An ultrastructural change in developing rat cerebral cortex: a morphometrical study.

Five age groups, each composed of four animals from each of the following ages, were used to assess age-related ultrastructural changes with development in the neuropil of the III layer of the frontal cortex (area 6) in rats; 1, 2, 3, 5 and 12 weeks old. Random samplings within the neuropil were taken to produce 25 electron micrographs in each rat (totaling 500). The profiles of axon terminals, dendrites and mitochondria in each element in the neuropil of each micrograph were traced. The percentage of their areas for the area of neuropil (relative volume fraction) was examined using the image analyzer system. The size and number of synaptic terminals were counted. The relative volume fractions of both the axon terminals and mitochondria in the terminals for the neuropil were found to have increased in the mature rats. On the other hand, the relative volume fraction of dendrites for the neuropil had been unchanged and the size of the terminals had gradually decreased. The number of terminals had progressively increased with development.