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The dentate gyrus is one of the regions where neurons are continuously born throughout life.In adult, the mechanism of neurogenesis has attracted much attention over the past years; however, how the generation and differentiation of granule cells in the dentate gyrus are controlled, and what is the role of radial glial scaffold during the ontogenetic development are not very clear.Here we have identified a sheath enveloping the whole dentate gyrus that controls the neurogenesis/cell migration and cell differentiation during postnatal dentate gyrus development.Fate mapping indicates that the sheath consists of CR cells and the fimbrial bundle that derived from the cortical hem.After the sheath was genetically abolished, with the fimbrial bundles were severely disrupted, the extension direction of the supragranular bundles was turned directly to the upper blade of the developing dentate gyrus rather than to the hilus, and this led to the progenitor cells could not be relocated in the tertiary matrix properly, instead directly migrated more fast than that of the wild type to the region where future SGZ will be formed.Cell proliferation and differentiation were severely affected as well and finally a bigger gentate gyrus was observed than that of wild type.In brief, we have identified a novel sheath enveloping the whole developing dentate gyrus controlling postnatal dentate formation.Further studies are needed to elucidate the mechanism underlying the sheath controlling in the postnatal dentate formation.