).
Angiogenesis: sprouting and branching of new vessels from pre-existing ones.
Blood islands form in the yolk sac → endothelial tubes develop → fuse into a primitive vascular network.
Vasculogenesis: formation of new blood vessels de novo from angioblasts (mainly in the yolk sac, chorion, and embryo proper).
Hemangioblasts → precursors of both blood cells and endothelium.
Within the blood islands, centrally-located cells become primitive blood cells, whereas outer cells give rise to endothelial cells (ECs).
ECs then form the vascular primary plexus which is subsequently remodeled to form the yolk sac vasculature.
In the embryo proper, mesodermal precursor cells differentiate into the vascular primary plexus and major vessels, aorta, and cardinal vein.
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First blood cells form in the yolk sac (primitive hematopoiesis).
Later, hematopoiesis shifts to:
Liver (6 weeks–birth, main site during fetal life).
Spleen (minor contributor).
Bone marrow (dominant after late fetal life, continues postnatally).
Aortic arches (pharyngeal arch arteries) develop and remodel into:
Arch 1 → maxillary artery (partly regresses).
Arch 2 → hyoid and stapedial arteries.
Arch 3 → common carotid and proximal internal carotid arteries.
Arch 4 → part of aortic arch (left) and right subclavian artery (right).
Arch 6 → pulmonary arteries and ductus arteriosus (left).
Specialized shunts allow oxygenated placental blood to bypass non-functioning lungs:
Ductus venosus: connects umbilical vein to inferior vena cava (bypasses liver).
Foramen ovale: shunts blood from right atrium → left atrium.
Ductus arteriosus: connects pulmonary trunk → descending aorta.
At birth, with the first breath and loss of placental circulation:
Increased left atrial pressure closes foramen ovale (→ fossa ovalis).
Ductus arteriosus constricts (→ ligamentum arteriosum).
Ductus venosus closes (→ ligamentum venosum).
Umbilical vein → ligamentum teres of liver.
✅ In summary:
The circulatory system develops very early (week 3), beginning with blood islands and a primitive vascular network. The heart tube forms, loops, and undergoes septation. Blood vessels remodel from pharyngeal arches. The fetus uses special shunts to bypass the liver and lungs until birth, when circulation transitions to the adult pattern.
Would you like me to also create a timeline diagram (week-by-week events) or a flowchart of fetal to postnatal changes for easier visualization?