Vascularization

Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels. In precise usage this is distinct from vasculogenesis, which is the de novo formation of endothelial cells from mesoderm cell precursors, and from neovascularization, although discussions are not always precise (especially in older texts). The first vessels in the developing embryo form through vasculogenesis, after which angiogenesis is responsible for most, if not all, blood vessel growth during development and in disease.

Angiogenesis is a normal and vital process in growth and development, as well as in wound healing and in the formation of granulation tissue. However, it is also a fundamental step in the transition of tumors from a benign state to a malignant one, leading to the use of angiogenesis inhibitors in the treatment of cancer. The essential role of angiogenesis in tumor growth was first proposed in 1971 by Judah Folkman, who described tumors as "hot and bloody," illustrating that, at least for many tumor types, flush perfusion and even hyperemia are characteristic.

Sprouting angiogenesis was the first identified form of angiogenesis. It occurs in several well-characterized stages. First, biological signals known as angiogenic growth factors activate receptors on endothelial cells present in pre-existing blood vessels. Second, the activated endothelial cells begin to release enzymes called proteases that degrade the basement membrane to allow endothelial cells to escape from the original (parent) vessel walls. The endothelial cells then proliferate into the surrounding matrix and form solid sprouts connecting neighboring vessels.

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