Isolated guinea pig megakaryocytes were cultured in the presence of plasma from normal or thrombocytopenic rabbits. Thrombocytopenic but not normal plasma stimulated formation of long cytoplasmic processes and cytoplasmic fragmentation. Activity was found in the 60% to 80% ammonium sulfate fraction of thrombocytopenic plasma but not in the 0% to 60% fraction. The 60% to 80% fraction of normal plasma contained a small amount of activity. Both colchicine and vincristine inhibited the morphogenesis stimulated by thrombocytopenic plasma. Cytochalasin B and D both mimicked the thrombocytopenic plasma-induced morphological change and affected more megakaryocytes than did the thrombocytopenic plasma. Cytochalasin and thrombocytopenic plasma together had a synergistic effect, causing many megakaryocytes to form processes and break into cytoplasmic fragments 3 to 6 microns in diameter. Immunofluorescence staining with antitubulin antiserum showed that cytoplasmic processes formed in the presence of thrombocytopenic plasma contain microtubules and that fragments released by the megakaryocytes contain microtubule rings. A model for the cytoskeletal basis of platelet formation is proposed.

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