Direct visualization of proplatelet/platelet release in culture. (A and B) Isolated proplatelets were diluted with a semisolid culture medium and maintained in custom BSA-coated chambers at 37°C. They were examined on an inverted microscope, and frames were captured at 30-s intervals. (A) The separation of proplatelet cytoplasm and associated release of a shorter proplatelet fragment. The arrows highlight the site of proplatelet division. (B) The release of an individual platelet (arrows) from the end of a larger released proplatelet. (C and D) Thin-section electron micrographs showing the ultrastructure of a barbell-shaped proplatelet in the process of platelet release. Black arrows highlight the formation of a constricted region resembling a cleavage furrow along the long shaft of a cultured proplatelet. (C, inset) A higher magnification view of the boxed area is shown. (D) A similarly shaped barbell proplatelet immediately after cleavage. The arrow highlights the site of proplatelet division.