The marginal zone/layer I as a novel niche for neurogenesis and gliogenesis in developing cerebral cortex

The cellular diversity of the cerebral cortex is thought to arise from progenitors located in the ventricular zone and subventricular zone in the telencephalon. Here we describe a novel source of progenitors located outside these two major germinative zones of the mouse cerebral cortex that contributes to neurogenesis and gliogenesis. Proliferating cells first appear in the preplate of the embryonic cerebral cortex and further increase in the marginal zone during mid and late neurogenesis. The embryonic marginal zone progenitors differ in their molecular characteristics as well as the size and identity of their clonal progeny from progenitors isolated from the ventricular zone and subventricular zone. Time-lapse video microscopy and clonal analysis in vitro revealed that the marginal zone progenitor pool contains a large fraction of oligodendrocyte or astrocyte progenitors, as well as neuronal and bipotent progenitors. Thus, marginal zone progenitors are heterogenous in regard to their fate specification, as well as in regard to their region of origin (pallial and subpallial) as revealed by in vivo fate mapping. The local environment in the marginal zone tightly regulates the size of this novel progenitor pool, because both basement membrane defects in laminin gamma1-/- mice or alterations in the cellular composition of the marginal zone in Pax6 Small Eye mutant mice lead to an increase in the marginal zone progenitor pool. In conclusion, we have identified a novel source of neuronal and glial progenitors in the marginal zone of the developing cerebral cortex with properties notably distinct from those of ventricular zone and subventricular zone progenitors.