Ultrastructure of platelet shedding MKs and platelets obtained in the presence of PEG-rHuMGDF from CD34⁺CD41⁺ cells. (a) A mature MK presenting signs of platelet formation. This MK displays dilatation of the demarcation membranes (dm) located at the periphery. They individualize a zone of cytoplasm (arrows) which will form a future proplatelet (N, nucleus). (b) Platelet-sized-fragments (P) exhibit the usual cytoplasmic organelles: α granules (A), smooth connected canalicular system (sccs), and endoplasmic reticulum (er).