The angiogenic switch refers to a key early stage in solid tumor progression during which an avascular tumor lesion first becomes vascularized (Fig. 1) (Harper and Moses 2006). The acquisition of this angiogenic phenotype is a rate-limiting step in a tumor’s development in that these new capillaries provide necessary nutrients and gas exchange for the nascent tumor. The acquisition of a capillary network is required for exponential growth of the tumor and for its metastasis. It has been proposed that the angiogenic switch occurs as a function of a perturbation in the balance between angiogenic stimulators and inhibitors in favor of the positive regulators (Folkman 2002; Harper and Moses 2006).
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