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Stem cells have the ability to self-renew and to differentiate into a variety of different cell types. However, it is not clear what determines the path taken by any particular stem cell. Discussing recent work with stem cells from the fruit fly testis (Yamashita et al.), Wallenfang and Matunis explain in their Perspective that, at least in the case of these stem cells, the trick is the asymmetric arrangement of the mitotic spindle during cell division. This asymmetric arrangement ensures that as the stem cell divides, one daughter cell remains in the environmental niche of the testis and continues to self-renew, whereas the other daughter cell is edged out of the niche and begins to differentiate.
R. M. Wallenfang is in the Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia, PA 19104, USA. E. Matunis is in the Department of Cell Biology, The Johns Hopkins Medical Institutions, Baltimore, MD 21210, USA. E-mail: matunis{at}jhmi.edu
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