Bcl-2 protein as a marker of neuronal immaturity in postnatal primate brain

The distribution of neurons expressing immunoreactivity for the protein Bcl-2 was studied in the brain of squirrel monkeys (Saimiri sciureus) of various ages. Several subsets of small and intensely immunoreactive neurons displaying an immature appearance were disclosed in the amygdala and piriform cortex. The piriform cortex exhibited clusters of various forms in which Bcl-2+ neurons appeared linked to one another by their own neurites. The subventricular zone, which is known to harbor the largest population of rapidly and constitutively proliferating cells in the adult rat brain, was intensely stained, particularly at the basis of the lateral ventricle. A long and dorsoventrally oriented Bcl-2+ fiber fascicle was seen to emerge from the subventricular zone, together with numerous Bcl-2+ cells that formed a densely packed column directed at the olfactory tubercle. In adult and aged monkeys, the small and intensely labeled neurons were progressively replaced by larger and more weakly stained neurons in the amygdala and piriform cortex. In contrast, Bcl-2 immunostaining did not change with age in the subventricular zone and olfactory tubercle, the islands of Calleja of which were markedly enriched with Bcl-2. The dentate gyrus contained only a few layers of intensely labeled granule cells in juvenile monkeys, but the number of these layers increased markedly in adult and aged monkeys. These findings suggest that Bcl-2 can serve as a marker of both proliferating and differentiating neurons and indicate that such immature neurons may be much more widespread than previously thought in postnatal primate brain.