Megakaryocyte ultrastructure. Megakaryocytes from wild-type (A and B), c-mpl-/- (C and D), and TPO-/- (E and F) mice display multilobulated nuclei, and the cytoplasm contains normal α-granules, some of which display dense cores (B, D, and F, arrows). The demarcation membrane system appears normal (arrowheads). Bar = 1 μm.