Stem cells in the developing and adult nervous system.

encompassing those primary neural precursor cells For those of us examining aspects of neural develthat are capable of producing differentiated progeny opment, it is both daunting and inspirational to take and of self-renewing to generate more stem cells. down a copy of Ramon y Cajal’s Histology of the At this stage, it is perhaps premature, and in fact Nervous System (Ramon y Cajal, 1911) and browse may be unrealistic and misleading, to provide a through the illustrations. Here is the most complex more concrete definition. The field is new enough biological tissue with numerous cells of diverse to be still at the stage of identifying the basic charactypes exquisitely arranged. How can we possibly teristics of cells that fall into our general concept. understand how this tissue forms, and how can we In this regard, significant progress has been made not try? in understanding neural stem cell specification, the It has become increasingly clear that solving the sites where they reside, their progeny, the factors puzzle of how cells in the central and peripheral that control their proliferation/differentiation in nervous systems (CNS and PNS) are generated revitro, and their fate after transplantation into the quires an understanding of the properties of the earembryonic or adult brain. It seems useful to pause at liest progenitors in the lineages, the neural stem this point and bring together a collection of reviews cells. A single definition of neural stem cells is not under the theme of neural stem cells. Our aim is to yet universally acknowledged—to some researchprovide an overview of our current understanding ers, these cells have defined properties that may of these cells from their initial specification in the include extensive self-renewal, asymmetric cell diembryo to their role in the adult, bringing into focus vision, the ability to regenerate adult tissues, multiissues and questions that could guide future studies. potency, slow division time, or a combination of