Stem Cells and Pattern Formation in the Nervous System The Possible versus the Actual

sometimes seems as if NSC biology has proceeded apace and apart, if not willfully ignorant, at least passively neglecting to integrate these discoveries into its perspective on neural development. And to some extent, the converse is also true. In part, the increasing divergence between the fields of neural pattern formation— neuroembryology, for short—and NSC biology reflects their focus on different questions. Neurembryology has Introduction been primarily concerned with understanding the con-Neural stem cells (NSCs) are broadly defined as multi-trol of cell fate: what do progenitor cells do, and how potent, self-renewing progenitor cells. In the central ner-do they do it? NSC biologists, however, are concerned vous system (CNS), these cells generate neurons, astro-not only with cell fate determination but also with devel-cytes, and oligodendrocytes. In the peripheral nervous opmental potential: what can stem and progenitor cells system (PNS), they generate neurons, Schwann cells, do, given the opportunity? This latter issue is crucial in and other neural crest derivatives such as smooth mus-applying NSC biology to the treatment of disease. A cle cells. Over the past few years, remarkable progress central dichotomy facing NSC biologists is, therefore, has been made in identifying and understanding the to understand the difference between fate and potential: properties of these elusive cells. Yet recently there has in a word, the actual versus the possible. been a quantal increase in the attention afforded stem In some respects, the ways that NSC biologists and cells, in both the scientific and lay communities. What neuroembryologists think about neural development seem accounts for this heightened level of interest? What do to have grown so divergent that one begins to wonder we really know about these cells, and what remains to whether they have anything to contribute to each other be learned? anymore. To state it in extreme terms, is NSC biology Much of the excitement about NSCs reflects their becoming a field of applied neuroscience, one in which promise for treating neurological illnesses such as Par-it does not matter whether the behavior of isolated stem kinson Disease (Bjorklund and Lindvall, 2000). There cells is an accurate reflection of their normal properties has also been a rediscovery of Joseph Altman's original in vivo, so long as they are therapeutically useful? And finding that neurogenesis persists into adulthood in se-have issues of cell lineage and commitment become lected regions of the brain (Gage, 2000). The confirma-conceptually irrelevant to understanding pattern formation …