AN ELECTRON MICROSCOPE STUDY OF RAT MEGACARYOCYTE .2. SOME ASPECTS OF PLATELET RELEASE AND MICROTUBULES

Rat megacaryocytes in the thrombocytogenic stage were studied in fetal liver and adult bone marrow. Megacaryocytes are spherical or ovoid cells during differentiation. Prior to the liberation of platelets in the thrombocytogenic stage, the cells become highly irregular in outline owing to the formation of cytoplasmic extensions protruding between neighbouring parenchymal cells or into sinusoids between sinusoidal lining cells. Pieces of cytoplasm of varying sizes are released from the cytoplasmic extensions; the pieces subsequently undergo further fragmentation into platelets. This process may take place either in the sinusoids or in the parenchyma. Pieces of cytoplasm released in the parenchyma pass into sinusoids, where they fragment further; single platelets, liberated in the parenchyma, also pass between sinusoidal lining cells into the circulation. Liberation of platelets from spherical megacaryocytes was never observed. Microtubules are present in abundance in megacaryocytes. In the spherical cells they are arranged at random; in the cytoplasmic extensions they are usually oriented roughly parallel to the long axis of the extensions and may play a role in the formation of these. © 1969 Academic Press, Inc.