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Differences in vascular permeability between normal tissue and a tumor. Vascular permeability, often in the form of capillary permeability or microvascular permeability, characterizes the capacity of a blood vessel wall to allow for the flow of small molecules (drugs, nutrients, water, ions) or even whole cells (lymphocytes on their way to the site of inflammation) in and out of the vessel.
Excessive or prolonged increases in permeability of the endothelium, as in cases of chronic inflammation, may lead to tissue swelling . Altered barrier function is also implicated in cancer extravasation. [6] Endothelial cells are involved in many other aspects of vessel function, including: Blood clotting (thrombosis and fibrinolysis).
The blood–brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system, thus protecting the brain from harmful or unwanted substances in the blood. [1]
Pericytes stabilize and monitor the maturation of endothelial cells by means of direct communication between the cell membrane as well as through paracrine signaling. [14] A deficiency of pericytes in the central nervous system can cause increased permeability of the blood–brain barrier. [6]
The permeability of a capillary wall is determined by the type of capillary and the surface of the endothelial cells. A continuous, tightly spaced endothelial cell lining only permits the diffusion of small molecules. Larger molecules and blood cells require adequate space between cells or holes in the lining.
TEM of rat kidney tissue shows a protein dense tight junction (three dark lines) at ~55,000x magnification.. Tight junctions provide endothelial and epithelial cells with barrier function, which can be further subdivided into protective barriers and functional barriers serving purposes such as material transport and maintenance of osmotic balance.
Angiopoietin-1 and angiopoietin-2 are modulators of endothelial permeability and barrier function. Endothelial cells secrete angiopoietin-2 for autocrine signaling while parenchymal cells of the extravascular tissue secrete angiopoietin-2 onto endothelial cells for paracrine signaling, which then binds to the extracellular matrix and is stored ...
VE-cadherin is known to be required for maintaining a restrictive endothelial barrier – early studies using blocking antibodies to VE-cadherin increased monolayer permeability in cultured cells [7] and resulted in interstitial edema and hemorrhage in vivo. [8]