Ultrastructural analysis of the mechanism of the contraction of splenic sinus endothelial cells

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Endothelial cells of the splenic sinus have been previously investigated as a critical site for controlling blood cell passage through the splenic cord, and they have also been shown to be ultrastructurally quite different from other vascular endothelial cells. Particularly prominent are their shape and arrangement, like the staves of a barrel, with a highly ordered network of contractile stress fibers running longitudinally in their basal part. in addition, tubulovesicular structures in close apposition to abundant caveolae and stress fibers are conspicuous. An elevation in intracellular free Ca2+ concentration plays a crucial role in the regulation of endothelial vascular functions and, furthermore, agonists and/or mechanical stress also evoke Ca2+ oscillations. The exact mechanisms contributing to Ca2+ oscillation are unknown, but oscillatory changes in IP3-receptor sensitivity, tune-resolved Ca2+ release from different Ca2+-pools, the rhythmic entry of Ca2+ through non-selective ca … More tion channels, changes in cellular morphology and the involvement of endoplasmic Ca2+-ATPases have been proposed. Considering that the stress **ers in sinus endothelial cells are contracted, caveolae, Ca2+storing ER, IP3R, and RyR in sinus endothelial cells may be involved in the constriction of stress. fibers. in addition, the presence and contribution to Ca2+-oscillations of RyR in endothelial cells have been shown, but the localization in endothelial cells is not quite clear. Therefore, the distribution of caveolac and Ca2+storing ER and the localization of IP3R and RyR in the sinus endothelial cells of the rat spleen were examined by confocal laser scanning and electron microscopy.Immunofluorescence microscopy of tissue cryosections revealed IP3R and RyR to be localized in the subplasmalemmal area and cytoplasm of sinus endothelial cells. By electron microscopy of tissue sections treated with osmium ferricyanide to selectively stain the sarcoplasmic reticulum and transverse tubules in muscle cells, electron-dense tubulovesicular structures were observed to be in close apposition to caveolac and the plasma membrane. Immunogold electron microscopy revealed IP3R and RyR to be present in the tubulovesicular structure in the subplasmalemmal area of sinus endothelial cells. It is speculated that IP3R and RyR in sinus endothelial cells is involved in the constriction of stress fibers. Less