A biochemical and quantitative morphologic study of presynaptic endings during postnatal development was carried out in subcellular fractions from cerebral cortex of 1, 4, 8, 12, and 18 day old and adult rats. Crude mitochondrial fractions were subfractionated in Ficoll gradients and all resulting fractions were examined in the electron microscope. Presynaptic terminals and other intact processes were counted. Protein content and enzyme activities were assayed in the fractions and in total brain homogenate. In the first and fourth day of life, most of the presynaptic terminals were found in two "light" fractions, between supernatant and 7.5% Ficoll, where they accounted, respectively, for 6 and 22% of all the processes. Progressively with age, more presynaptic terminals were found in the traditional "synaptosomal" fractions between 7.5 and 13% Ficoll. In that region of the gradient, 40, 54, 75, and 89% of the processes were presynaptic endings at 8, 12, and 18 postnatal days and in the adult animal, respectively. A similar shift from the lighter to the heavier fractions was observed in the distribution of choline acetyltransferase and acetylcholinesterase between days 8 and 12. The rate of increase of the specific activity of these two enzymes paralleled that of the percentage of the presynaptic endings after day 8. This study indicates that subcellular fractions can be used to study formation and maturation of synapses during postnatal development.

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