Abnormal ultrastructure of Nbeal2^−/− bone marrow megakaryocytes. (A) Thin-section transmission electron micrographs of representative WT MKs show typical platelet territories (white arrowheads) and platelet-like structures containing α-granules. (B) In contrast, Nbeal2^−/− MK are deficient in both α-granules and platelet territories (magnification ×4000, scale bars = 2 µm). (C) Higher-magnification (×15 000) images of WT MK reveal multiple α-granules (white arrowheads) and platelet territories, whereas (D) α-granules (white arrowheads) are rare in Nbeal2^−/− MK, which show poorly defined platelet territories (scale bars = 500 nm). (E) Example of the emperipolesis that was frequently observed in Nbeal2^−/− MK; here, 3 exogenous cells (white arrowheads) are present in the MK (magnification ×3000, scale bar = 10 µm).