Aging in the neuropil of cerebral cortex--a quantitative ultrastructural study.

Six age groups, each composed of four animals from each of the following ages, were used to assess age-related ultrastructural changes in the neuropil of the III layer of the frontal cortex in rats: 3, 6, 12, 18, 24 and 30 months old. Random samplings within the neuropil were taken to produce 40 electron micrographs in each rat (totaling 960). The profiles of axon terminals, dendrites and astroglial processes in the neuropil of each micrograph were traced. Then the percentage of their areas for the area of neuropil (relative volume fraction) was examined using the image analyzer system. The relative volume fractions of both the axon terminals and dendrites for the neuropil were found to have decreased in the aged rats. On the contrary, the relative volume fraction of astroglial processes for the neuropil had progressively increased with aging.