High magnification of the constriction zones observed on mature MKs (grown for 7 days from CD34⁺CD38⁺ progenitors in the presence of Mpl-l). (a) Some elongated parallel microtubules (mt) are present along the length of the cytoplasmic extension separating the platelet buds. (b) Immunogold labeling for tubulin confirms its presence in the length of the elongated arm separating platelet territories (arrowheads). (c) On each side of the constriction area, transverse microtubule fragments (mt) are observed near the longitudinal microtubules (arrows). (d) Immunogold labeling for tubulin confirms its concentration on each side of the constriction zone (arrowheads). (e, f, and g) A vacuole of increasing size (v) is formed at the constriction zone, which may lead to detachment of the platelet fragment.