Two illustrations of MK progenitors characterized by immunogold staining with polyclonal antibody reacting with murine GPIIb-IIIa on ultrathin frozen sections of MIIb-tk bone marrow 4 days after discontinuing GCV. The size of these MK progenitors is between 10 to 15 μm in diameter. (a) Example of an immature MK having a high nucleus/cytoplasm ratio and a thin cytoplasmic rim. Some staining for GPIIb-IIIa is visible on the plasma membrane (arrowheads) and on the membrane of the few -granules present (arrows). (b) In this immature MK characterized by the presence of GPIIb-IIIa on the plasma membrane (arrowheads), the gold particles are also associated with internal membranes possibly representing a developing demarcation membrane system (arrow). Bars = 0.5 μm.