Megakaryocyte maturation is inefficient in pIpC-treated Runx1-excised mice. BM from representative (A) Runx1^F/F and (B) Runx1^F/F—Tg(Mx1-Cre) mice 143 days after pIpC injection shows an absence of normal megakaryocytes in Runx1-excised mice (hematoxylin and eosin [H&E] staining; original magnification, × 600). (C) BM from representative Runx1^F/F—Tg(Mx1-Cre) recipient mice 98 days after transplantation (H&E; original magnification, × 600). (A-C) Yellow arrows indicate representative megakaryocytes. Red and green arrowheads indicate representative erythroid and myeloid elements, respectively. Sections demonstrate a notable absence of megakaryocytes and an increased ratio of maturing myeloid-to-erythroid forms in Runx1-excised versus nonexcised marrows. (D) Histograms depicting number of cultured Runx1^F/F and Runx1^F/F—Tg (Mx1-Cre) bone marrow cells stained with propidium iodide to show ploidy. Colors indicate CD41 gates: green, CD41⁺; red, CD41^–. (E, left) Plotted are mean ± SD numbers of acetylcholinesterase-positive colonies per 5 × 10⁴ BM cells plated from pIpC-treated Runx1^F/F (□) and Runx1^F/F—Tg(Mx1-Cre) (▪) mice. Results shown are for 3 experiments performed in quadruplicate. 1 and 3, fresh marrow; 2, previously frozen marrow. (Right) Plotted is the average fold increase for Runx1^F/F—Tg(Mx1-Cre) (▪) relative to Runx1^F/F (□) in the 3 experiments (P ≤ .034). Representative megakaryocyte colonies from (F) Runx1^F/F and (G) Runx1^F/F—Tg(Mx1-Cre) mice show acetylcholinesterase staining (brown) of megakaryocytic cells (original magnification, × 100).