Effects of increasing concentrations of PEG-rHuMGDF on proplatelet formation (A) and platelet production (B) from CD34⁺ CD41⁺ cells. CD34⁺CD41⁺ cells sorted from marrow were grown in serum-free conditions in the presence of increasing concentrations of PEG-rHuMGDF (0.01 to 500 ng/mL). A murine soluble Mpl receptor was added at day 4 (time of differentiation into proplatelet-bearing MKs, ⧫) or day 6 (time of platelet shedding, •) to cultures stimulated with 1 ng/mL PEG-rHuMGDF. PEG-rHuMGDF (500 ng/mL) was also added at day 4 to cultures initiated with 1 ng/mL of this cytokine (▴). MKs bearing proplatelets and platelets were enumerated at days 6 and 7, respectively, as in Figs 3 and 4. Asterisks denote a significative difference (P < .05) in comparison with culture with 1 ng/mL PEG-rHuMGDF alone.