Hippocampal adult neurogenesis occurs in a microenvironment provided by PSA-NCAM-expressing immature neurons.

Neurons continue to be generated in the adult hippocampus. In the present study, the early developmental processes of newly generated neurons in the adult rat hippocampus were examined by confocal laser scanning microscopy using a combination of bromodeoxyuridine (BrdU) labeling and immunohistochemistry for highly polysialylated neural cell adhesion molecule (PSA-NCAM) and NeuroD, which are markers for immature neurons, and glial fibrillary acidic protein (GFAP). Rats were injected with BrdU and 2 hours, 1, 3, and 7 days after the injection, the hippocampus was processed for immunohistochemistry. One day after the injection, BrdU-labeled cells were found frequently in clusters consisting of dividing cells, putative undifferentiated cells, NeuroD-positive differentiated neurons, and GFAP-positive astrocytes. Three days later, BrdU-labeled cells were loosely aggregated and BrdU-positive fragmented nuclei were sometimes observed, suggesting that apoptosis occurred in the clusters. These BrdU-labeled nuclei were frequently associated in various ways with the processes of immature PSA-positive granule cells. They are positioned along PSA-positive apical and basal dendrites or surrounded by these processes. By 7 days after the injection, the number of the clusters was reduced and the BrdU-labeled cells had developed dendrites. These cell-to-cell associations support the hypothesis that the clustering and a microenvironment provided by the PSA-expressing immature neurons contribute to the early developmental events of adult neurogenesis, such as proliferation, differentiation, apoptosis, and neurophilic migration in the adult hippocampus.