Mechanism of controlling blood-cell passage between the sinus endothelial cells

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Junctions between the sinus endothelial cells of rat spleen were examined by electron microscopy, using both freeze-fracture and detergent-extraction techniques. Adherens and tight junctions were observed. Adherens junctions were the predominant junctional structures between endothelial cells and were located on basolateral and lateral surfaces. At the basolateral adherens junctions, actin filaments were associated with the junctional membrane and were continuous with actin filaments in stress fibers. Cross-bridges were present in the interspaces of the adherens junctions, and spacings of the bridges were fairly regular. A form of tight junction, macula occludens, was also observed between the endothelial cells, it was not observed at every cellular apposition. Electron-dense material, adjoiningthe cytoplasmic surfaces of membranes in the tight junctions, separated the junctional membranes from masses of thin filaments. At basolateral tight junctions, the actin filaments were continuous with those in the stress fibers. Based on these observations, these two intercellular junctions were considered to play important roles in sinus functions.