The mechanism of platelet release has been studied in mouse bone marrow, using scanning and transmission electron microscopy. Cytoplasmic processes extended into sinusoids by megakaryocytes were found to undergo attenuation, and at the distal end develop constrictions between segments of platelet size. Rupture at sites of constriction is thought to release platelets. Microtubules were present in the processes, orientated longitudinally, and became concentrated in the constrictions. The narrowest constrictions observed were about 0.2 micrometer in diameter and several microns in length. The structural similarity of the narrowest constrictions with those found in bridges separating midbody and cell in the terminal phase of cytokinesis suggests that platelets and cells share a common mechanism for release. The presence of a centriole in each cytoplasmic process was revealed by serial sectioning. It is suggested that the microtubules in each cytoplasmic process originate from an organizing center associated with the centriole, and that the number of cytoplasmic processes that emerge from a megakaryocyte is governed by its ploidy.

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