Electronmicroscopic Studies on Human Thrombocytogenesis

The mechanism of platelet production and platelet release in human bone marrow tissues was studied electronmicroscopically. Promegakaiyocytes and megakaryocytes were classified morphologically into 4 stages. Out of the 126 cells studied, 3.2% were in stage I, which was featured by active synthesis of protein and other cellular constituents; 8.2% were in stage II, in which specific granules were produced; 57.9% were in stage III, in which the demarcation membrane was formed; 21.5% were in stage IV, in which platelets were released. Eleven (8.7%) were unclassified. Seventy-three or 75% of the colls in each stage I, II and III existed in the parenchyma. But almost all megakaryocytes in stage IV existed as free cells presumably in a sinusoid. It is assumed that, after formation of the demarcation membrane in the whole cytoplasm, megakaryocytes in the parenchyma move into the sinusoids, and then the whole cytoplasm of megakaryocytes is broken explosively into small pieces to be designated as platelets.