Granule cell proliferation and axon terminal degradation in the dentate gyrus of gerbils (Meriones unguiculatus) during maturation, adulthood and aging

The objective of the present study was to examine both naturally occurring degrading events in axon terminals of the dentate gyrus and granule cell pr oliferation in the dentate gyrus of gerbils (Meriones unguiculatus) through out postnatal life. For that purpose, (1) a selective silver staining techn ique was applied to analyze neuronal lysosome accumulation (LA), indicating synaptic degradation during development. LA was quantified by counting sil ver grains in the inner third and outer two thirds of the molecular layer, granular layer, subgranular layer and the hilus of the dentate gyrus. (2) P roliferation of granule cells was identified by in-vivo labeling with 5-bro mo-2'-desoxyuridine (BrdU). BrdU-labeled granule cell nuclei were identifie d in consecutive horizontal slices along the mid-septotemporal axis of the hippocampus and light-microscopically quantified 4h after the BrdU-labeling . It was found (1) that in young animals LA significantly increased within all layers and reached adult levels after about 3 months. During subsequent development LA kept on this level throughout aging with highest values wit hin the inner molecular layer. (2) There was a highly significant temporal gradient in granule cell proliferation with numbers of BrdU-labeled cells e xponentially declining during juvenile life. Nevertheless, granule cell pro liferation occurred throughout adult life and aging. The present results ar e discussed (1) with concepts of ongoing neuroplasticity and remodeling of neuronal networks in the developing and adult brain, and (2) with regard to pharmacologically induced neuromorphogenesis.