Angiogenesis is the term used to describe the process in which cells in the blood vessel walls degenerate, proliferate and re-arrange to form new blood vessels and these processes mainly take place at the level of capillaries. Although this is a general explanation of the angiogenic process, the modern terminology for angiogenesis states that it is the ‘formation of thin-walled endothelium-lined structures with muscular smooth muscle wall and pericytes’. Many intrinsic factors would be regulating the process of angiogenesis and it is vital in early development as well as in events such as wound healing. But, many pathological processes including cancers grow and disseminate through this process and certain other medical conditions would also manifest due to angiogenesis.
The angiogenesis usually take place in stages and going through each of these stages would enable us to understand what exactly is taking place during new blood vessel formation. Thus, following is a simplified version of the angiogenesis process.
• A cell deprived of oxygen releases stimuli to attract inflammatory and endothelial cells.
• The attracted inflammatory cells proliferate and while being attracted would also release stimuli to further intensify the angiogenic stimulus.
• Endothelial cells migrate towards the angiogenic stimulus by secreting metaloproteases which would digest the blood vessel walls and allow easy passage for these cells.
• Digestive elements such as proteins would lead to a formation and morphological re-arrangement of the basement membranes thus giving rise to a tubular structure while intensifying the angiogenic stimulus and proliferation of endothelial cells.
• Anastomosis will take place between capillaries extending from arterioles and vennules to complete the system.
As you can see, the process is rather complicated and at times the scientists even describe another type of an angiogenesis known as the ‘intussuceptive’ angiogenesis. In this event, existing blood vessels would be split into two to give rise to a new vessel and therefore also known as ‘splitting angiogenesis’.
As mentioned at the beginning, although this is a very useful mechanism to sustain life, it often becomes the culprit to some of the most serious medical problems including malignant cancers. Apart from cancers, psoriasis and macular degeneration have also shown the unwanted effects of angiogenesis.
Lastly, it is also interesting to note what makes the angiogenesis a possibility and it is remarkable to state, that more than 20 endogenous proteins in the form of enzymes, co-factors…etc would be assisting the process in many different ways. Among these factors, metalloproteinases, integrins. , cytokines, such as vascular endothelial growth factor (VEGF), fibroblast growth factors (FGF), transforming growth factors (TGF-beta) are some of the well known ones.