Basement membrane of central nervous system capillaries lacks ruthenium red-staining sites

We used the cationic dye ruthenium red to examine the distribution of anionic macro-molecules (presumably proteoglycans) in the basement membranes of the fenestrated capillaries and epithelium of the choroid plexus, and of the continuous capillaries forming the blood-brain barrier. Both the endothelial and epithelial basement membranes of choroid plexus displayed discrete, 10- to 20-nm-diameter, electron-dense sites after exposure to ruthenium red. These sites were similar in size, appearance, and distribution to those found in the vascular and epithelial basement membranes of a variety of tissues outside the central nervous system. In contrast, the basement membranes of continuous, blood-brain barrier capillaries did not display electron-dense sites following exposure to ruthenium red, even after measures had been taken to enhance penetration of the dye across the endothelial cells. The lack of discrete ruthenium red staining in the basement membrane of continuous blood-brain barrier capillaries could be due to a relative paucity of anionic macromolecules, or may be the result of the compact architecture of this particular basement membrane. Regardless of the final explanation, these findings suggest that the basement membrane of the blood-brain barrier, like its endothelium, is structurally (and perhaps functionally) unique.