 Tie-1 heterodimerizes with Tie-2 to enhance and modulate signal transduction of Tie-2 for vascular development and maturation.Tyrosine kinase receptors are typically expressed on vascular endothelial cells and specific macrophages for immune responses.Angiopoietin-1 is a growth factor produced by vascular support cells, specialized pericytes in the kidney, and hepatic stellate cells (ITO) cells in the liver.