Electron micrographs (EMs) of central nervous tissue perfused with glutaraldehyde and post-fixed with osmium tetroxide exhibit more extracellular space in superficial than in deep regions of the brain. It was postulated that this is due to the availability to the surface tissue of the cerebrospinal fluid in the subarachnoidal space. It was assumed that in addition to the extracellular material this fluid is taken up by dendritic elements in the surface layer of the cortex during the glutaraldehyde perfusion. Indeed, dendritic structures in EMs of the surface layer of glutaraldehyde-perfused and OsO4-post-fixed cortex exhibited a more pronounced swelling than similar structures in deeper cortical regions, where only extracellular material was available to the dendrites. The extracellular space in such preparations consisted of narrow slits surrounding the tissue elements. These slits were formed during the post-fixation since they were not present in freeze-substituted, glutaraldehyde-perfused tissue, which showed a paucity of extracellular material. The slits were in general wider in the superficial than in the deep cortical tissue. That this difference is related to the amount of material taken up by the dendrites during the glutaraldehyde perfusion was supported by experiments in which the exposed cortical surface was flooded with Ringer's solution during the glutaraldehyde perfusion. This resulted in a ballooning of dendritic elements in the cortical surface due to the availability of an unlimited amount of fluid on the cortex. Post-fixed material of these flooded cortices yielded EMs with unusually wide extracellular slits around the tissue elements.

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