Permeability of the external glial limiting membrane of rat parietal cortex.

As the relevance of previous reports demonstrating the permeability of the glia limitans appears to be not generally accepted, the problem is reinvestigated by examination of the ultrastructure of the glia limitans in combination with tracer studies using Ruthenium Red. The results clearly show that gap junctions are the only type of intercellular contact present in this layer. Measurements of the lengths of cross-sections of junctional regions reveal a decrease in size in absolute terms as well as in relation to total astrocytic surface if one proceeds from the more superficial layers into the depth of the tissue. The intercellular clefts leading from the subarachnoidal space into the interspaces of brain tissue are patent thus allowing the tracer to penetrate to a depth of 0.4-0.6 mm in one hour. Although these observations confirm the idea of an ultimate communication between the cerebrospinal fluid compartments and the extracellular space of brain the discussion shows how the particular arrangement of the intercellular channels and the cells bordering them may be suited to control the local composition of interstitial fluid independently from that of the cerebrospinal fluid.