Definitions and criteria for stem cells.

The working definition of a stem cell includes self-renewal and the ability to differentiate into several cell types. There are also aspects of clonality and potency. Stem cells can be derived from early embryos after the formation of the blastocyst or from fetal, postnatal, or adult sources. Neural stem cells (NSCs) arise from embryonic ectoderm that forms neuroepithelial cells. The neuroepithelial cells generate radial glia that produce fetal and adult NSCs within the central nervous system (CNS). Adult NSC and restricted progenitors are found in the several regions of the CNS throughout life. Human embryonic stem cells, with their ability for self-renewal, clonal capacity, normal karyotype, and potential to form NSCs, easily may be the best source of NSCs and progenitors for treating disease. However, the complexity of NSCs, neural patterning, and the formation of multiple populations of neurons, astrocytes, and oligodendrocytes warrant the need for intense studies to characterize these cells and to define the microenvironment that will be needed to support them in the diseased CNS. Ways to produce well-defined populations, avoid oncogenicity, and ensure survival need to be clarified before clinical application can begin.