Megakaryocytes (MKs) and platelets from SG^−/− mice. (A) Electron micrographs of SG^−/− platelets and megakaryocytes. (i) Two average-size platelets with scroll-like membranes called “cigars,” elongated intracellular membranous structures decorated with glycogen particles. (ii) Large hypogranulated platelet and small platelet with cigars. (iii) Large heavily granulated platelet with cigar. No cigars were seen in normal platelets. (iv) Immature MKs with neutrophil. (v) Mature MKs with several neutrophils. “Cigar” is just to left of *. (vi) Detail showing “cigar” from panel v; neutrophil is the one directly left of * in panel v. (vii) Neutrophil surrounded by demarcation membrane system in mature MKs. α-Granules had nucleoids, but many granules were elongated (Figure 1A center; Figure 1C platelet on right). Magnification (at microscope objective): Platelets: (i) ×4200; (ii) ×5500; (iii) ×4200. MKs: (iv,v) ×1000; (vii) ×1600. Additional image information available in “Morphology.” (B) Wright-Giemsa–stained blood smear. Photographs were taken electronically and are presented in their original form. Magnification was ×1000. Note identical appearance of red blood cells in the WT and KO smears.