Differential impact of aging on ELPs and PDCs in the BM. (A) BM cells from 3-month-old and 12-month-old C57BL/6 mice were stained with the indicated antibodies before collection of 500 000 events on an LSRII flow cytometer. Lineage (Lin) cocktail included antibodies to B220, CD3ϵ, TER-119, Gr-1, and CD11b. Initial gating of Lin^–IL-7Rα^– and Lin^–IL-7Rα⁺ was as previously described.²⁰  (B) BM cells from the mice in panel A were also stained with the indicated antibodies for resolution of BM PDCs as shown. Nonviable cells were excluded from analyses as described in Figure 1. (C) Absolute numbers of cells within the indicated population were calculated by multiplying the frequency of cells within the gates indicated in panels A and B times the total number of viable BM cells harvested from each mouse. Data are the mean and the SEM with 3 mice per group. The age-related loss of EBPs/CLPs and Flt3 ⁺ LSKs was also judged to be statistically significant as determined by unpaired Student t test (P < .01).