Dendritic development of dentate granule cells in the absence of their specific extrinsic afferents

Dendrites and spines are postsynaptic structures that develop in association with presynaptic fibers. Recent studies have shown that granule cells of the fascia dentata survive in slice cultures and differentiate in a manner known from in situ studies. However, all extrinsic afferent fibers are absent under culture conditions. In the present study, we study whether dendrites and spines of granule cells in slice cultures differentiate normally, although they are not contacted by their normal layer-specific afferents. Slices of hippocampus were prepared from rat pups at the day of birth. After 5, 10, 15, and 20 days of incubation, granule cells in these cultures were Golgi impregnated. For comparison, perfusion-fixed hippocampal sections of 5-, 10-, 15-, and 20-day-old rats were impregnated the same way. Our results show that the total density of spines on granule cell dendrites in culture increased as in perfusion-fixed animals. However, after 20 days of incubation, the absolute number of dendritic spines on cultured neurons was reduced because of a reduction of peripheral dendrites. This reduction was accompanied by an increase in the number of stem dendrites originating from the perikaryon. The density of spines on these proximal dendrites was larger in cultured granule cells than in controls. Our results suggest that the lack of major extrinsic (entorhinal) afferents that normally terminate on peripheral granule cell dendrites causes retraction of these dendrites. At the same time, there is growth of proximal dendritic portions. Proximal dendrites are targets of associational fibers, which are known to sprout under these culture conditions.