Neonatal and adult megakaryopoiesis.

PURPOSE OF REVIEW: It has become increasingly clear that there are substantial biological differences between fetal/neonatal and adult megakaryopoiesis. Over the last 18 months, studies challenged the paradigm that neonatal megakaryocytes are immature and revealed a developmentally unique uncoupling of proliferation, polyploidization, and cytoplasmic maturation. Several studies also described substantial molecular differences between fetal/neonatal and adult megakaryocytes involving transcription factors, signaling pathways, cytokine receptors, and microRNAs. RECENT
FINDINGS: This review will summarize our current knowledge on the developmental differences between fetal/neonatal and adult megakaryocytes, and recent advances in the underlying molecular mechanisms, including differences in transcription factors, in the response to thrombopoietin (Tpo), and newly described developmentally regulated signaling pathways. We will also discuss the implications of these findings on the way megakaryocytes interact with the environment, the response of neonates to thrombocytopenia, and the pathogenesis of Down syndrome-transient myeloproliferative disorder (TMD) and Down syndrome-acute megakaryoblastic leukemia (DS-AMKL).
SUMMARY: A better characterization of the molecular differences between fetal/neonatal and adult megakaryocytes is critical to elucidating the pathogenesis of a group of disorders that selectively affect fetal/neonatal megakaryocyte progenitors, including the thrombocytopenia-absent radius (TAR) syndrome, Down syndrome-TMD or Down syndrome-AMKL, and the delayed platelet engraftment following cord blood transplantation.