IL-1α-induced MK rupture yields larger platelets. (A–C) Time-lapse images of thrombopoiesis in living BM from 6-wk-old CAG-eGFP mice treated with IL-1α (10 µg/mouse s.c. daily for 5 d). (A, Video 8; B, Video 9; and C, Video 10.) (D) Quantification of MK dynamics and numbers and platelet counts in 6-wk-old CAG eGFP mice treated with TPO (10 µg/mouse s.c. daily for 5 d [TPO10], or with 70 µg/mouse daily for 3 d [TPO70]) or IL-1α (10 µg/mouse s.c. daily for 5 d). The BM was visualized and platelet counts were analyzed 7 d after the first administration. n = 50 high-power fields from 5 animals for each group. *, P < 0.05 versus vehicle treated mice. (E and F) Platelet counts in isolated blood (E) and the CD41⁺CD42b⁺ MK fraction among Lin⁻ BM cells (F) treated with vehicle (CTRL), low-, or high-dose TPO, IL-1α, anti–IL-1α neutralizing antibody (IL-1Ab), anti–IL-1R neutralizing antibody (IL-1RAb), or isotype-matched control antibody (IgG1 for IL-1Ab, IgG2 for IL-1RAb). All antibodies were used at 100 µg/mouse administered i.p. daily for 3 d. n = 8 animals in each group. (G) Identification of newly produced MKs using MX-Cre-GFP mice. GFP-labeled cells were analyzed among Lin⁻CD41⁺CD42b⁺ BM cells 2 d after PIPC injection. The data shown are from a single representative experiment from among three repeats. (H) Quantification of thrombopoiesis under physiological conditions in 6-wk-old IL-1α^+/+, IL-1 α^−/−, IL-R1^+/+, and IL-1R1^−/− mice. (I) MK dynamics in chimeric mice. n = 50 high-power fields from 5 animals in each group. *, P < 0.05 versus control mice. (J) Fractions of thiazole orange^high platelets in isolated blood from WT mice treated with vehicle, TPO, IL-1α and/or clodronate. n = 5 mice. (K) Flow cytometric size analysis of thiazole orange^high and thiazole orange^low platelets in WT mice treated with TPO or IL-1α. (L) Platelet lifetimes in WT mice treated with vehicle, low-dose TPO, IL-1α, and/or clodronate. Platelets were labeled in vivo with anti-CD42c antibody (X488). During the gradual disappearance of circulating X488⁺ platelets, we were able to distinguish newly generated platelets from previously circulating ones based on their X488 negativity. n = 5 mice. Bars, (red) 20 µm. *, P < 0.05.