Neural Stem Cells Are Blasting Off

ability to produce differentiated cell types and additional stem cells. Cells capable of proliferation and neurogen-*Division of Developmental Biology National Institute for Medical Research esis in such a paradigm, but which fail to self-renew, are best termed progenitor cells rather than stem cells. The Ridgeway, Mill Hill London NW7 1AA Detailed analyses demonstrating self-renewal have been accomplished only within a few neural structures: United Kingdom † Ontogeny, Inc. the neural crest, the embryonic cerebral cortex, and the embryonic striatum. 45 Moulton Street Cambridge, Massachusetts 02138 Peripheral Neural Stem Cells Clonal primary cultures of rat neural crest have revealed that single cells that coexpress nestin and low affinity Introduction nerve growth factor receptor can produce large colonies Over the past 5 years, there has been a flurry of excite-containing neurons, Schwann cells, and smooth muscle ment over the identification of putative neural stem cells. Why all the fuss? Stem cells, as the name implies, are Subcloning experiments showed that these cells self-cells from which other cell types arise and thus are of renew, and hence these cells were designated neural great interest to developmental biologists. The most crest stem cells (NCSCs). The fact that these cells can general type of stem cell is the fertilized egg; from this be grown in clonal culture has allowed the direct assess-single cell arise the myriad cell types that constitute a ment of the activity of purified growth factors. In recent mature multicellular animal. Operationally, however, the experiments, NCSCs were found to differentiate in re-appellation is given to cells with far less developmental sponse to several different growth factors along differ-potential; in a given tissue, one can speak of stem cells ent pathways (Shah et al., 1994; 1996). In response to that have the ability to give rise to all of the differentiated neuregulin, nearly 80% of NCSCs respond to form only cell types associated with that specific tissue. Several Schwann cells. By contrast, neurogenesis is induced properties of these cells provide a useful operational by bone morphogenic proteins 2 and 4 (BMP2/4), and definition. Specifically, stem cells are undifferentiated smooth muscle cells are generated in response to trans-cells capable of proliferation, self-renewal, and " asym-forming growth factor ␤1. It appears that these growth metric " division. The latter allows for self-renewal (i.e. factors act directly to convert NCSC into differentiated the production of new stem cells) as well as the produc-progeny. This is most …