Telomere shortening accompanies increased cell cycle activity during serial transplantation of hematopoietic stem cells

Reactivation of telomerase and maintenance of telomere length can lead to t he prevention of replicative senescence in some human somatic cells grown i n vitro. To investigate whether telomere shortening might also play a role in the limitation of hematopoietic stem cell (HSC) division capacity in viv o, we analyzed telomere length during serial transplantation of murine HSCs . Southern blot analysis of telomere length in donor bone marrow cells reve aled extensive shortening (similar to7 kb) after just two rounds of HSC tra nsplantation. The number of cycling HSCs increased after transplantation an d remained elevated for at least 4 mo, while the frequency of HSCs in the b one marrow was completely regenerated by 2 mo after transplantation. Direct analysis of telomeres in HSCs by fluorescent in situ hybridization during serial transplantation also revealed a reduction in telomere size. Together , these data show that telomeres shorten during division of HSCs in vivo, a nd are consistent with the hypothesis that telomere shortening may limit th e replicative capacity of HSCs.