Cell kinetics of hemopoietic colony-forming units in spleen (CFU-S) in young and old mice.

The growth potential of the hemopoietic progenitor cells from young and old mice is similar. Previous studies of their cell-kinetics showed no significant differences between them when measured by the incorporation of tritiated thymidine ([3H]TdR). A different approach for exploring stem-cell kinetics in aged animals is provided by another method; the incorporation of bromodeoxyuridine (BrdUrd) during DNA synthesis followed by exposure to near-ultraviolet light (near-UV) kills BrdUrd labelled cells in DNA-synthesis (S-phase). This BrdUrd-near UV cytocide (BUUV) reveals the size of cycling fractions at the level of hemopoietic progenitor cells; it also demonstrates the velocity of the cells entering S-phase when cells are labeled by a continuous infusion of BrdUrd by an osmotic pump, followed by an appropriate colony-assay for each progenitor cell. We compared the cell kinetics of young and old hemopoietic progenitor cells (CFU-S) by this approach. Osmotic pumps were implanted subcutaneously in the backs of young (2 months) and old (22 months) male C57BL/6CrSlc mice for 2, 4, 8, 12, and 16 days to continuously infuse BrdUrd at a flow-rate of 1 mg/h per kg. Cells were harvested from femoral marrow of the infused mice, plated in non-coated bacterial plates, and exposed to a single dose of near-UV at 4000 J/m2. After BUUV cytocide, bone-marrow cells were assayed for 8- and 13-day CFU-S colonies. In the 8-day colonies, the cytocide fraction of CFU-S from young mice increased rapidly, whereas the fraction from old mice showed flatter curves. Both curves reached a plateau at 52.6% for young, and 43.9% for old mice, and then converged 4 days after labeling. In the 13-day colonies, the curve for the aged was much flatter than that for the young; however, the plateaus in both young and old are similar, but at much lower values than earlier, i.e. 24.5% and 16.0%, respectively. The size of the cycling fraction of progenitor cells was close in the two groups during the steady-state cell-cycle. However, the velocity of the cell cycle at a progenitor level was very different, old mice being much slower. Further, within the progenitors, the cell cycle was much slower in the primitive ones and became faster when the stem cells differentiated into mature progenitor cells.