The construction of new blood vessels is an important phase in the growth of any new tissue, both during embryogenesis and in vitro models, because without enough perfusion, the tissue will not be able to grow beyond the size where nourishment and oxygenation can be managed solely by diffusion. Endothelial cells are the fundamental building blocks of blood vessels, and while they can form tube-like structures on their own, they are unable to produce functional vasculature capable of carrying blood flow. Endothelial cells constitute the endothelium, a one-cell thick-walled layer that borders all of our blood arteries. Endothelial cells have diverse functions depending on where they originate. Vasculogenesis and angiogenesis play critical roles in embryonic development as well as adult vascular homeostasis.