Regulation of PLA₂ activity during differentiation of CD34⁺ cells. (A) Freshly purified CD34⁺ cells were incubated for up to 8 days with IL-3, IL-6, and SCF (all at 10 ng/mL). After 18 hours of incubation without growth factors, cells were primed for 10 minutes with either TNF (500 U/mL), IL-3 (10 ng/mL), GM-CSF (10 ng/mL), SCF (100 ng/mL), or diluent (0.01% vol/vol FCS), followed by activation with 1 μmol/L A23187 for 20 minutes, and arachidonate release was measured. Basal release in unstimulated samples was subtracted from the A23187-stimulated values and the data are expressed as a percentage of total cell radioactivity. The data shown are the mean ± 1 SE of 3 to 8 experiments. The statistical significance of the differences between cytokine and diluent-treated cells at each time interval are given (Student’s paired t-test). (B) Morphological analysis of cytospin preparations from the cultures as used in (A).