# Enhancement of EPR Effect for Passive Tumor Targeting: Current Status and Future Perspectives
## 3 Morphological Characteristics of the Vascular Network
### Functional Characteristics of the Tumor Vascular System
#### 3.4.1 Angiogenesis  
The vasculature, in both normal and cancerous tissues, is required to supply and regulate oxygen, nutrients, growth factors, and cells, as well as to remove waste, supporting tissue nutrition and tumor growth [74,89]. Tumors exploit the host vasculature to meet their increased nutritional requirements due to their high rate of proliferation [80]. During tumor growth, reduced blood supply, due to heterogeneity in blood vessel size and arteriovenous drainage, leads to nutrient deprivation, the accumulation of acidic metabolic by-products, and high oxygen consumption, creating a hypoxic microenvironment [74,90,91]. Hypoxia triggers a vascular switch by promoting growth and facilitating tumor metastasis, allowing cancer cells to migrate through newly formed blood vessels, creating secondary tumors [74,90,91,92]. Without vascular support, tumors fail to grow and become necrotic or apoptotic. Angiogenesis, the formation of new capillaries, is essential for tumors to grow beyond their minimum size (diameter up to 0.8-1.0 mm), providing oxygen, nutrients, and waste disposal and facilitating their migration [74,77,89,93,94]. It is carried out through three processes: (1) the sprouting of new capillaries (neovascularization) through the remodeling of existing daughter vessels, namely endothelial cell proliferation, migration, and stabilization by pericytes and smooth muscle cells; (2) de novo angiogenesis, where new blood vessels are formed from endothelial progenitor cells (EPCs) from bone marrow or vascular walls; (3) the division of large \"mother\" vessels into smaller daughter vessels [78,80,81,87,92,93,95]. Angiogenesis is a fundamental process during embryonic development and in various physiological and pathological conditions in adults [89]. It consists of four stages: (1) tumor cells release cytokines and growth factors that activate neighboring normal cells; (2) angiogenesis is stimulated, with the removal of pericytes and degradation of the basement membrane and extracellular matrix (ECM); (3) endothelial cells proliferate and migrate with the help of soluble factors; (4) consolidation with pericytes and the maturation of the initial microvessels into complete tubular structures and eventually into capillaries, where blood flow is established [77,92,93,96]. In the revised model of angiogenesis, endothelial cells form filopodia and migrate through the ECM, followed by proliferating stem cells that form lumen and recruit pericytes for stabilization. The apical cells from two migrating endothelial cells fuse to form a blood vessel [96,97]. The tumor microenvironment and normal tissue play a crucial role in these stages [92]. Angiogenesis is tightly regulated by a dynamic balance between the stimulators and inhibitors of angiogenesis (pro- and anti-angiogenic factors, respectively) [94,95]. Under physiological conditions, this balance tightly controls endothelial cell activation when necessary [95]. During pre-tumor development, genomic instability of the endothelial cells leads to the expression of multiple angiogenic factors at high levels. Angiogenesis inhibitors, either direct or indirect, inhibit tumor growth and cause tumor regression by targeting individual angiogenic factors [87,94]. Most angiogenic factors involved in normal and pathological angiogenesis are classical peptide growth factors that bind to specific tyrosine kinase receptors on the surface of endothelial cells [94]. Some factors involved in the regulation of angiogenesis in rapidly growing tumors are discussed below. Positive Regulators of Angiogenesis  
Stimulators of angiogenesis include the vascular endothelial growth factor family (VEGF-A, -B, -C, and -D) and placental growth factor (PIGF), which bind with different affinities to VEGF receptors (VEGFRs) on endothelial cells [77; 94; 98]. VEGF-A, the main mediator of angiogenesis, interacts with VEGFR-1 and VEGFR-2, inducing angiogenesis and vascular permeability, with VEGFR-2 dominating [77; 87; 98].