Dynamics and inner structure of the “caps.” (A) Formation of the “balloon” and of the “cap” (indicated with arrow) in the procoagulant platelet at indicated times after activation. See also supplemental Video 2. Scale bars, 1 μm. (B) Distribution of calcein in procoagulant platelets at 5 minutes after the stimulation (upper row) and after the procoagulant platelet formation (at 20 minutes after the stimulation) (lower row). (C) The same for the nonprocoagulant platelets. (D) Transmission electron microscopic images of PS-positive platelets and of the “caps” indicated with white arrows (numbered 1). Discernible mitochondria are indicated with black arrows (numbered 2). Scale bars, 1 μm.