Early spinogenesis in postnatally-born hippocampal granule cells.

Introduction: The development of early postnatally-born hippocampal granule cells (pbGCs) can be studied in organotypic entorhino-hippocampal slice cultures (OTCs) in which local dentate gyrus (DG) and entorhinal projections are established within the first 10 days Methods: In this study, we analyzed the development of small dendritic protrusions (SDPs) on complete reconstructions of dendritic trees from retrovirally (RV) labeled pbGC born at postnatal day 4-5 during the phase of early spinogenesis in OTCs. Results: We found SDPs with head (spines) and without head (other protrusions) as early as 12 days post RV injection (dpi) in all parts of the growing dendritic tree of pbGCs. At 14 - 20 dpi, while dendritic reorganization was still ongoing, SDP density as well as spine tip size increased moderately. At 16 dpi, spines with larger, bulbous heads appeared. Interestingly, the percentage of spines stayed constant at 60% of all SDPs over time. While spines were evenly distributed in all parts of the dendritic tree with no preference for any layer, other SDPs were more abundant at proximal parts of the dendritic tree within the granule cell layer (GCL). Discussion: Our findings support the idea that SDPs appear early in dendritic development and are associated with dendritic growth rather than the establishment of specific synaptic connectivity. The uniform distribution of spines across the dendritic tree during the phase of dendritic maturation and reorganization suggests that early spinogenesis may be guided in part by intrinsic cellular programs, while not excluding a contribution of layer-specific cues.