Fig. 1. Physiological blood vessel formation. The first step of angiogenesis is vessel sprouting. Angiogenic growth factors trigger endothelial cells to degrade the basement membrane (A) and
select for tip and stalk cells. Tip cells then migrate into the tissue stroma while stalk cells proliferate (B). A lumen is formed by fusion of tip cells (C–D) and flow initiates differential branchmaturation (E).