A systematic histochemical investigation in mammals of the dense glycocalyx glycosylations common to all cells bordering the interstitial fluid compartment of the brain.

Microanatomical evidence is presented that the intercellular fluid (ICF) compartment of the central nervous tissue is lined entirely and exclusively by heavily glycosylated cells, with glycoconjugates exposed primarily at the apical cell surface, fronting the CSF or blood. On both common ependymal cells and on those specialised to form the choroid plexus epithelium, oligosaccharides coat the cilia and microvilli at the apical surface, and also the smoother lateral and basal cell surfaces. In the ependyma, folded and wrinkled structures seem especially associated with freely exposed carbohydrates. On cerebral endothelial cells, oligosaccharides coat the luminal surface densely and the basal surface lightly. The patterns of carbohydrate distribution thus vary from one cell type to another, but the different cell types all bear essentially the same set of oligosaccharides, variations being due largely to degree of terminal sialylation. Furthermore, the same set of oligosaccharides borders the brain in a broad spectrum of mammals, including pouched and placental mammals. In both epithelia and endothelia, the lectin binding sites visualised in fixed and embedded preparations were shown to be exposed likewise at the cell surfaces in unfixed tissues and so able to bind molecules present in the fluid (CSF or blood) bathing the cells in vivo. This phylogenetically ancient enclosure of the ICF compartment in a "ring of sugars" is suggested to relate to regulation of the central neuronal microenvironment.