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The extracellular space of the rat brain was visualized by electron roscopy in sections of cerebral cortex fixed by freeze-substitution. The volume tissue occupied by the extracellular space was estimated stereomnetrically, and found to decrease from 20.8 percent in 3-month-old rats to 9.6 percent in escent animals, 26 months of age. This decrease in extracellular space indicates age-associated change in the microenvironment of nerve cells.
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Bondareff et al. (1972) studied this question.
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