Representative transmission electron micrographs of resting platelets and platelets incubated with HIT serum.

Resting platelets or platelets incubated with HIT serum in the presence of 0.1 U/mL heparin were fixed with 0.5% glutaraldehyde and embedded in glycolmethacrylate. (A) The morphology of a representative normal resting platelet. Alpha granules (a), mitochondria (m), and channels of the open-cannalicular system (ocs) can be seen throughout the platelet. The platelets appear round or discoid in shape, and there is an absence of pseudopodia. (B-F) The morphology of representative platelets incubated with HIT serum in the presence of heparin. Platelets are activated with a centralized clustering of alpha granules (a) and mitochondria (m). (B) Pseudopodia (psd) can be seen extending from the platelet body, and several distinct membrane-bound structures resembling small vesicles (v) are observed near the platelet. Numerous pseudopodia demonstrated a region of bulging (E) at the terminal end of the pseudopod or (C) along the body of the pseudopod. Frequently, several areas of bulging were observed on (D) the same pseudopod and (C) the platelet body itself. (F) In these platelets, microparticles appeared to be released by the budding of pseudopods. (Original magnification (A, B) × 15 000; (C, D) × 25 000; and (E, F) × 50 000.)