Immunoelectronmicroscopy of platelet and megakaryocytes sections. Washed platelets and megakaryocytes were proceeded for transmission electron microscopy as described in Materials and Methods. Sections incubated with 75 μg/mL of 4PCI (A through E) showed positive staining in the OCS, in the  granules (arrowheads), and on the plasma membrane. Sections incubated with nonimmune-IgG (negative control, F) were devoid of gold staining. PCI labeling in the  granules (A through E, arrowheads) was distributed over all  granules and predominantly in the electron dense zone or nucleoid, although sometimes they were found to bind to the  granule membrane (A through E, arrowheads). The PCI staining pattern in megakaryocytes (G and H) was very similar to that seen in platelets. Gold particles often appeared in granules identifiable as  granules, as well as on the plasma membrane, and in the open canalicular system. Experiments of colocalization of PCI with PAI-1 (I through M) were performed as described in Materials and Methods. The gold particles of 10 and 5 nm indicate the presence of PCI and PAI-1, respectively. Colocalization demonstrates that the granules observed were indeed  granules. (A and C through G) Bars = 200 nm; (B and H through M) bars = 100 nm.