Junctions between the sinus endothelial cells of rat spleen

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 membranes and were continuous with the actin filaments in stress fibers. Cross-bridges were present in the interspaces of the adherens junctions and spacing of the bridges was fairly regular. A form of tight junction, the macula occludens, was also observed between the endothelial cells, but it was not observed at every cellular apposition. Electron-dense material, adjoining the 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, the two intercellular junctions were considered to play important roles in sinus functions.