Stem and precursor cells in the nervous system.

The early-formed neural tube consists of proliferating, morphologically homogeneous cells, termed "neuroepithelial (NEP) stem cells" which generate neurons, astrocytes, and oligodenrocytes through a series of intermediate precursor cells. In addition to NEP cells, a second class of stem cells-the neurosphere-forming cell-can be isolated at later stages of development. NEP cells can differentiate into neural crest stem cells, which in turn generate PNS derivatives. NEP cells and neurosphereforming stem cells and more restricted precursors express a characteristic spectrum of markers that can be used to characterize them. Each of these cell types can be isolated from embryonic stem (ES) cell cultures, and their behavior appears similar to cells isolated at later developmental ages. The relative advantages and disadvantages of these cells for cell replacement therapy are discussed.