Extravasation endothelial cells

The activation of a chemokine receptor is more complex than the traditional agonist-receptor paradigm. For example, chemokine activity is mediated by GAGs (heparin, heparan, and heparin sulfate chondroitin sulfate and dermatan sulfate) at various sites during the chemotactic process. Chemokines released by tissue injury, infection, or inflammation activate adjacent endothelial cells and induce rolling and extravasation of leukocytes. These interactions between... 

Fig. 11.1. Atherogenesis is a persistent inflammatory response that occurs in response to conditions that cause endothelial damage (e.g., hypercholesterolemia and oxLDL). After endothelial cells are activated, they elaborate cytokines, chemokines, and other mediators that recruit mononuclear cells (monocytes and T lymphocytes) to extravasate into the vessel wall where they are activated and release additional proinflammatory factors. Macrophages are able to take up oxLDL via scavenger receptors causing them to differentiate into foam cells and form a fatty streak that progresses to an atheroma with a necrotic lipid core and a fibrous cap. Chemokines can lead to weakening of the fibrous cap and eventual plaque rupture leading to thrombosis and occlusion of the involved vessel.

The endothelium has many diverse functions that enable it to participate in in-flammatoiy reactions (H27). These include modulation of vascular tone, and hence control of local blood flow changes in structure that allow leakage of fluids and plasma proteins into extravascular tissues local accumulation and subsequent extravasation into tissues of leukocytes and synthesis of surface molecules and soluble factors involved in leukocyte activation (B43). The endothelial cells themselves can modulate vascular tone by the release of vasoactive substances such as prostacyclin, nitric oxide (NO), ET. Endothelium-derived vasoactive substances... 

Apart from inducing vascular damage via infiltration and degranulation of various blood cells, PAF and TNF exert also direct effects in the endothelium. In vitro, both substances cause contraction of endothelial cells, which may partially account for the increased vascular permeability and plasma extravasation observed in many species following PAF or TNF administration [311]. While it has been known for some time that endothelial cells produce PAF when stimulated with various agonists such as thrombin, it has recently been established that TNF and IL-1 also induce cultured endothelial cells to synthesize PAF, the majority of which remains associated with the cells [317]. 

Variations on the filter-based assay have been designed to approximate more physiological contexts. Such assays include tumor cell invasion across a confluent cell monolayer (e.g., endothelial cells (EC) as a surrogate for intravasation or extravasation during hematogenous metastasis (24)) and ovarian carcinoma invasion of mesothelial cell monolayers (25). Additionally, 1 mm thick slices of human brain tissue have been used as a tissue barrier on Transwell filters with invasion of GFP-labeled glioma cells measured by confocal microscopy (26). 

There are differences in the ease of extravasation of macromolecules from the bloodstream into different tissues [14, 104, 105]. Capillaries in the liver, spleen, and bone marrow have incomplete basal membranes and are lined with endothelial cells which are not continuously arranged. Capillaries in the muscle have a somewhat tighter arrangement, and there is an almost impermeable barrier which isolates the central nervous system from circulating blood. The rate of glomerular filtration of macromolecules depends on their hydrodynamic radius, the threshold being approx. 45 A [106]. Structure of the macromolecule is of utmost importance, since shape, flexibility, and charge influence the penetration and possible readsorption in the tubular epithelia [100]. 

The conjunctival ischemia results from the necrosis of the endothelial cells of the conjunctival and scleral vessels. There is often a blood extravasation so that the conjunctival hemorrhages are often associated to this ischemia. 

IFN-y also induces the costimulatory molecules on the macrophages, which increases cell-mediated immunity. As a consequence, there is activation and increase in the tumoricidal and antimicrobial activity of mononuclear phagocytes, granulocytes and NK cells. The activation of neutrophils by IFN-y includes an increase in their respiratory burst. IFN-y stimulates the cytolytic activity of NK cells. It is an activator of vascular endothelial cells, promoting CD4+ T lymphocyte adhesion and morphological alterations, which facilitates lymphocyte extravasation. IFN-y promotes opsonization by stimulating the production of IgG subclasses that activate the complement pathway. A summary of the characteristics of selected cytokines is shown in Table 2.3. 

