Size of megakaryocyte clonal progeny, TPO responsiveness, and Mpl expression recorded for CD42a⁺ and CD42^– fractions of CD34⁺CD41⁺ megakaryocyte progenitors. (Left) Histogram columns indicate the percentages of progenitors of each fraction that generated clones of the sizes indicated. Data are the means of 3 duplicate experiments ± 1 SD. Distributions of percentages in the 2 fractions differed by chi-square analysis (P < .001). (Middle) The bell-shaped curves are TPO threshold distributions for the CD42a⁺ (left) and CD42a^– (right) fractions. The means and 95% confidence intervals (95% CIs) of the corresponding TPO₅₀ were 1.13 pg/mL (95% CI, 0.427-1.82 pg/mL) and 2.75 pg/mL (95% CI, 1.91-3.96 pg/mL), respectively. Because the CI did not overlap, TPO₅₀ values differed significantly from each other at the 5% level. (Right) Flow cytometric analysis of Mpl expression on CD42a⁺ (right) and CD42a^– (left) megakaryocyte progenitors. Mononuclear bone marrow cells were labeled with anti-Mpl followed by antimouse IgG-PE, washed with mouse serum, and further labeled with anti-CD34–PC5, anti-CD41–APC, and anti-CD42a–FITC. Median Mpl expression was 2.8-fold higher on CD42a⁺ than on CD42a^– progenitors (P < .001).