Frequency of occurrence of perforated synapses in developing rat neocortex

The frequency of occurrence of perforated synapses in the molecular layer of rat parietal cortex has been determined using the disector method. Perforated and non-perforated synapses were examined in electron micrographs, all quantitative procedures being carried out on adjacent sections, as required by the disector procedure. The numerical densities of perforated, non-perforated and all synapses were determined at nine ages, ranging from 0.5 to 22 months of age. The frequencies of perforated synapses were subsequently calculated. These ranged from 18.1% at 0.5 months to a peak of 37.2% at 10 months of age, and remained at around 34% up to 19 months. A frequency of 26.7% was found at 22 months of age. The mean projected height of the total perforation increased from 66 nm at 0.5 months to 133 nm at 22 months of age. These results suggest that perforated synapses are present throughout the life-span of the animal, and increase in complexity with increasing age.