Regulation of megakaryopoiesis. TPO mRNA expression in BM stromal cells is enhanced by TGF-β1, which suppresses the TPO-promoted CFU-Meg with relative specificity at midmegakaryopoiesis by interacting with receptors I and II, which were induced by TPO itself in normal subjects (A). Increased TGF-β1 from destroyed platelets or megakaryocytes markedly upregulates TPO mRNA expression in stromal cells. The subsequent production of TPO stimulates stem cells to commit to the megakaryocyte lineage. The expression of the TGF-β receptor on megakaryoblasts renders them susceptible to suppression by TGF-β1 in patients with ITP (B).