The nervous system—which is comprised of both the central nervous system (CNS), including the brain, and a network of extensions called the peripheral nervous system (PNS)—is comprised of several distinct classes of cells including neurons and glial cells; oligodendrocytes, astrocytes, and microglia. It is believed that common neural stem cells (NSCs) and common neural progenitor cells (NPCs) are the predecessors of all neural cell types except the microglia. The morphological differences arise, as they do in other tissues, based on location of the cells and a program of distinct genetic upregulation. Specifically, the release of sonic hedgehog (Shh), retinoic acid (RA), and fibroblast growth factors (FGFs) from surrounding tissues are critical for neural development and differentiation. The key cell types distinctive to the neural system ...