Sprouting angiogenesis was originally the only known form of angiogenesis. It is very simple and occurs in several stages. First, the basement membrane degrades to allow endothelial cells to escape from the vessel walls. The endothelial cells then proliferate into the surrounding matrix and form solid sprouts connecting neighboring vessels. These sprouts then become a full-fledged vessel lumen as cells migrate to the site of angiogenesis. Sprouting is slow as it is reliant on cell proliferation but is able to spread new vessels across gaps in the vasculature. It is markedly different from splitting angiogenesis, however, because it forms entirely new vessels as opposed to splitting existing vessels (Burri, Hlushchuk, & Djonov, 2004).

Intussusception

Intussusception is the process whereby a new blood vessel is created by the splitting of an existing blood vessel in two. It is one of the three ways that blood vessels are known to be formed in the human body, the other two being angiogenesis and vasculogenesis.

Intussusception, also known as splitting angiogenesis, was first observed in neonatal rats. In this type of vessel formation, the capillary wall extends into the lumen to split a single vessel in two. There are four phases of intussusceptive angiogenesis. First, the two opposing capillary walls establish a zone of contact. Second, the endothelial cell junctions are reorganized and the vessel bilayer is perforated to allow growth factors and cells to penetrate into the lumen. Third, a core is formed between the two new vessels at the zone of contact that is filled with pericytes and myofibroblasts. These cells begin laying collagen fibers into the core to provide an extracellular matrix for growth of the vessel lumen. Finally, the core is fleshed out with no alterations to the basic structure. Intussusception is important because it is a reorganization of existing cells. It allows a vast increase in the number of capillaries without a corresponding increase in the number of endothelial cells. This is especially important in embryonic development as there are not enough resources to create a rich microvasculature with new cells every time a new vessel develops.