Neurons in the central nervous system are separated by extracellular spaces, the distribution, composition and function of which are not unequivocally known. Earlier electron microscopic studies of chemically-fixed tissues demonstrated extracellular spaces composed of uniformly narrow, apparently empty channels constituting 3-5% of the brain volume. The results of more recent morphological and non-morphological studies support the existence of less uniform intercellular channels, varying in dimension from 100 Å to almost 1 μ, and constituting 20-25% of the brain volume. Although this space is not revealed in electron micrographs of chemicallyfixed nervous tissues, it is demonstrated readily in specimens fixed by freeze-drying or freeze-substitution (Fig. 1). The dynamic nature of those extracellular spaces, as visualized in electron micrographs of nervous tissue fixed by freeze-substitution, was demonstrated in studies of normalbrain maturation. An extracellular space of 40% became gradually smaller as development proceeded to reach the smaller extracellular space characteristic of mature animals.
96. Electron Microscopic Studies of the Brain in Experimental Edema and Dehydration