The substructure of tight junctions is investigated by direct freezing techniques that avoid any chemical fixation and serve to increase the resolution of individual membrane components. The backbone of the tight junction is a pair of rod-shaped structures embedded in the central lipophilic domain of each of the paired component membranes. This conclusion replaces the previous view that tight junctions are comprised of rows of intramembrane proteins. Instead, the rod-shaped structures, which are comparable to cylindrical micelles in liposomes, are now interpreted as inverted cylindrical micelles of membrane lipids. These results lead to an understanding of how tight junctions serve in the blood-brain barrier system to prevent small charged solutes from entering the brain. Similar techniques are being applied to understand the substructure of specific glial membrane structures which are regarded as components of the blood-brain barrier system.