Adult stem cell hideouts can be identified by the presence of extra long telomeres, according to findings from Ignacio Flores, Maria Blasco (Spanish National Cancer Center, Madrid, Spain), and colleagues.
Because of their location at chromosome tips, telomeres shorten with every cell division. Stem cells divide more slowly than other cell types, so Blasco's group reasoned that they might have the longest telomeres. Using a precise, quantitative version of FISH with telomere sequences, the group found that cells with the longest telomeres corresponded to known stem cell niches in skin, brain, testis, and other tissues.
The approach bypasses the need to identify distinguishing stem cell markers in each tissue type. Until now, the only other generally applicable method was the slower label-retaining technique. The authors will now test other tissues to identify unknown stem cell populations or resolve controversial ones.
Telomeres in the stem cell niches and elsewhere were dramatically shorter in two-year-old mice than in those just a year younger. The group hypothesizes that telomere maintenance mechanisms may go awry in old age. If stem cells with stubby telomeres are unable to function normally, this shortening may be a cause of aging. The group would now like to try to extend lifespan in mice by delaying telomere shortening. 
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