High Sodium Content of Cortical Astrocytes: Electron Microscopic Evidence

FOR MORE than a decade, an unexplained disparity has existed between the values for extracellular space in the cerebral cortex obtained by different methods. Electron microscopy of thin sections has revealed, with rare exceptions, a space between cells or cell processes measuring only 150 to 200 Angstrom wide.^1-5 More recently, localized obliterations of even this small gap by the formation of so-called tight junctions^6-8 has been described. Calculations based on measurement of the cortical extracellular space in electron micrographs⁹ yield a maximum value of 5% for this compartment as a fraction of total cortical volume. Not only has a consistent average value of 200 A for the width of the intercellular gap been measured in electron micrographs of normal cerebral cortex fixed and embedded in various ways, but brain slices incubated in vitro^10,11show astrocytic swelling without change in the dimensions of the extracellular space. A