Transgene copy number and increased marrow megakaryocytosis in the Tg(mpl) mouse. (A) Southern blot determination of transgene copy number in DNA isolated from the marrows of Tg(mpl) mice and copy number standards generated by dilution of plasmid containing the mpl transgene into genomic DNA from the marrows of mpl^−/− mice. Transgene copy number in Tg(mpl) mice was determined to be 38 based on band intensities quantitated by PhosphorImager. (B) Hematoxylin-eosin–stained sections of femurs from WT (C57BL/6J), mpl^−/−, and Tg(mpl) mice were examined by light microscopy (original magnification, × 200). (C) Morphologically recognizable megakaryocytes were counted in whole femur sections from each strain. Numbers represent the mean plus or minus SE of 6 femurs taken from 3 mice in each group. *P ≤ .003, Tg(mpl) versus WT and Tg(mpl) versus mpl^−/−. (D) Representative 2-color flow cytometric analysis of indirect immunofluorescence staining of 8- to 10-week-old WT, mpl^−/−, and Tg(mpl) mice bone marrow populations stained with anti-CD41–FITC, anti-CD14–PE, and isotype-matched control antibodies. The percentage of cells staining CD41⁺/CD14⁻ is shown in the bottom right of each histogram and is representative of 3 independent experiments (4 mice from each strain were pooled in each experiment).