Megakaryopoiesis in the mammalian embryo is associated with rapid lineage maturation and synthesis of platelets with altered activation

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We have previously determined that megakaryocyte (MK) progenitors first emerge in the yolk sac of E7.5 mouse embryos and are subsequently found in the E10.5 liver, concomitant with initiation of platelet release into the bloodstream (Tober, Blood 2007). While megakaryopoiesis is established prior to hematopoietic stem cell emergence, little is known regarding the differentiation and function of the MK lineage during embryogenesis. We have now compared E12.5 fetal liver versus the adult marrow and find that primary embryonic MKs have lower mean ploidy than adult MKs. Unlike adult MKs, low-ploidy embryonic MKs reveal evidence of maturation, including mature cell surface marker expression and the presence of a punctate pattern of a-granule components and demarcation membranes in the cytoplasm. These differences between embryonic and adult MKs appear to be cell intrinsic, since MKs derived from the in vitro culture of E9.5 yolk sac versus adult marrow have lower ploidy and late-stage cell surface marker expression. While we had previously shown that embryonic platelets are larger, more reticulated, with small a-granules than adult platelets, our recent histochemical studies reveal similar distribution patterns of the a-granule components VEGF and endostatin. To begin to determine if there are functional differences between embryonic and adult platelets, we first compared expression of transcripts associated with signaling cascades regulating platelet activation. While the transcripts for many components are similar, embryonic platelets express much lower levels of ADP-receptors (P2Y12, P2Y1) and P-selectin. Consistent with these results, E12.5 platelet activation failed to increase P-selectin on the platelet surface. Taken together, our findings indicate that embryonic MKs are characterized by low ploidy and rapid maturation leading to the generation of platelets with marked differences in size, structure and function compared to adult platelets. Hypothesizing that differentiating ES cells recapitulate early embryonic events, we find that MKs generated from ES cells have the low ploidy and rapid maturation characteristics of primary embryonic MKs. A better understanding of embryonic megakaryopoiesis will lay a foundation for the development of clinically useful platelet therapies from ES/iPS cell sources.