Revising concepts about adult stem cells

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Science  09 Feb 2018:
Vol. 359, Issue 6376, pp. 639-640
DOI: 10.1126/science.aar7732

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The term “stem cells” is one of the most disputed terms in science. The general definition that stem cells are at the origin of a lineage, self-renewing and multipotent—generating all cell types of a given tissue or even organism (if totipotent)—is agreed upon. But how many cell divisions are required to be called a stem cell? Among the adult stem cells, hematopoietic stem cells are the champions because they can repopulate the immune systems of five generations of mice, living beyond the life of the organism from which they originated. But what about neural stem cells (NSCs), the founder cells of all cells in the central nervous system? Are a few cell cycles of self-renewal, such as nine in embryonic neurogenesis of the neocortex (1), sufficient to call them stem cells? And how many different cell types need to be generated by a stem cell? Do cells making only neurons, as occurs in embryonic neurogenesis, qualify as stem cells (2)? However, when cultured in vitro, they generate neurons and several other glial cell types, which qualifies them as multipotent (2). This is also the case for adult NSCs, but their behavior in vivo is even less well understood. On page 658 of this issue, Pilz et al. (3) track, for the first time, adult NSCs live in the mammalian brain, gaining exciting new insights that prompt revision of how we define stem cells.