Impaired ability of stromal cells from aged mice to support the proliferation of IL-7–dependent pre-B–cell lines. The IL-7–responsive pre-B–cell lines were used to compare the ability of stromal cells to support IL-7–mediated proliferation. Ten thousand pre-B cells were cocultured with 10,000 FACS-purified stromal cells (SC) from 1-month-old mice (▪) or 24-month-old mice () for 4 days. The pre-B cells were then harvested and enumerated. (A) The results from one of four or five experiments with each cell line. (B) The relative ability of aged SC to support IL-7–mediated proliferation as compared with young SC. The amount of proliferation supported by the young SC for each experiment was set as 1.00. The results shown are the average ± SD from four or five experiments with each pre-B–cell line. *P < .02; **P < .05.