Intercellular junctions between specialized ependymal cells in the subcommissural organ of the rat

The permeability of intercellular junctions in specialized ependymal cells in the rat subcommissural organ (SCO) has been studied ultrastructurally by freeze-fracturing and tracer experiments with horseradish peroxidase (HRP). In addition to normal smooth membrane, areas which could be classified as a leaky tight junction are found within the ependymal junctional region. This consists of only one or two relatively continuous strands but with interruptions in the apical portion. Some strands are perpendicular to the apical membrane surface and often form hairpin-like bends in the basal portion of the junction. The junctional region also shows areas with no strands but only a rippled membrane structure which may be equivalent to very close appositions without fusion of adjacent ependymal cell membranes. The relative proportions of normal smooth membrane, strands and rippled structure in the junctional region is approximately 3:4:6 including two parts overlapping of the strands and rippled structure. Intraventricularly infused HRP passes through many junctions but is occasionally stopped, leaving unstained intercellular spaces of various lengths between membrane fusions of tight junctions. Even when it is stopped, the intercellular space below the junction is densely stained by the enzyme. Orthogonal arrays of intramembrane particles are found to be distributed on the basal and lateral cell membranes below the junctional region in the SCO ependyma.