Measurements of the uptake of compounds that ordinarily do not penetrate into cells have been a source of data on the size of the extracellular space in nervous tissue. The distribution of one such compound, ferrocyanide, has been studied in the toad retina by means of the light and electron microscopes. At the level of the light microscope, ferrocyanide, detected as Prussian blue, appears to penetrate predominantly within the inner processes of Müller cells. A diffuse background staining by Prussian blue can be noticed also at the inner retinal layers. At the level of the electron microscope, Müller cells exhibit an extensively developed system of channels which are formed by infoldings of the plasma membrane. Ferrocyanide, detected as copper ferrocyanide deposits, is found occupying the lumina of these channels and in the narrow intercellular gaps of the retina. These observations indicate that in the toad retina the extracellular medium includes the intercellular spaces plus a glial compartment formed by the infoldings of the plasma membrane of the Müller cells.

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