Figure. Angiogenesis is triggered by reduced oxygen delivery to the tissue, which induces the elaboration by ischemic cells of angiogenic factors such as vascular endothelial growth factor (VEGF).

Angiogenesis is characterized by capillary sprouting, endothelial cell migration, proliferation, and luminogenesis to generate new capillaries. Adult vasculogenesis is mediated by the action of circulating cells such as endothelial progenitor cells (EPCs). EPCs are a heterogenous population of cells, largely of hematopoietic lineage, and are characterized by cell-surface antigen markers including CD34, CD133, and VEGF receptor 2 (VEGFR-2). These circulating cells contribute to the expansion of the microvasculature via multiple mechanisms, secretion of paracrine factors playing a prominent role. A small subset of these cells seem to incorporate into the vasculature (inosculation). Arteriogenesis is a positive remodeling of pre-existing collateral channels in the limb. There is little to no flow through these narrow, high resistance channels in healthy individuals. However, when major conduits become severely narrowed or occluded, more flow becomes directed through the collateral channels. Under the influence of vascular shear stress, the diameter of these channels increase. This positive remodeling seems to be because of endothelial factors, as well as infiltrating macrophages. The remodeling process is characterized by dynamic restructuring of the extracellular matrix with degradation and synthesis; vascular smooth muscle cell apoptosis as well as proliferation, which lead to an increased diameter and thickness of the vessel.