What are the types of angiogenesis?

The two types of angiogenesis are sprouting angiogenesis and splitting angiogenesis. Angiogenesis is regulated by chemical signals in the body (e.g. VEGF). Once VEGF binds to receptors on the surface of normal endothelial cells, signals within these cells become initiated, inducing growth and survival of new blood vessels. This process is known as sprouting angiogenesis, as new growths of endothelial cells are derived from blood vessels to reach areas of tissues which do not have other blood supply. Activated endothelial cells release proteases which break down the basement membrane. This forms an opening in the existing blood vessel which allows for the outpouring of activated endothelial cells from the existing parent blood vessel. Once the endothelial cells exit, they multiply into the surrounding matrix and generate solid sprouts. These assist in connecting the adjacent blood vessels. The sprouts then extend towards the source of the angiogenic stimulus. The newly formed endothelial cells use adhesion molecules that help them bind to one another to form chains. The sprouts then produce loops to become tubular blood vessels. Sprouting may occur rapidly at a rate of several millimeters per day.  

The process in which a capillary forms without sprouting is known as splitting angiogenesis. One blood vessel splits into two to create the new capillary. Endothelial cells move into the blood vessel’s lumen and generate pillars instead of sprouting outward. The pillars help to induce the growth of new capillary branches. In splitting angiogenesis, capillary walls facing one another first must make contact. Then the endothelial cell junctions are modified so that a bilayer of blood vessel walls is formed. The bilayer is then punctured by angiogenesis factor activated endothelial cells which allows growth factors to enter into the lumen. A core is formed at the point of contact between the two walls and is filled up with pericytes and myofibroblasts. The new cells then begin forming collagen fibers in the core and provide an EC matrix for growth of the vessel lumen. The core finally splits the existing blood vessel lumen into two.