Giant electron dense chains, clusters and granules in megakaryocytes and platelets with normal dense bodies: an inherited thrombocytopenic disorder III. Platelet analytical electron microscopy.

A woman and her son were referred because of prolonged thrombocytopenia. Ultrastructural studies revealed the presence of giant organelles in their cells never observed previously in human platelets. Our initial reports described the evolution of two types of giant organelles in patient megakaryocytes and platelets. The last report also demonstrated that the large organelles in platelet whole mount preparations were inherently electron opaque like serotonin-rich dense bodies in normal and patient platelets. In the present study analytical electron microscopy of whole mount preparations from the patients and controls revealed that the inherently electron opaque dense and hexagonal precursor fragments, chains, clusters and giant organelles contained high concentrations of calcium and phosphorous, the major elements in normal dense bodies. The ratio of calcium to phosphorous in patient giant organelles was nearly the same as in normal platelet dense bodies. However, as shown in the previous study, levels of serotonin and adenine nucleotides were normal in patient platelets, and giant organelles and their contents were not secreted following activation of platelets with thrombin. Thus, despite the similarity in mineral content responsible for electron density, the giant organelles in patient platelets do no appear to be aberrant serotonin storage organelles.