The neural progenitor cells isolated from the hippocampus of adult female Fischer 344 rats used in this work have been characterized previously (Gage et al., 1995 (link); Palmer et al., 1997 (link)). The whole brain–derived neural stem cells from P10 ICR mice were isolated and cultured according to the methods as described, with slight modifications. Cells were cultured as described previously (Ray et al., 1993 (link); Gage et al., 1995 (link)). Cells between passages 10 and 20 were used for in vitro differentiation analyses. To induce differentiation, cells were plated into 4-well chamber slides at a density of 55,000–75,000 cells/well and were allowed to proliferate in insulin-containing N2-supplemented (Invitrogen) DME:Ham's F12 (Omega Scientific) medium containing 20 ng/ml FGF-2 (PeproTech, Inc.) for 24 h. FGF-2 was then withdrawn and cells were subsequently treated with differentiation media. Differentiation conditions were either N2 medium with 1 μM RA (Sigma-Aldrich) and 1% FBS (Omega Scientific) for 4 d (mixed); 1 μM RA and 5 μM forskolin (Sigma-Aldrich) for 4 d (neuronal), or 50 ng/ml leukemia inhibitory factor (CHEMICON International, Inc.) and 50 ng/ml BMP2 (R&D Systems) for 6 d (astrocytic; Nakashima et al., 1999 (link)). For IGF induction experiments, cells were trypsinized, washed with 1× PBS, and plated into insulin-free N2 medium. Either 500 ng/ml human recombinant IGF-I or IGF-II (R&D Systems) or 500 ng/ml insulin (Sigma-Aldrich) was added for 4 d, except when indicated otherwise. In some cultures, 2.5 μM BrdU (Sigma-Aldrich) was added to label dividing cells, and 2 μM Q-VD-OPh (Enzyme Systems Products) was added to prevent apoptosis. Recombinant mouse Noggin (R&D Systems) was added in some cultures to a final concentration of 500 ng/ml.