Staining of cytocentrifuge preparations of wild-type (a and b; +/+) and NF-E2-null (c; −/−) cultured megakaryocytes to show cytoplasmic acetylcholinesterase activity (a) and immunoreactivity with rabbit antimouse platelet antiserum (b and c). Proplatelets, indicated by the large arrows in (a) and (b), are hence shown to represent extensions of the megakaryocyte cytoplasm and membranes in wild-type cells and are not detected in the mutant megakaryocytes. Background staining of nonmegakaryocytic cells (short open bars) in (b) and (c) and absence of cholinesterase staining in granulocytes and monocytes (short straight arrows) in (a) provide internal controls for the antiserum and enzyme staining, respectively. Original magnification × 200.