Angiogenesis, the sprouting of endo

Angiogenesis, the sprouting of endothelial cell capillaries from preexisting blood vessels, is multifactorial including processes such as proliferation and directed migration. These processes in turn are influenced not only by autocrine but also by paracrine stimuli mediated by, for example, pericytes, fibroblasts, immune cells, and tumor cells in the case of cancer. This intercellular communication by stimuli includes growth factors, chemo- and cytokines as well as other secreted factors, like enzymes, extracellular basement membrane components, and the exchange of genetic material (miRNA, mRNA, DNA) via extracellular vesicles [161–163]. Anatomically, these various cell types are fine-tuned and orchestrated to become organized 3D structures to ultimately allow and control the flow of blood. However, the field of angiogenesis has typically focused on studying angiogenesis-associated processes in vitro in pure EC cultures. Although studying EC in isolation has some advantages in investigating certain EC-specific elements and to certain defined processes within angiogenesis, 3D co-culture spheroids allow one to study the direct intercellular cues