Blood platelets are assembled principally at the ends of proplatelet processes produced by differentiated megakaryocytes

Megakaryocytes release mature platelets in a complex process. Platelets are known to be released from intermediate structures, designated proplatelets, which are long, tubelike extensions of the megakaryo-cyte cytoplasm. We have resolved the ultrastructure of the megakaryocyte cytoskeleton at specific stages of proplatelet morphogenesis and correlated these struc-tures with cytoplasmic remodeling events defined by video microscopy. Platelet production begins with the extension of large pseudopodia that use unique cortical bundles of microtubules to elongate and form thin pro-platelet processes with bulbous ends; these contain a peripheral bundle of microtubules that loops upon it-self and forms a teardrop-shaped structure. Contrary to prior observations and assumptions, time-lapse micros