Actin polymerization in megakaryocytes after SDF-1 addition. Cultured cells were incubated at 37°C for 30 seconds and 2 minutes in the presence of either SDF-1 (300 ng/mL) or thrombin (Stago, 1 U/mL). Intracellular F actin (phalloidin) or G actin (oregon green conjugated DNase 1) was determined by flow cytometry after gating the CD41a⁺ cells. Results of one representative experiment are shown. Two other experiments gave similar results.