Cultured MK grown for 7 days from CD34⁺CD38⁺ progenitors in the presence of Mpl-l, and presenting signs of platelet formation. (a) This mature MK displays alignment and dilatation of some peripheral demarcation membranes, individualizing an outer ring of cytoplasm (arrows). (b) The peripheral sheet of cytoplasm has unfolded from the cell core. Constriction zones are regularly disposed along this cytoplasmic extension (arrows).