Sample of cultured MK grown for 7 days from CD34⁺CD38⁺ progenitors in the presence of Mpl-l, showing some evidence of platelet formation and shedding. (a) A cytoplasmic process extends straight from the cell core. Constriction zones seem to individualize already distinct platelet fields of large size, close to the MK core (arrows). (b) The appearance of this mature MK is consistent with the process of shedding platelets: The nucleus (N) displays condensed heterochromatin, and its shape is indented. Several platelet-sized cytoplasmic fragments have clearly detached from the MK (P). A large cytoplasmic fragment deprived of nuclear material and possibly corresponding to a detached proplatelet (PP) is also shown. (c) Before releasing platelets, a constriction zone (arrowheads) is formed between the mother cell and the forming platelet (P′). A few detached platelets (P) seem to have already detached, as well as numerous microparticles (arrows). (d) Considerable dilatation of the demarcation membrane system (dm) leads to the demarcation of future platelet fields (P).