The percentage of small-sized megakaryocytes, which produce fewer numbers of proplatelets, was increased in c-Myc^−/− mice compared with WT controls. BM nucleated cells from c-Myc^−/−and WT control mice were seeded at a final concentration of 5 × 10⁵/mL in MegaCult-C Collagen-Medium (StemCell Technologies) supplemented with 50 ng/mL rh TPO,10 ng/mL rm IL-3, 20 ng/mL rh IL-6, and 50 ng/mL rh IL-11. Cells were incubated at 37°C, 5% CO₂. The megakaryocyte maturation process was observed 48 hours later. We found that significant numbers of megakaryocytes had matured, as indicated by the development of proplatelet projections and, later (A,C) by platelet production (B,D). Shown are representative photomicrographs of megakaryocytes from c-Myc^−/−mice (A-B) and WT control mice (C-D). Based on the number of proplatelet projections, megakaryocytes were classified into 3 groups as summarized in panel I. We found that the percentage of megakaryocytes producing fewer proplatelets (< 5) was significantly increased, whereas the percentage of megakaryocytes producing larger numbers of proplatelets (> 10) was significantly decreased in c-Myc^−/− mice compared with WT controls. A total of 83 and 95 megakaryocytes were counted in c-Myc^−/− and WT control mice, respectively. Megakaryocytes were confirmed by AChE staining as shown in panels E to H. Megakaryocytes cultured as described in panels A to D were harvested 72 hours after incubation and stained with AChE. Shown are representative photomicrographs of AChE-positive megakaryocytes from c-Myc^−/− (E-F) and WT control mice (G-H). The size of AChE-positive megakaryocytes was measured. Based on diameter, megakaryocytes were classified into 3 groups as summarized in panel J. *Significant difference compared with WT control mice. Bar represents 100 μm. Panels A-D, 40×/0.8 air; panels E-H, 20×/.7 air.