Electron micrographs of negatively stained platelets activated in situ with HIT sera.

Platelets were allowed to settle on BSA-coated formvar grids and then incubated with patient serum or platelet agonists. (A) Control platelets were incubated with patient serum, which tested negative for HIT, or heparin alone (data not shown). (B) Platelets were inoculated with HIT serum in the presence of 0.1 μ/mL heparin. As a positive control, platelets were also incubated with (C) 1 U/mL thrombin or (D) 10 μm calcium ionophore A23187. Platelets were then fixed with 2% glutaraldehyde and negatively stained with 2% phosphotungstic acid. (A) Control platelets demonstrated a round or discoid shape, which is characteristic of resting platelets. (B) Platelets incubated with HIT serum demonstrated numerous microparticles surrounding the platelet body. Frequently, these platelets were observed with pseudopodia extending from the platelet body. Microparticles ranged in size from less than 0.1 to 1.0 μm in diameter and appeared as discrete membrane-bound particles. Platelets incubated with (C) thrombin and (D) calcium ionophore demonstrated a similar platelet morphology, with microparticles surrounding the platelet body. (Original magnification (A-E) × 13 000.) (E) These particles were observed to be near the terminal ends and body of the pseudopods, and numerous points of bulging or swelling along the body and terminal ends of the pseudopodia were observed.