The vascular endothelial growth factor is produced by tumor cells, macrophages, and endothelial and smooth muscle cells. It induces vascular endothelial cell migration, enhances vascular permeability, and promotes extravasation of plasma proteins from tumor vessels to form an extracellular matrix, facilitating inward migration of endothelial cells (Callagy et al., 2000). These characteristics impart selectivity to VEGF for endothelial cells. 

Continuous capillaries are found in the skin, all types of muscle, mesenteries, and the central nervous system (blood-brain barrier). These capillaries are characterized by tight junctions between the endothelial cells and an uninterrupted basement membrane. The restrictive capacity of the capillary walls barely allows extravasation of macromolecules into the parenchyma of these tissues. 

Discontinuous capillaries are present in the liver, bone marrow, and spleen. These capillaries have relatively large gaps between the endothelial cells (30-500 nm), and have incomplete basement membranes, which make them best suited for extravasation of macromolecules. 

M. B. Furie, B. L. Nnprstek, and S. C Silverstein. Migration of neutrophils across monolayers of cultured endothelial cells an in vitro model for leucocyte extravasation. 3. Cell ScL 8 161-175 f19871. 

The recent discovery of endokinins and hemokinins provides an alternative pathway that may take over from neuronally derived substance P as an increasing proportion of vessels are noninnervated. These novel tachykinins are expressed by bone marrow-derived inflammatory cells and endothelial cells themselves [75]. Both hemokinin-1 and endokinins A and B are selective and full agonists at NK, receptors in man, rat, and mouse, inducing acute plasma extravasation and vasodilation [76], It is likely that these novel tachykinins will also, like substance P, induce angiogenesis. 

The inflammation response to tissue injury involves infiltration of damaged areas by leukocytes from the blood stream (1-4). Unregulated extravasation of leukocytes can result in inflammatory disorders such as reperfusion injuries, stroke, psoriasis, rheumatoid arthritis and respiratory diseases. An early step in the cascade of events leading to influx of leukocytes is the recognition of the tetrasaccharide sLex 1 (found on the terminus of leukocyte surface glycoproteins), by E, P and L selectins that are expressed by endothelial cells... 

The importance of TNF-a is noted in the activation of the endothelium. TNF-a induces endothelial cells to present E-selectin and intercellular adhesion molecule 1 (ICAM-1), both of which are ceU-adhesion molecules that mediate the mechanism of leukocyte extravasation, termed diapedesis (see Chapter 14). While this process is essential for the recruitment... 

The majority of more recent structures relate to the sialyl Lewis X blood group carbohydrate receptor function for the selectin family of eukaryote lectins that mediate leucocyte-endothelial cell interaction during the initial stages of extravasation ( neutrophil rolling ) (20). These structures are the sialyl Le , Le, and Le sequences and several 6 sulfo GlcNAc and 3 sulfo Gal derivatives found in both protein and sphingolipid glycoconjugates (21-24). 

The arrest of tumor cells in the capillary bed of secondary organs and their subsequent extravasation occur through interactions with the local microvascular endothelium and the subendothelial matrix. The specificity of these interactions, depending on the heterogeneity of both microvascular endothelial cells (EC) and tumor cells, may favor in a selective way the initial adhesive events in preferred metastatic sites, and may consequently facilitate metastatic dissemination to those organs (Blood and Zetter, 1990 Pauli et al., 1990 Rusciano and Burger, 1992), similarly to what happens... 

During intravasation and extravasation, tumor cells must traverse not only the BM barrier but also the continuous layer of endothelial cells which lie on the BM. Adhesion to, separation of and intrusion into endothelial (or epithelial) cells are as crucial to tumor spread as degradation of the extracellular matrix. Crissman et al. (1985) estimate that the endothelial cell layer may delay tumor cells from contacting the BM in vivo for up to 2 days. 

Numerous in vitro and in vivo tests have been developed to study specific steps of the neoangiogenic process. Several of these experimental protocols use the same techniques applied for assessment of tumor cell behavior (Albini, 1998). The activated endothelial cell acts similarly to a metastatic cell in the initial phases of angiogenesis, degrading the capillary basement membrane (BM), extravasating, digesting the extracellular matrix and moving toward the angiogenic stimulus (Liotta et al., 1991). For this reason... 

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