Vasculogenesis and Angiogenesis
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Direct answer
Blood vessels in the embryo appear before the heart has finished looping: during the third week (around days 17-18), mesodermal cells in the wall of the yolk sac aggregate into blood islands, whose outer cells flatten into endothelium while inner cells become the first haemoglobin-carrying precursors — the bipotential ancestor being the haemangioblast. New vessels forming de novo from these angioblasts is vasculogenesis; sprouting and remodelling of pre-existing vessels thereafter is angiogenesis, driven by VEGF acting on VEGFR-2 receptors and refined by Notch-Dll4 signalling. The first intraembryonic vessels are the paired dorsal aortae, connected to the developing heart tube by the first aortic arch arteries; the vitelline, umbilical and cardinal venous systems then undergo remodelling into the portal vein, ductus venosus and caval systems. Every vascular anomaly you meet clinically — infantile haemangioma, arteriovenous malformation — is a frozen frame of one of these steps.
What you must remember
- Where it starts: extraembryonic mesoderm of the yolk sac, days 17-18 — "blood islands" with peripheral endothelial and central haematopoietic precursors; examiners want the site and the timing.
- The two words: vasculogenesis = new vessels from angioblasts (de novo); angiogenesis = sprouting from existing vessels — the distinction that survives into postgraduate pathology.
- Molecular triad: VEGF from mesoderm acting on VEGFR-2 (flk-1) drives endothelial differentiation; Notch-Dll4 lateral inhibition prunes tip cells; PDGF and TGF-beta stabilise walls with pericytes and smooth muscle.
- First intraembryonic vessels: paired dorsal aortae; first aortic arch arteries connect them to the heart tube even before the aorticopulmonary septum divides the outflow.
- Haematopoietic relay: yolk sac (primitive erythroblasts, mostly nucleated) then liver (from week 6) then aortagonad-mesonephros region and finally bone marrow — the relay examiners ask as a sequence.
- Clinical freeze-frames: infantile haemangioma (GLUT-1 positive, appears after birth, proliferates then involutes — "strawberry naevus"); hereditary haemorrhagic telangiectasia from aberrant TGF-beta signalling; PHACE association with large facial haemangiomas.
- Remodelling leftovers: failure of involution of embryonic vessels explains persistent sciatic artery and the vitellointestinal remnants — vasculogenesis questions often pivot into these.
Tracing one vessel from island to ligamentum
Follow the right vitelline vein as a worked example of vascular remodelling. Sprouting angiogenesis wraps the duodenum with venous loops; anastomoses form, segments regress, and what survives is the portal vein with the liver sinusoids it feeds, while the paired vitelline trunks around the duodenum vanish. Meanwhile the left umbilical vein, which brings placental blood to the ductus venosus and liver, is obliterated after birth into the ligamentum teres hepatis — so a structure you palpate nowhere becomes visible in every port vein dissection and every paraumbilical varix of portal hypertension (the caput medusae is vasculogenesis history rerouted by disease). The cardinal veins do the same dance: the precardinal channels on the right become the superior vena cava, the postcardinal system largely regresses leaving the azygos and the renal-to-iliac segments, and the subcardinal anastomosis beneath the future kidneys becomes the left renal vein crossing between aorta and superior mesenteric artery — the very segment compressed in nutcracker phenomenon. One developmental principle — grow, anastomose, regress — explains a dozen named adult vessels.
Where students slip
The recurring error is treating vasculogenesis and angiogenesis as synonyms. They are sequential and different: vasculogenesis builds the first vessels from scratch during week 3; angiogenesis — sprouting, intussusceptive growth, pruning — sculpts everything thereafter, and in the adult is reactivated in wound healing, tumour growth and diabetic retinopathy, which is why anti-VEGF therapy exists for macular disease. The second slip is the haematopoiesis relay order; recite yolk sac, liver, aortagonad-mesonephros, bone marrow with rough weeks (liver from week 6, marrow active by the fifth month) and name the yolk sac cells as primitive, largely nucleated erythroblasts making haemoglobin Gower. Indian viva panels also love asking which adult structure is the adult derivative of the umbilical arteries — the medial umbilical ligaments (the obliterated intra-abdominal segments), while the proximal segments persist as the superior vesical arteries; candidates who reverse the two lose easy marks.
Frequently asked questions
What is the difference between vasculogenesis and angiogenesis?
Vasculogenesis forms new vessels de novo from differentiating angioblasts; angiogenesis sprouts new vessels from pre-existing ones — first during embryonic remodelling and later in wound healing and tumours.
Where and when do the first blood vessels appear?
In the extraembryonic mesoderm of the yolk sac around days 17-18, as blood islands whose outer cells become endothelium and inner cells haematopoietic precursors.
Which growth factor system drives blood vessel formation?
VEGF acting on VEGFR-2 receptors on endothelial cells, with Notch-Dll4 signalling pruning the sprout — the axis targeted by anti-VEGF drugs in ocular neovascular disease.
What are the first intraembryonic blood vessels?
The paired dorsal aortae, connected to the early heart tube through the first pair of aortic arch arteries.
Which cells give rise to both endothelium and blood cells?
The haemangioblast, a bipotential mesodermal precursor of the blood islands that generates angioblasts and haematopoietic stem cells.