Stress fiber networks in sinus endothelial cells in the rat spleen

The wall of the splenic sinus is well known to be a critical site in the control of the blood-cell passage through the splenic cord. However, there is little information on what mechanism controls the blood-cell passage between the sinus endothelial cells adhered with intercellular junctions. Stress fibers of sinus endothelial cells in the rat spleen were examined by transmission electron microscopy and immunofluorescence microscopy. After extraction with saponin, the stress fibers were found to be conspicuously electron-dense in the basal portion of the cells and demonstrated two characteristic types and the mixed type of these. One type of the stress fibers was corrugated and formed thick, electron-dense bundles, and the other type had straight bundles with electron-dense areas at intervals. Statistically, the lengths of two types of stress fibers are significantly different. Although most of the stress fibers were segmented by the ring fibers, some of them ran successively beyond the attachment site of the ring fibers and thus stretched longitudinally and tangentially in the basal part of the cell to form a widespread network. The peripheries of the networks were attached to the basal plasma membrane at the ring fibers and the lateral membranes at the focal adhesions and the cell-cell adherens junction, respectively. Phosphotyrosine is localized in the basal part and at the intercellular adhesion site of the endothelial cells. Two types of stress fibers were organized in the sinus endothelial cells, and they form a widespread network. The possibility might be considered that stress fibers play important roles in the signal transduction and the regulation of cellular attachment and detachment.