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Blood vessels are the structures of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from the tissues. Blood vessels are needed to sustain life, because all of the body's tissues rely on their functionality.
A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the site of the exchange of many substances from the surrounding interstitial fluid, and they convey blood from the smallest branches of the arteries (arterioles) to those of the veins (venules). Other substances which cross capillaries include water, oxygen, carbon dioxide, urea, glucose, uric acid, lactic acid and creatinine. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in microcirculation.
Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature mainly by processes of sprouting and splitting, but processes such as coalescent angiogenesis, vessel elongation and vessel cooption also play a role. Vasculogenesis is the embryonic formation of endothelial cells from mesoderm cell precursors, and from neovascularization, although discussions are not always precise. The first vessels in the developing embryo form through vasculogenesis, after which angiogenesis is responsible for most, if not all, blood vessel growth during development and in disease.
A thrombus, colloquially called a blood clot, is the final product of the blood coagulation step in hemostasis. There are two components to a thrombus: aggregated platelets and red blood cells that form a plug, and a mesh of cross-linked fibrin protein. The substance making up a thrombus is sometimes called cruor. A thrombus is a healthy response to injury intended to stop and prevent further bleeding, but can be harmful in thrombosis, when a clot obstructs blood flow through a healthy blood vessel in the circulatory system.
The microcirculation is the circulation of the blood in the smallest blood vessels, the microvessels of the microvasculature present within organ tissues. The microvessels include terminal arterioles, metarterioles, capillaries, and venules. Arterioles carry oxygenated blood to the capillaries, and blood flows out of the capillaries through venules into veins.
An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries.
A venule is a very small vein in the microcirculation that allows blood to return from the capillary beds to drain into the venous system via increasingly larger veins. Post-capillary venules are the smallest of the veins with a diameter of between 10 and 30 micrometres (μm). When the post-capillary venules increase in diameter to 50μm they can incorporate smooth muscle and are known as muscular venules. Veins contain approximately 70% of total blood volume, while about 25% is contained in the venules. Many venules unite to form a vein.
The lymphatic vessels are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessels are lined by endothelial cells, and have a thin layer of smooth muscle, and adventitia that binds the lymph vessels to the surrounding tissue. Lymph vessels are devoted to the propulsion of the lymph from the lymph capillaries, which are mainly concerned with the absorption of interstitial fluid from the tissues. Lymph capillaries are slightly bigger than their counterpart capillaries of the vascular system. Lymph vessels that carry lymph to a lymph node are called afferent lymph vessels, and those that carry it from a lymph node are called efferent lymph vessels, from where the lymph may travel to another lymph node, may be returned to a vein, or may travel to a larger lymph duct. Lymph ducts drain the lymph into one of the subclavian veins and thus return it to general circulation.
The glomerulus is a network of small blood vessels (capillaries) known as a tuft, located at the beginning of a nephron in the kidney. Each of the two kidneys contains about one million nephrons. The tuft is structurally supported by the mesangium, composed of intraglomerular mesangial cells. The blood is filtered across the capillary walls of this tuft through the glomerular filtration barrier, which yields its filtrate of water and soluble substances to a cup-like sac known as Bowman's capsule. The filtrate then enters the renal tubule of the nephron.
Mesangial cells are specialised cells in the kidney that make up the mesangium of the glomerulus. Together with the mesangial matrix, they form the vascular pole of the renal corpuscle. The mesangial cell population accounts for approximately 30-40% of the total cells in the glomerulus. Mesangial cells can be categorized as either extraglomerular mesangial cells or intraglomerular mesangial cells, based on their relative location to the glomerulus. The extraglomerular mesangial cells are found between the afferent and efferent arterioles towards the vascular pole of the glomerulus. The extraglomerular mesangial cells are adjacent to the intraglomerular mesangial cells that are located inside the glomerulus and in between the capillaries. The primary function of mesangial cells is to remove trapped residues and aggregated protein from the basement membrane thus keeping the filter free of debris. The contractile properties of mesangial cells have been shown to be insignificant in changing the filtration pressure of the glomerulus.
Pericytes are multi-functional mural cells of the microcirculation that wrap around the endothelial cells that line the capillaries throughout the body. Pericytes are embedded in the basement membrane of blood capillaries, where they communicate with endothelial cells by means of both direct physical contact and paracrine signaling. The morphology, distribution, density and molecular fingerprints of pericytes vary between organs and vascular beds. Pericytes help to maintain homeostatic and hemostatic functions in the brain, one of the organs with higher pericyte coverage, and also sustain the blood–brain barrier. These cells are also a key component of the neurovascular unit, which includes endothelial cells, astrocytes, and neurons. Pericytes have been postulated to regulate capillary blood flow and the clearance and phagocytosis of cellular debris in vitro. Pericytes stabilize and monitor the maturation of endothelial cells by means of direct communication between the cell membrane as well as through paracrine signaling. A deficiency of pericytes in the central nervous system can cause increased permeability of the blood–brain barrier.
Neovascularization is the natural formation of new blood vessels, usually in the form of functional microvascular networks, capable of perfusion by red blood cells, that form to serve as collateral circulation in response to local poor perfusion or ischemia.
