Scl^–/Δ mice show abnormal stress thrombopoiesis. (A) Response of Scl^–/Δ mice to thrombopoietin. Mice of the indicated Scl genotypes were injected intraperitoneally with 2 μg pegylated recombinant human thrombopoietin (PEG-rhTPO) daily for 5 days as indicated (arrows), and platelet counts were determined periodically. Results are the mean ± standard deviation of 4 mice analyzed. (B) Determination of platelet turnover during TPO administration. Mice were administered 2 μg/mL PEG-rhTPO on days 0 to 5 and N-hydroxy-succinomidyl biotin on day 2 and the percentage of biotinylated platelets determined daily using flow cytometry (top panel). Relating this value to the determined platelet count yielded the number of nonbiotinylated platelets (bottom panel). Data are mean ± standard deviation of 3 mice analyzed. (C) Normal expansion of Scl^–/Δ megakaryocytes in response to thrombopoietin. Mice of the indicated Scl genotypes were either untreated (□) or treated with 2 μg PEG-rhTPO for 5 days, then analyzed on day 8 (▪). Sternal bone marrow and spleen sections were stained with hematoxylin and eosin and analyzed using light microscopy. The number of megakaryocytes per 400 × microscopic HPF is indicated for each genotype. Results are the mean + standard deviation of 15 adjacent fields analyzed for 4 mice. (D) Increased ploidization of megakaryocytes in response to TPO. Mice of the indicated Scl genotypes were treated with 2 μg/mL PEG-rhTPO for 5 days and the DNA content of megakaryocytes determined using propidium iodide staining and flow cytometry. The level of staining corresponding to the numbers of haploid genomes (N) contained in each cell is indicated. Data are representative of 3 separate mice of each genotype analyzed. (E) Response of Scl^–/Δ mice to 5FU. Mice of the indicated Scl genotypes were injected intraperitoneally with 150 mg/kg 5FU, and platelet counts were determined periodically. Results are the mean ± standard deviation of 3 mice.