Neurogenesis is a constitutive activity in the adult dentate gyrus whereby new cells are created in the subgranular zone, before becoming neurons in the dentate gyrus granule cell layer. New granule cells are thought to migrate from the subgranular zone outwards to the edge of the cell layer as they mature. In these experiments we examined the dendritic morphology of granule cells in the subgranular zone, and the inner and outer regions of the granule cell zone in Sprague-Dawley rats with low and high rates of neurogenesis. In animals with lower rates of neurogenesis, the number of primary dendrites, degree of dendritic complexity and total dendritic length was lowest in cells located in the subgranular zone, higher in inner granule cell zone neurons, and highest in outer granule cell zone granule cells. Subgranular zone granule cells typically extended one primary dendrite and had a simple, immature dendritic tree, while granule cells in the outer granule cell zone had an increased number of primary dendrites, greater dendritic complexity, and greater total dendritic length. Animals that engaged in voluntary exercise showed increased neurogenesis, and the proportion of cells with one or two primary dendrites was increased in all of the granule cell zones. Despite having fewer primary processes, these cells showed enhanced dendritic complexity and an overall increase in their total dendritic length. These results indicate that granule cell dendritic morphology may be indicative of the age and position of a cell in the granule cell layer, but that in animals with increased rates of neurogenesis, the proportion of cells exhibiting what is considered an immature phenotype is increased throughout the all regions of the dentate gyrus cell layer.
