Origin of the neocortical subependymal cells speculated by Emx1 and GAD67 expression.

More than a decade has passed since adult neurogenesis in the murine telencephalon was described and accepted widely. The adult neurogenesis has been attributed to the progenitors in the subgranular layer in the dentate gyrus and the subependymal layer of the telencephalon. However, the developmental origin of the progenitors in the dentate gyrus and the subependymal layer is poorly understood. It is well known that the neurogenesis in the embryo brains occurs in the ventricular zone (VZ). Recently it has been proposed that the progenitors in the VZ of the telencephalon turn into the subventricular zone (SVZ) astrocytes (Tramontin et al., 2003). The subependymal layer of the neocortex has been regarded as a source of the granule cells and periglomerular cells in the adult olfactory bulb. The subependymal layer produces neurons that migrate to the olfactory bulb as the rostral migratory stream, and the newly produced neurons are going to be GABAergic inhibitory neurons in the olfactory bulb. However, we felt discontinuity in the idea that the neocortical VZ cells turn into SVZ astrocytes and then produce GABAergic neurons. By now, it is well known that most of the neocortical GABAergic neurons were supplied by tangential cell migration originating in the subpallial structures (Anderson et al., 1997; Tamamaki et al., 1997). It is also speculated that the neocortical VZ almost lack the ability to produce GABAergic neurons. Under these circumstances, we speculated that the GABAergic neurons for the olfactory bulb may be produced by progenitors derived from other than the neocortex. In this paper we will show that the subependymal cells in the adult neocortex are heterogeneous and may have different origin.