State-of-the-Art Methods for Evaluation of Angiogenesis and Tissue Vascularization

V ascular dysfunction is causally contributing to many dis- eases, including but not limited to cardiovascular disease, which is still the leading cause of death in the Western world.The endothelium that lines the inner wall of the blood vessels plays a critical role in the pathobiology of these illnesses.Particularly after ischemia or injury, the growth of new blood vessels, driven by endothelial expansion, is essential to maintain oxygen supply to the ischemic or injured tissue.Recent studies additionally suggest that the endothelium acts as a paracrine source for signals that determine tissue regeneration versus fibrosis after injury.Excessive vascularization, however, might also be unwanted, as in the case of cancer, neovascular eye diseases including diabetic retinopathy, atheroma growth, or the expansion of vasa vasorum, which leads to adverse vessel wall remodeling.Neovascularization is a tightly regulated and essential process that results in the formation of new blood vessels.Specific types of neovascularization include angiogenesis, the formation of new capillaries from existing capillaries, and arteriogenesis, the formation of new arteries from preexisting collaterals or de novo.Although endothelial cells (ECs) certainly are essential for both processes, the formation of functionally active vessels requires a complex molecular cross-talk of ECs with perivascular cells such as pericytes, smooth muscle cells, and macrophages.Simple in vitro models are best suited to examine specific aspects of particular processes involved in angiogenesis such as the biochemical interactions that regulate EC proliferation, motility, and apoptosis or lumen formation.However, in vivo models are required to understand the complex cellular interactions that enable the generation of functionally active and hierarchical blood vessel networks capable of providing an appropriate blood supply and paracrine stimuli to organs.In addition, the development of therapeutic strategies to either promote or inhibit vessel growth depends on reproducible measures and end points in experimental models that are relevant for the treatment of human diseases.Accordingly, the selection of an appropriate experimental model is critical to study specific aspects of the molecular and cellular mechanisms that are of physiological or pathological relevance.This scientific statement summarizes in vitro assays and in vivo models suitable for gaining insights into the basic mechanisms of vessel growth and provides an overview of models that are particularly useful for clinical translation. In Vitro AssaysConsiderable progress has been made over the past 20 years in the development of in vitro strategies to investigate the complex processes of vascular morphogenesis and maturation events in