(a) Platelet-sized cytoplasmic fragments, shed by 7-day cultured MKs, exhibit the usual platelet cytoplasmic organelles: α-granules (A), open canalicular system (ocs), and microtubule transverse sections (mt) at each extremity of the cell. Note that microtubules also extend along the length of the cell (arrows), which is unusual in circulating platelets. Immunogold labeling for tubulin occasionally shows the presence of the protein at (b) the 2 platelet poles (arrowheads) and (c) more frequently along the platelet length (arrowheads). (d) GPIIb-IIIa has the classic platelet-like distribution on the platelet fragments shed by MK: plasma membrane (arrowheads), open canalicular system (ocs), and α-granules (A).