QUIESCENCE, CYCLING, AND TURNOVER IN THE PRIMITIVE HEMATOPOIETIC STEM-CELL COMPARTMENT

Continuous oral bromodeoxyuridine (BrdU) administration was used for t he non-invasive measurement of the in vivo cell cycling characteristic s of hematopoietic stem and progenitor cell populations of increasing maturity, isolated on the basis of their relative levels of Rhodamine 123 (Rh) and Hoechst 33342 (Ho) fluorescence. The results showed that whereas primitive hematopoietic stem cells (PHSCs) are hierarchically ordered on the basis of quiescence, the most primitive of these, chara cterized by their Rh/Ho-dull phenotype and their capacity for long-ter m hematopoietic reconstitution, are not dormant, but cycle slowly in n ormal steady-state bone marrow (BM). Cell cycle analysis showed that 3 0 +/- 7% of Rh/Ho-dull PHSCs had cycled and incorporated BrdU followin g continuous administration over 1 week, whereas 60 +/- 14% and 89 +/- 3% of these cells were BrdU positive at 4 and 12 weeks, respectively. Linear regression analysis of these data showed that Rh/Ho-dull PHSCs cycle with an average turnover time of 4.3 weeks (30 days), and a t(1 /2) (time to 50% cycled) of 2.75 weeks (19 days).