The crude mitochondria1 fraction P2 and subfractions of it were prepared from the brain stem, hemispheres, hypothalamus and striatum of adult rats and from the hemispheres and brain stem or diencephalon-mesencephalon and pons-medulla oblongata of 1-day-old rats. Samples were fixed in glutaraldehyde-osmium, KMnO, or by Tranzer's triple fixation method (aldehydes-chromate-dichromate-osmium) and examined by electron microscopy. With all these fixation techniques 80-95% of processes in the adult samples were found to be mature synaptosomes. After incubation with either 10-100 pg/ml of 5-hydroxydopamine or a-methylnoradrenaline, and after permanganate or triple fixation, monoamine synaptosomes, containing small granular vesicles were detected.They were found in all the brain regions studied and in the fraction P2 and subfractions C and D but only a t a frequency of less than 1%. In samples from l-dayold rats using the different fixatives, synaptosomes comprised 10-30% of all processes. Nerve endings were a t very different stages of maturation in sharp contrast to the rather uniform type of adult synaptosomes. They varied from processes still without any substructures to mature synaptosomes containing synaptic junctions. Monoamine synaptosomes containing small granular vesicles were detected after amine incubation and KMnO, or triple fixation. Their frequency was lower than that in the adult. Most common were very immature terminals containing smooth endoplasmic reticulum and a very few small granular vesicles, indicating a very early appearance of the capacity to form small granular vesicles under experimental conditions. The immature terminals had all the characteristics of the growth cone and might partly have originated from growth cones a t the tips of growing axons and dendrites. Less frequent were more mature synaptosomes containing a greater number of small granular vesicles and no longer any smooth endoplasmic reticulum. The crude microsomal fraction P3 was also examined in the adult brain regions and the brain stem of 1-day-olds.This fraction contained a small number of small agranular vesicles originating from cell bodies and nonterminal axons; their number was smaller in 1-day-old than in adult rats. The present results are in agreement with the high proportion of immature monoamine terminals in newborn rat brain detected earlier by fluorescence histochemistry.Earlier studies have demonstrated that density, contrasting to the rather uniform synaptosomes in a low proportion can be iso-adult type of synaptosomes (Spence and lated from the brain of newborn rats.

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