The ultrastructure of resting and stimulated platelets from a FHL5/Munc18b/STXBP2 patient. Platelets from a biallelic patient (P1; A and C) and normal control (B and D) were prepared as described under “Ultrastructure analysis.” (A and B) Platelets under resting conditions. Thrombin (final concentration 0.1 U/mL) was added to samples shown in panels C and D. After 2 minutes at room temperature, samples were fixed and analyzed by transmission electron microscopy. (D) Some α-granules are marked with white asterisks. The scale bar is indicated in all images.