Vascular Development - Anatomy & Physiology

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Overview of Vascular Development

Blood vessel formation is a combination of the following three processes:

• Vasculogenesis: the formation of blood vessels from endothelial progenitor cells.
• Angiogenesis: the sprouting of new capillaries from pre-existing vessels.
• Arteriogenesis: the remodelling of newly formed or pre-existing vascular channels into larger and more muscular arterioles.

Vasculogenesis

Vasculogenesis occurs during the third week of gestation in domestic mammals, initially in the yolk sac and afterwards in the allantois. The blood vessels form from blood islands containing haemangioblasts which develop from mesodermal cells in the presence of fibroblast growth factor. Exposure to angiopoietin and vascular endothelial growth factor (VEGF) stimulate differentiation of haemangioblasts to angioblasts which ultimately develop into endothelial cells.

Angiogenesis

The sprouting of new capillaries from existing blood vessels occurs when surrounding cells become hypoxic. A low partial pressure of oxygen causes an increase in levels of hypoxia-inducible factor (HIF), this in turn stimulates cells to release vascular endothelial growth factor (VEGF) leading to new vessel formation. Other growth factors such as angiopoietin also play an important role. Angiopoietin-1 interacts with the Tie-2 receptor on endothelial cells at the sites where sprouting occurs. Platelet derived growth factor (PDGF) and transforming growth factor-β (TGF-β) influence the maturation of the capillary network. Angiopoietin-2 expression in the absence of VEGF leads to vessel regression and apoptosis.

Arteriogenesis

The volume and pressure of blood flowing in the network dictates the degree of development of individual vessels. The channels which carry blood at high pressure develop additional tissue layers from the surrounding mesoderm thereby becoming thick-walled. The diameter of these vessels also increases and they become known as arteries. Differentiation of mesenchymal cells to smooth muscle is driven by growth factors released from endothelial cells.