The study redefines the configuration of certain junctions between ependymal cells, astrocytes, and electrically coupled neurons in the vertebrate brain. These junctions, previously thought to be tight, pentalaminar occlusions, are now characterized by a median gap 20-30 Å wide, which is continuous with the interspace. The gap junctions are further demonstrated to be patent, as shown by the penetration of horseradish peroxidase or lanthanum into the median gap. However, these tracers can also circumvent the gap junctions because they are plaque-shaped rather than complete, circumferential belts. In contrast, tight junctions, which retain a pentalaminar appearance after uranyl acetate treatment, are restricted to the endothelium of parenchymal capillaries and the epithelium of the choroid plexus. They form extensive, overlapping occlusions and do not interfere with the intercellular movement of tracers. The study concludes that the interspaces of the brain are generally patent, allowing intercellular movement of colloidal materials. Endothelial and epithelial tight junctions occlude the interspaces between blood and parenchyma or cerebral ventricles, forming the structural basis for the blood-brain and blood-cerebrospinal fluid barriers.The study redefines the configuration of certain junctions between ependymal cells, astrocytes, and electrically coupled neurons in the vertebrate brain. These junctions, previously thought to be tight, pentalaminar occlusions, are now characterized by a median gap 20-30 Å wide, which is continuous with the interspace. The gap junctions are further demonstrated to be patent, as shown by the penetration of horseradish peroxidase or lanthanum into the median gap. However, these tracers can also circumvent the gap junctions because they are plaque-shaped rather than complete, circumferential belts. In contrast, tight junctions, which retain a pentalaminar appearance after uranyl acetate treatment, are restricted to the endothelium of parenchymal capillaries and the epithelium of the choroid plexus. They form extensive, overlapping occlusions and do not interfere with the intercellular movement of tracers. The study concludes that the interspaces of the brain are generally patent, allowing intercellular movement of colloidal materials. Endothelial and epithelial tight junctions occlude the interspaces between blood and parenchyma or cerebral ventricles, forming the structural basis for the blood-brain and blood-cerebrospinal fluid barriers.