Comparison of the relative amount of IL-7 within the cytoplasm of stromal cells from young and old mice. The relative amount of IL-7 within stromal cells was determined by cytoplasmic flow cytometry as described in Fig 7 and in Materials and Methods. The histograms compare the levels of cytoplasmic staining for normal Ig and anti–IL-7 antibody in stromal cells from (A) 1-month-old mice and (B) 24-month-old mice. (C) Net specific IL-7 staining was calculated as the difference in MFIs between staining with anti–IL-7 and control Ig antibodies. The average ± SD was calculated from four separate experiments using stromal cells from two independent culture initiations is shown. In each experiment, two to four pooled LTBMC-Bs were used as the source of stromal cells.