Are SGCs immature or ectopic granule cells?
Because granule cells develop postnatally (Schlessinger et al., 1975), it is possible that the SGCs we find represent immature GCs. However, several lines of evidence suggest this is unlikely. First, granule cell neurogenesis occurs in the subgranular zone in the hilus, and newly born GCs incorporate into the GCL at the hilar border (Schlessinger et al., 1975). Second, intracellular recordings show that immature GCs have similar electrophysiological properties to mature GCs, except for a higher input resistance (Schmidt-Hieber et al., 2004). The SGCs we report have a lower input resistance than control GCs in the GCL and a constellation of different intrinsic properties. Immature GCs also have less extensive dendritic arborization than mature cells, and often have dendrites that fail to reach the fissure (Schmidt-Hieber et al., 2004; Shapiro and Ribak, 2005). In contrast, dendrites of SGCs have more extensive lateral spread than GC dendrites and extend to the fissure. As several studie
