Ultrastructure of MKs by transmission electron microscopy. Human MKs of different ploidy levels were flow sorted after anti-human CD41-FITC and Hoechst 33342 staining and examined by transmission electron microscopy. (A) Representative flow-sorted 2N/4N CB-MKs. The majority of these MKs were mature and exhibited abundant platelet granules and a well-developed DMS. (B) In contrast, 77% of flow-sorted 2N/4N PB-MKs were immature, as evidenced by the absence of a DMS and the paucity of granules (Gr) seen in this representative electron photomicrograph. These immature forms also exhibited multivesicular bodies (MVBs) and dense compartments (DCs), representing the precursors of platelet granules and of the DMS, respectively. (C) Fifty-four percent of all flow-sorted 8N PB-MKs were ultrastructurally similar to the mature neonatal 2N/4N MKs, although they were larger. All images were taken on a G2 Spirit BioTWIN transmission electron microscope (Tecnai) at an accelerating voltage of 80 kV, and recorded with a 2k CCD camera (Advanced Microscopy Techniques).