Transmission electron microscopy of megakaryocytes and platelets. Representative electron microscopic views of platelets and cultured megakaryocytes of Pf4/Srf WT and KO mice. Pf4/Srf KO mice contain a population of giant platelets, and Pf4/Srf KO megakaryocytes have structural alterations. (A) Representative image of a normal population of platelets from a WT mouse. (B-D) Electron micrographs showing giant platelets isolated from the blood of Pf4/Srf KO mice. Scale bar represents 500 nm (A-D). (E) Representative image of a normal megakaryocyte with a well-structured, uniform demarcation membrane system from WT mouse. (F-H) Electron micrographs of megakaryocytes with altered structures from Pf4/Srf KO mice. A small area of demarcation membrane system concentration, resembling a maze of small tubules, is observed in a population of Srf-deficient megakaryocytes (F-G). (H) Representative Pf4/Srf KO megakaryocyte micrographs in which the cellular contents are not evenly distributed throughout the cytoplasm, including areas with missing granules and organelles. Scale bar represents 2 μm (E-H).