Abnormal ultrastructure of femoral BM Vps33b^fl/fl-ER^T2 MKs. (A) Representative transmission electron micrographs of femoral BM sections showing the classification of MKs in 3 main maturation stages. Scale bars, main image 5 μm; inset 1 μm. (B) Quantification of the percentage of MKs present per maturation stage in Vps33b^fl/fl and Vps33b^fl/fl-ER^T2 mice (n = 32 to 44 MKs analyzed in 3 mice per genotype). (C) Vps33b^fl/fl MKs showing nice distribution of granules in maturation stages II and III (top panels), whereas Vps33b^fl/fl-ER^T2 MKs were devoid of morphologically distinct α-granules but were abundant of small α-granule–like structures (bottom panels). Clusters of lamellar structures were also evident in Vps33b^fl/fl-ER^T2 MKs. Scale bar, 0.5 μm. (D-E) Quantification of α-granules and immature granules at maturation stage II (D) and III (E) reveals a marked decrease in their numbers in Vps33b^fl/fl-ER^T2 mice when compared with controls (n = 32 to 44 MKs analyzed in 3 mice per genotype). All values are mean ± SEM. **P < .01; ***P < .001. α, α-granule; δ, δ-granule; G, α-granule-like structure; im, immature granule; Lm, lamellar structure; M, mitochondrion.