MPL and JAK2 levels are upregulated during megakaryocytic differentiation. The increase in MPL/JAK2 protein expression could regulate megakaryopoiesis. (A) Megakaryopoiesis can be regulated by a single cytokine, namely TPO, which exerts both proliferative and antiproliferative effects. (B) Human CD34⁺ cells from cytapheresis were cultured in a serum-free medium (MSS) in the presence of TPO at 10 ng/mL. MPL and JAK2 mRNA expression (relative to HPRT) was determined by Taqman during megakaryopoiesis at days 0, 3, 6, 9, and 18. (C) MPL, JAK2, p53, p21^CIP1 expression and STAT5, STAT3, ERK phosphorylation status were analyzed by immunoblotting during human megakaryopoiesis at days 0, 3, 4, 5, 7, 9, 11, and 13. (D) Quantification of JAK2 and MPL protein expression (normalized with HSC70 expression and in comparison with D3) during megakaryopoiesis in vitro. Error bars represent mean ± standard deviation.