Kinetics of MK growth (A), proplatelet formation (B), and platelet production (C) from marrow or blood CD34⁺ cells. CD34⁺ cells from marrow (◊) and blood CD34⁺ cells (□) were grown in serum-free conditions in the presence of PEG-rHuMGDF (10 ng/mL). MKs were enumerated as CD41⁺ cells by flow cytometry at different days of culture. In this and all subsequent figures, the number of MKs bearing proplatelets was determined using an inverted microscope and hemocytometer as MKs showing one or more cytoplasmic expansions with constriction areas. In this and all subsequent figures, culture platelets were enumerated by flow cytometry as CD41⁺events with the same scatter properties as blood platelets shown in Fig1. For the kinetics of MK growth and proplatelet formation, results are the average of three experiments. For determination of platelet production, the entire kinetics was only performed in one experiment from a blood CD34⁺ cell-derived culture.