Characterization of Vps33b^fl/fl-ER^T2 MKs in primary culture. (A) Distribution of Vps33b^fl/fl and Vps33b^fl/fl-ER^T2 BM-derived MKs ploidy after 5 days in culture. The percentage of cells with 2N to 128N ploidy was quantified by propidium iodide staining and flow cytometry (n = 5 mice per genotype). Mean ± SD. (B) Proplatelet formation was unaltered in Vps33b^fl/fl-ER^T2 MKs (n = 3 mice per genotype). Mean ± SD. (C) Transmission electron micrographs of BM-derived MKs showing representative images of MVB I (left) and MVB II (right) in control MKs. Scale bar, 0.5 μm. (D) Representative transmission electron micrographs of BM-derived MKs. Vps33b^fl/fl MKs had normal α- and δ-granules, whereas MBV I and MBV II were also present (left panels). Note the presence of large vacuolar structures in Vps33b^fl/fl-ER^T2 MKs (right panels). Scale bar, 0.5 μm. (E-F) Quantification of organelles present in MK sections. Number of MVB I, α-, and δ-granules (E), and number of classical and atypical MVB II and vacuoles (F) per MK section. Twenty to 27 MKs imaged per genotype, 4 to 5 fields of view (4.98 × 3.32 μm) per MK taken at a magnification of ×30 000. Mean ± SEM. *P < .05. A, atypical MVB II; α, α-granule; δ, δ-granule; M, mitochondrion; ns, not significant; V, empty vacuole.