Cytoplasmic maturation of CB-MKs. At day 14 of culture, MK cytoplasmic maturation and polyploidization were analyzed by flow cytometry and immunofluorescence. (A) Representative flow cytometric measurements of the percentage of CD42b⁺ MKs at each ploidy level in CB-MK and PB-MK cultures. (B) Compared with PB-MKs, CB-MKs exhibited significantly higher percentages of CD42b⁺ (mature) cells at each ploidy level. The bars represent the means ± SEM of 3 independent experiments. *P < .05. (C) When the intensity of CD42b staining was measured by MFI, CB-MKs exhibited almost equally intense expression at all ploidy levels. In contrast, among PB-MKs, the intensity of the staining increased gradually in parallel to the ploidy level. The figure shows a representative result of 3 independent experiments. (D) Like mature PB-MKs, low-ploidy CB-MKs formed α granules coexpressing P-selectin and VWF, as indicated by immunofluorescence staining. Images were taken on an Eclipse fluorescent microscope equipped with a DXM1200F camera (both from Nikon) at a magnification of 600×.