Myelopoietic defects are present in old mice. (A) The frequency of CMPs and GMPs is increased in the BM of old (78- to 105-week-old) compared with young (5-week-old) mice both at steady state and at day 8 after 5-FU treatment. Steady-state frequencies are presented as the mean plus or minus SEM of 16 young and 12 old mice analyzed in 4 independent experiments. 5-FU frequencies are presented from the pooled BM of 9 young and 7 old mice analyzed in 2 independent experiments. (B) The number of Annexin V⁺ CMPs and GMPs was increased in the BM of old compared with young mice. Cell numbers are presented as the mean plus or minus SEM of 12 young and 8 old mice analyzed in 3 independent experiments and normalized per 5 × 10⁴ CMPs or GMPs, respectively. Myeloid colonies derived from whole BM cells (C) and sorted CMPs (D) isolated from young mice contained more cells than those derived from BM and CMPs isolated from old mice. Whole BM cells (5 × 10⁴ per dish) and CMPs (250 per dish) were plated. Cell numbers shown in C are presented as the mean plus or minus SEM of 24 colonies derived from young BM cells and 24 colonies derived from old BM cells picked in 2 independent experiments. Cell numbers shown in D are presented as the mean of 12 colonies derived from young CMPs and 12 colonies derived from old CMPs plus or minus SEM picked in 1 of 2 representative experiments. BM cells (1.5 × 10⁵) (E) and CMPs (2 × 10³) (F) plated in liquid culture supplemented with myelopoietic cytokines isolated from young mice produced more Gr-1⁺CD11b⁺ myeloid cells compared with their old counterparts. Numbers are presented as the mean plus or minus SEM of 3 to 6 wells analyzed in 1 of 2 representative experiments.