Early vessel development

Blood vessel formation within the embryo begins from about 22 days gestation. Endoderm deep to the visceral mesoderm induces the latter to form angioblasts. Angioblasts cluster throughout the yolk sac to form angiocysts, regions of differentiating cells:

• the outer cells become thinner and are the progenitors of endothelial cells; they proliferate and ramify to make contact with other angiocysts both intra- and extra-embryonically
• the inner cells are progenitors of blood cells; the groups they form within the confines of the endothelial cells are termed blood islands

At about day 18 of development, lateral folding divides the visceral mesoderm and the angiocysts in the same regions into two groups of cells:

• the more lateral groups of endothelial cells have formed tubes on each side which now move ventrally and medially to fuse; fusion results in the primitive heart tube
• the more medial groups of endothelial cells also form primitive vessels: however, they are excluded from lateral folding; eventually, these become a pair of dorsal aortae extending along the length of the embryo dorsally to the primitive gut tube

The development of the branchial arch arteries provides continuity between the primordial heart and the dorsal aortae. Budding and proliferation of endothelial cells also ensures that the dorsal aortae make contact with the vitelline and umbilical arteries and veins to complete the circulation of the early embryo.