Vascularization is the connection of an organ to the blood system and thus can also correspond to a new formation of smaller vessels. In the case of pathologic neovascularization, such as the systemic connection of a tumor, it is also referred to as neovascularization. In medical practice, vascularization plays a role mainly therapeutically.
What is vascularization?
Vascularization is the connection of an organ to the blood system and thus can also correspond to a new formation of smaller vessels. By the term vascularization, medicine refers to two different contexts. On the one hand, the term refers to the overall vascular connection of a particular organ. Even more frequently, however, physicians use the word to refer to what is known as angiogenesis. This process corresponds to the formation of new vessels in the human body. Angiogenesis is thus the growth of vessels that are formed by sprouting or fission on the basis of preformed blood vessels. The formation of new vessels from endothelial progenitor cells is distinct from this type of vascularization and is also referred to as vasculogenesis. Vasucogenesis is most relevant to the development of the vasculature in the embryonic period. Angiogenesis plays a role especially for repair processes of wound healing. The last type of new vessel formation is arteriogenesis, in which arteries and arterioles are formed on the basis of smooth muscle cells. All forms of new blood vessel formation within an adult organism are also referred to as neovascularization. Neovascularization may also be referred to in the case of new vessel formation with pathologic value.
Function and Purpose
Vascularization as a blood vessel system connection refers to the circulatory system as a flow system. The system extends from the heart through the body in a network of individual blood vessels, ensuring survival. The blood vascular system ensures the metabolism of every organ, tissue and cell of the body. It thus maintains the chemical physiological level of body fluids. The blood primarily transports oxygen from the lungs to the individual cells and removes carbon dioxide from there. Nutrients from digestion are also transported to the organs and tissues via the blood. In this way, the individual cells receive fats, sugars and proteins, which they consume, process or store. The resulting waste products are transported away with the blood to other tissues. In addition, messenger substances such as hormones or immune cells are transported to their place of action in the blood system. The totality of the vessels of a given organ fulfills all of the above tasks and is called vascularization. Vascularization in the sense of new formation processes with small blood vessels as a result corresponds to the formation of vascular structures with endothelial cells, pericytes and smooth muscle cells. These new formation processes are relevant in the context of wound healing and the associated repair processes. In the broadest sense, the two meanings of vascularization overlap. The common point of overlap corresponds to the supply of tissue sections with a system of vessels and blood capillaries. The liver is considered a well-vascularized tissue. It is particularly rich in blood vessels. Thus, significantly more bleeding occurs in this type of tissue during an injury than in weakly vascularized tissue such as tendons.
Diseases and complaints
Vascularization in the sense of angiogenesis is of great importance in the medical clinic, for example, in the context of tumors. A solid tumor depends on the co-growing network of capillaries. In this context, there is talk of tumor-induced angiogenesis. This capillary network supplies the tumor with nutrients and oxygen. Every tumor of two mm³ or more is dependent on the formation of new vessels. Without vascular connectivity, tumors remain asymptomatic and have no clinical relevance. Suppression of vascularization accordingly limits tumor growth. Anti-angiogenic therapeutic approaches reduce vascularization and thus blood flow to tumors. VEGF-neutralizing monoclonal antibodies such as bevacizumab have been allowed for metastatic colorectal cancer since 2004. Today, this type of therapy is also used for breast cancer, lung cancer or kidney cancer. Pro-angiogenic therapy is to be distinguished from this.It is based on angiogenic growth factors and is used, for example, to treat arteriosclerosis. In particular, the potent angiogenic growth factor FGF-1 is used. Pro-angiogenic therapies can also play a role in chronic wound healing disorders. Vascularization-promoting therapy corresponds to either protein therapy, gene therapy, or cell therapy. The use of growth factors corresponds to protein therapy. So far, the gene therapy studies to promote vascularization have mainly used the gene encoding angiogenic growth factor in DNA. On this basis, the gene therapy pathway may correspond to, for example, adenovirus-mediated gene transfer. To date, however, unresolved problems plague gene therapy. For example, gene transfection, which may be accompanied by an adverse immune system response, occurs more frequently with these therapeutic approaches. The potential toxicity of carrier viruses is also an unresolved problem of these approaches. In turn, vascularization-promoting cell therapy is based on the transfer of different cell types. To date, this therapeutic approach is still in its infancy. The current stage corresponds to an initial stage. Studies with small numbers of patients are available. However, these studies show relatively contradictory results. So far, cells of different types have been used for transfer. In addition to various forms of adult stem cells, such as endothelial progenitor cells, hematopoietic and mesenchymal stem cells have been used in the respective pilot studies.
