The extracellular compartment of the cerebral cortex

Specimens of mouse and rat cerebral cortex, fixed by immersion in osmium tetroxide and by perfusion with formalin‐osmium tetroxide were prepared for electron microscopy. The extracellular compartment, consisting of interendothelial cell spaces, capillary basement membranes and extracellular spaces between adjacent elements of neuropil, formed a morphological continuum with capillary lumina. A delicately fibrillar, extracellular matrix characterized the basement membranes and extracellular spaces of the neuropil. Desmosome‐like junctional complexes occurred between endothelial cells and between adjacent neuroglial cells. It is suggested that in the cerebral cortex molecular species from blood which pass through the basement membranes are further distributed in the cytoplasm of astrocytes. Astrocytic cytoplasm appears, then, to have assumed the histophysiological function of the connective tissue ground substance in other parts of the body. The relative constancy of the extracellular compartment as determined physiologically may relate to electro‐osmotic phenomena.