Specimens of rat cerebral cortex were fixed in solutions containing ruthenium red and prepared for electron microscopy by a method described by Luft ('65a). In specimens taken from a narrow zone 5–10 m̈ thick and about 5 m̈ beneath the pial surface the ruthenium red reaction was found to be uniform and reproducible. In this zone a particulate reaction product was distributed extracellularly. The reaction product accumulated in intercellular gaps, filling them with closely-packed, dense particulates which adhered closely to the outer leaflets of apposed plasma membranes and filled also the intercellular spaces with additional particulates.
The selective, extracellular distribution of the ruthenium red reaction product suggested the presence of an intercellular acid mucopolysaccharide, which may be different in capillary basal laminae, intercellular gaps and synaptic clefts, as suggested by the variable reactivity of these extracellular sites.
The presence of an intercellular acid mucopolysaccharide in the cerebral cortex would controvert neither ion transport by glial cytoplasm nor intercellular channels. It is suggested that such an intercellular substance might facilitate ion transport and the histophysiological significance of this is discussed.