Endothelial stem cells (ESCs) are one of three types of stem cells found in bone marrow. They are multipotent, which describes the ability to give rise to many cell types, whereas a pluripotent stem cell can give rise to all types. ESCs have the characteristic properties of a stem cell: self-renewal and differentiation. These parent stem cells, ESCs, give rise to progenitor cells, which are intermediate stem cells that lose potency. Progenitor stem cells are committed to differentiating along a particular cell developmental pathway. ESCs will eventually produce endothelial cells (ECs), which create the thin-walled endothelium that lines the inner surface of blood vessels and lymphatic vessels. The blood vessels include arteries and veins. Endothelial cells can be found throughout the whole vascular system and they also play a vital role in the movement of white blood cells
Angiopoietin is part of a family of vascular growth factors that play a role in embryonic and postnatal angiogenesis. Angiopoietin signaling most directly corresponds with angiogenesis, the process by which new arteries and veins form from preexisting blood vessels. Angiogenesis proceeds through sprouting, endothelial cell migration, proliferation, and vessel destabilization and stabilization. They are responsible for assembling and disassembling the endothelial lining of blood vessels. Angiopoietin cytokines are involved with controlling microvascular permeability, vasodilation, and vasoconstriction by signaling smooth muscle cells surrounding vessels. There are now four identified angiopoietins: ANGPT1, ANGPT2, ANGPTL3, ANGPT4.
Endothelial progenitor cell is a term that has been applied to multiple different cell types that play roles in the regeneration of the endothelial lining of blood vessels. Outgrowth endothelial cells are an EPC subtype committed to endothelial cell formation. Despite the history and controversy, the EPC in all its forms remains a promising target of regenerative medicine research.
Platelet-derived growth factor receptor beta is a protein that in humans is encoded by the PDGFRB gene. Mutations in PDGFRB are mainly associated with the clonal eosinophilia class of malignancies.
Angiogenesis is the process of forming new blood vessels from existing blood vessels, formed in vasculogenesis. It is a highly complex process involving extensive interplay between cells, soluble factors, and the extracellular matrix (ECM). Angiogenesis is critical during normal physiological development, but it also occurs in adults during inflammation, wound healing, ischemia, and in pathological conditions such as rheumatoid arthritis, hemangioma, and tumor growth. Proteolysis has been indicated as one of the first and most sustained activities involved in the formation of new blood vessels. Numerous proteases including matrix metalloproteinases (MMPs), a disintegrin and metalloproteinase domain (ADAM), a disintegrin and metalloproteinase domain with throbospondin motifs (ADAMTS), and cysteine and serine proteases are involved in angiogenesis. This article focuses on the important and diverse roles that these proteases play in the regulation of angiogenesis.
Neuroangiogenesis is the coordinated growth of nerves and blood vessels. The nervous and blood vessel systems share guidance cues and cell-surface receptors allowing for this synchronised growth. The term neuroangiogenesis only came into use in 2002 and the process was previously known as neurovascular patterning. The combination of neurogenesis and angiogenesis is an essential part of embryonic development and early life. It is thought to have a role in pathologies such as endometriosis, brain tumors, and Alzheimer's disease.
Tumor-associated endothelial cells or tumor endothelial cells (TECs) refers to cells lining the tumor-associated blood vessels that control the passage of nutrients into surrounding tumor tissue. Across different cancer types, tumor-associated blood vessels have been discovered to differ significantly from normal blood vessels in morphology, gene expression, and functionality in ways that promote cancer progression. There has been notable interest in developing cancer therapeutics that capitalize on these abnormalities of the tumor-associated endothelium to destroy tumors.
VINE-seq is a method to isolate and molecularly characterize the vascular and perivascular cells of the human brain microvessels at single-nuclei resolution. This technique is achieved by combining various known laboratory-based strategies involving the mechanical dissociation of brain tissue samples into single cells, density gradient centrifugation and filtration to isolate nuclei of microvessels, fluorescence-activated cell sorting (FACs) of cellular populations and droplet-based single-nuclei RNA sequencing (drop-snRNA-seq). Altogether, this generates a single-nuclei transcriptomic profile of the various cell types present in the vasculature of the brain. Through processing and analyzing the single-nuclei transcriptomic data, the heterogeneity within and between cell types can be distinguished to construct the molecular landscape of the human brain vasculature that was not previously done before.
References
- ↑ Fujimoto, Akihisa, Onodera, Hisashi, Mori, Akira, Isobe, Naoki, Yasuda, Seiichi, Oe, Hideaki, Yonenaga, Yoshikuni, Tachibana, Tsuyoshi & Imamura, Masayuki (2004) Vascular endothelial growth factor reduces mural cell coverage of endothelial cells and induces sprouting rather than luminal division in an HT1080 tumour angiogenesis model. International Journal of Experimental Pathology 85 (6), 355-364.
- ↑ Levéen, P.; Pekny, M.; Gebre-Medhin, S.; Swolin, B.; Larsson, E.; Betsholtz, C. (1994-08-15). "Mice deficient for PDGF B show renal, cardiovascular, and hematological abnormalities". Genes & Development. 8 (16): 1875–1887. doi: 10.1101/gad.8.16.1875 . ISSN 0890-9369. PMID 7958863.
- ↑ Abramsson A, Berlin O, Papayan H, Paulin D, Shani M, Betsholtz C. (2002). Analysis of Mural Cell Recruitment to Tumor Vessels. Circulation 105:112.
- ↑ Armulik, Annika; Genové, Guillem; Betsholtz, Christer (August 2011). "Pericytes: Developmental, Physiological, and Pathological Perspectives, Problems, and Promises". Developmental Cell. 21 (2): 193–215. doi: 10.1016/j.devcel.2011.07.001 . ISSN 1534-5807. PMID 21839917.
- ↑ Vanlandewijck, Michael; He, Liqun; Mäe, Maarja Andaloussi; Andrae, Johanna; Ando, Koji; Del Gaudio, Francesca; Nahar, Khayrun; Lebouvier, Thibaud; Laviña, Bàrbara (2018-02-14). "A molecular atlas of cell types and zonation in the brain vasculature". Nature. 554 (7693): 475–480. Bibcode:2018Natur.554..475V. doi:10.1038/nature25739. hdl: 10138/301079 . ISSN 0028-0836. PMID 29443965. S2CID 205264161.
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