Scanning electron microscopy analysis of activation-independent aggregates. Blood containing PPACK and PG E₁ was perfused over immobilized VWF (see legend to Figure 2). This was immediately followed by perfusion-fixation with a buffered 4% paraformaldehyde solution before processing for electron microscopic analysis. The transition from activation-independent rolling aggregates to fixed formations on the surface was documented by microscopic evaluation in real time. Panels A and B show elongated aggregates formed by discoid (nonactivated) platelets with membrane protrusions that link platelets to one another. In panel B, 2 such protrusions originating from adjacent platelets (highlighted by a dotted line) appear to be in contact. Panel C shows a rolling aggregate in which platelets are in close contact with one another with only few and short membrane protrusions.