Absent α-granules and increased δ-granules in platelets from patients containing mutations in VPS33B. Thin-section transmission electron micrographs of representative platelets from related ARC fetal (A) and neonatal (B) blood, control fetal (C), and unrelated ARC neonatal (D) blood. Magnifications are ×30 000 (A-C) and ×25 000 (D). The black bar represents 500 nm. Multiple α-granules (white arrows) are seen in the control platelet (C), and are absent in ARC platelets (A, B, D). The dark-staining structures seen in ARC platelets (A, B, D) contain distinct internal membranes and may represent mitochondria. Prominent Golgi structures are also visible in ARC platelets (eg, near the center of the platelet in panel B). Multiple dark densities encircled by a lighter halo representing δ-granules (arrows) are visualized in a single thin-section EM image of a neonatal ARC platelet (D), where one would expect to see less than the average of 0.4 δ-granules per platelet thin section observed in adults.³⁴