Mouse strain-dependent changes in frequency and proliferation of hematopoietic stem cells during aging: correlation between lifespan and cycling activity.

We have quantified the frequency and proliferation of five subsets of primitive hematopoietic cells, using the cobblestone area forming cell (CAFC) assay, in marrow of five strains of mice with lifespans ranging from about 500 to 800 days. Stem cell characteristics were determined in young (6 weeks) and old (12 months) mice. We report striking effects of both intrinsic strain lifespan and individual mouse age on stem cell populations. First, the relative and absolute numbers of the most primitive stem cell subsets was threefold to fourfold higher in old than in young mice. Second, a considerable strain-to-strain variation in the number of primitive cells was observed: when absolute frequencies were calculated, there was a trend for longer lifespan to be correlated with a larger stem cell pool. Third, stem cells from old mice had a far lower cycling activity than cells from young mice. However, this was highly strain dependent: short-lived C3H/He and CBA/J mice showed a stronger reduction in cycling activity during aging than long-lived C57BL/6 mice. Finally, a significant negative correlation was demonstrated in young mice between maximal lifespan and proliferative activity. These data show that aging has a major impact on the frequency and cell-cycle kinetics of primitive hematopoietic cell compartments. In addition, the observation that cycling activity of stem cells is related to the maximal lifespan of the mouse strain may open ways to identify the genetic mechanisms of both strain- and age-dependent variation in the structure of primitive hematopoietic cell compartments.