The frequency (and the range defined by ±SEM) of growth factor-responsive cells (cells able to make clones of viable progeny) and pre-CFC in the lin⁻CD34⁻CD38⁻ was determined by LDA. For each marrow tested, 3 or 4 different numbers of lin⁻CD34⁻CD38⁻ cells were stimulated with 5 growth factors for 10 days, and individual wells were scored for the presence of viable cells that were then harvested and assayed in methylcellulose cultures for the presence or absence of CFC. The overall mean values were calculated as geometric means, with the range shown as ±SEM for the intersample variation.

LTC-IC frequencies of lin⁻CD34⁺CD38⁻ cells from the same samples were calculated by dividing bulk assay CFC outputs by 18, which is the mean 6-week CFC output per LTC-IC value previously established for adult marrow LTC-IC assayed under the conditions used here.22 The mean value is the geometric mean shown with the range defined by ±SEM.