Ultrastructural analysis of Vps33b^fl/fl-ER^T2 platelets reveals an α-granule deficiency. (A) Representative transmission electron micrographs of platelets from Vps33b^fl/fl (left panels) and Vps33b^fl/fl-ER^T2 (right panels) mice at low (scale bar, 1 μm) and high magnification (scale bar, 0.5 μm). Images were obtained using a Tecnai G2 Spirit TEM. (B) Percentage of α-granule containing platelets per ultrathin section (70 to 90 nm) in Vps33b^fl/fl and Vps33b^fl/fl-ER^T2 mice (n = 200 platelets per genotype). (C) Platelets containing small granules were observed in TEM from Vps33b^fl/fl-ER^T2 mice. Those platelets were devoid of α-granules. Scale bar, 0.5 μm. (D) Representative transmission electron micrographs of an ARC patient with a VIPAS39 mutation (p.Arg270*, top right panel) and 2 ARC patients with VPS33B mutations (p.Pro412Argfs*7 or p.Pro147Argfs*4, bottom left and right panel respectively). Small α-granule–like structures are shown in white arrowheads. Scale bar, 1 μm. (E) Immunogold labeling (IEM) using the Tokuyasu method for the presence of VWF (10 nm gold) in platelets from Vps33b^fl/fl (left panel) and Vps33b^fl/fl-ER^T2 (right panel) mice. Scale bar, 200 nm. All values are mean ± SD. ***P < .001. α, α-granule; δ, δ-granule; G, α-granule-like structure; M, mitochondrion.