Expression of spα2N1 inhibits proplatelet elaboration by MKs and prevents IMS maturation. (A) The introduction of spα2N1-GFP in MKs through retroviral infection inhibited proplatelet formation. Most MKs expressing spα2N1-GFP failed to make proplatelets (A top panel), although a few developed primitive proplatelets (A bottom panel). Left panels show phase contrast images and right panels show fluorescence images. (B) Control uninfected MKs, identified by lack of green fluorescence, form normal proplatelets. (C) Fluorescence images of proplatelet formation by control MKs expressing GFP alone. Scale bars indicate 7.5 μm. (D) Quantitative analysis of proplatelet formation in spα2N1-GFP–expressing and control, GFP-expressing MKs. spα2N1-GFP–expressing MKs show a dramatic reduction in the percentage of proplatelets formed (8% compared with 44% in control cells). Bars represent the standard deviations. (E-G) Representative electron micrographs of a noninfected MK (E), a MK expressing GFP alone (F), and a MK expressing spα2N1-GFP (G). Control MKs (E-F) show an extensive, open IMS that fills the cell cytoplasm, whereas spα2N1-GFP–expressing MKs (G) do not. Insets show low-magnification views of the corresponding cells. Scale bars indicate 4μm.