Figure 1.

Schematized tyrosine kinase with Ig and EGF homology domain 2 (TIE2)–angiopoietin system. There are two kinds of ligand for TIE2: angiopoietin‐1 (Ang‐1) and Ang‐2. (A) Ang‐1 produced by mural cells (MC) stimulates TIE2 and promotes adhesion between endothelial cells (EC) and MC. In this situation, blood vessels are structurally stable. (B) Ang‐2 is an antagonist of Ang‐1 and inhibits the binding of Ang‐1 to the TIE2 receptor. Ang‐2 is produced in EC under tissue hypoxia and promotes dissociation between EC and MC. This inactivation of TIE2 becomes a trigger for initiation of sprouting angiogenesis.