Transendothelial channel

Numerous variations in the microvasculature bed (i.e., arterioles, capillaries, and venules) exist, which can affect permeability. For example, venular portions of the capillaries have thin endothelial cells (170 nm), with frequent interendothelial discontinuities. The gap in about 30% venular junctions is about 6 nm. Arterioles, in contrast, have endothelial cells that are linked by the tight junctions and communicating junctions, whereas the capillary endothelium contains only occluding junctions. Communicating gaps are small and rare in muscular venules and are absent in capillaries and pericytic venules. Endothelial cells in capillaries have more vesicles than those in arterioles (1000/pm vs. 190/pm ). The intercellular sealing is strong in arterioles, weU developed in capillaries, and particularly loose in venules. Furthermore, capillaries and venules have more transendothelial channels. 

PMN binding to and migration across endothelium initiates a sequence of events that resembles that following histamine treatment [14,30], namely, an increase in the permeability of the EC monolayer. For anionic plasma macromolecules, the plasmalemmal vesicles and transendothelial channels are suitable candidates for exit from the vessel lumen. Indeed, receptor-mediated transcytosis of insulin and transferrin has been identified for brain capillaries [31-33]. The insulin carriers have not yet been identified, but it is speculated that they could be coated vesicles and/or plasmalemmal vesicles. Transferrin [34] as well as ceruloplasmin [35,36] binding has been localized to coated pits and vesicles in the endothelium of bone marrow and liver capillary endothelium, respectively. 

N. Simionescu, M. Simionescu, and G. E. Palade. Permeability of muscle capillaries to small hemepeptides. Evidence for the existence of patent transendothelial channels. J. Cell Biol. 64 586-607 (1975). 

Fig. 3. Transvascular exchange. Transport pathways in normal capillary endothelium. (1) endothelial cell (2) lateral membrane diffusion (3) interendothelial junctions—(a) narrow, (b) wide (4) endothelial fenestrae—(a) closed, (b) open (5) vesicular transport—(a) transcytosis, (b) transendothelial channels. Note that water and lipophilic solutes share pathways (1), (3), and (4). Lipophilic solutes may use pathway (2) as well. Hydrophilic solutes and macromolecules use pathways (3) and (4). Macromolecules may also follow pathway (5). Note that in tumors these pathways have a leakier structure. [From Jain (1987a), with permission.]...

The density of plasmalemmal vesicles as well as stability of the transendothelial channels varies in different tissues, and is accompanied by a variation of the permeability of the vascular endothelium for polymers, which is, in general, inversely de-... 

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