Fig. 2. Maintenance of neural stem cells and promotion of astrocyte formation by Hes genes. (A) Regulation of cell differentiation by bHLH genes. Neuroepithelial cells initially form the neural plate. These cells gradually develop into radial glial cells, which have a cell body in the ventricular zone (VZ) and a radial fiber reaching the pial surface. Radial glial cells give rise to neurons by asymmetric cell divisions. After the production of distinct types of neurons, radial glial cells finally differentiate into astrocytes. Hes genes maintain neuroepithelial cells and radial glial cells during early development, and promote astrocyte formation during late development. Proneural genes such as Mash1, Ngn2 and Math1 promote neurogenesis. Unlike other Hes genes, Hes6 also promotes neurogenesis. The broken line indicates the border of the VZ. (B) Transfection experiments in mouse embryonic brains. Many of the cells transfected with a control vector that only drives enhanced green fluorescent protein (EGFP) expression in radial glial cells have migrated out of the VZ (at the bottom of the image) and differentiated into neurons, whereas cells transfected with a vector that directs the co-expression of Hes1 and EGFP, remain in the VZ and display radial glial cell morphology [from Ohtsuka et al. (Ohtsuka et al., 2001)]. Scale bars: 100 µm.