How Blood Vessels Form in the Embryo

Blood vessels form very early in embryonic development. The vascular system begins about 15 to 16 days after conception. It starts outside the embryo in the mesoderm of the yolk sac, chorion, and connecting stalk. About two days later, blood vessel formation begins inside the embryo itself.

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The development of blood vessels starts outside the embryo on day 15 or 16 in the mesoderm. Two days later, blood vessels start to form inside the embryo.

The early vascular precursor cells called hemangioblasts give rise to angioblasts, which subsequently aggregate to create structures with cords and masses known as blood islands.