Newly born cells in the ageing dentate gyrus display normal migration, survival and neuronal fate choice but endure retarded early maturation.

Addition of new granule cells to the dentate gyrus (DG) from stem or progenitor cells declines considerably during ageing. However, potential age-related alterations in migration, enduring survival and neuronal fate choice of newly born cells, and rate of maturation and dendritic growth of newly differentiated neurons are mostly unknown. We addressed these issues by analysing cells that are positive for 5'-bromodeoxyuridine (BrdU), doublecortin (DCX), BrdU and DCX, and BrdU and neuron-specific nuclear antigen (NeuN) in the DG of young adult, middle-aged and aged F344 rats treated with daily injections of BrdU for 12 consecutive days. Analyses performed at 24 h, 10 days and 5 months after BrdU injections reveal that the extent of new cell production decreases dramatically by middle age but exhibits no change thereafter. Interestingly, fractions of newly formed cells that exhibit appropriate migration and prolonged survival, and fractions of newly born cells that differentiate into neurons, remain stable during ageing. However, in newly formed neurons of the middle-aged and aged DG, the expression of mature neuronal marker NeuN is delayed and early dendritic growth is retarded. Thus, the presence of far fewer new granule cells in the aged DG is not due to alterations in the long term survival and phenotypic differentiation of newly generated cells but solely owing to diminished production of new cells. The results also underscore that the capability of the DG milieu to support neuronal fate choice, migration and enduring survival of newly born cells remains stable even during senescence but its ability to promote rapid neuronal maturation and dendritic growth is diminished as early as middle age.