Angiogenesis is the development of new blood vessels that form from pre-existing vessels. This process is a normal part of growth and healing, involving the migration, growth, and differentiation of endothelial cells, which line the inside wall of blood vessels.
Angiogenesis plays a role in several diseases, including cancer. Once a tumor grows to a certain size, it needs nutrients and oxygen from the blood to grow and spread. Angiogenesis becomes cause for worry when it occurs within tumors and cancers, feeding and sustaining them. Antiangiogenic agents that treat cancer target the angiogenesis factors, preventing the growth of new blood vessels, starving the cancer cells, and eventually killing the cancer cells.
Types of angiogenesis include:
- Sprouting angiogenesis
- Intussusceptive angiogenesis
Sprouting angiogenesis is different from intussusceptive angiogenesis because it forms entirely new vessels as opposed to splitting existing vessels. Sprouting angiogenesis occurs in several characterized stages:
- Angiogenic growth factors (biologic signals) activate receptors on endothelial cells present in pre-existing blood vessels.
- Activated endothelial cells begin to release enzymes called proteases that allow endothelial cells to escape from the original vessel walls.
- The endothelial cells form solid sprouts connecting neighboring vessels.
- Sprouting enables new vessels to grow across gaps in the vasculature.
Intussusceptive angiogenesis, also known as splitting angiogenesis, occurs when a new blood vessel is created by splitting an existing blood vessel in two. This process is important because it is reorganizing existing cells, allowing a large increase in the number of capillaries without a corresponding increase in the number of endothelial cells. There are four phases of intussusceptive angiogenesis:
- The two opposing capillary walls establish a zone of contact.
- The endothelial cell junctions are reorganized and the vessel bilayer is perforated to allow growth factors and cells to penetrate into the lumen.
- A core is formed between the two new vessels at the zone of contact. Cells begin laying collagen fibers into the core to provide extracellular matrix for growth of the vessel lumen.
- The core is fleshed out with no alterations to the basic structure.