The ontogeny of the megakaryocyte lineage

Megakaryocytes are the large cellular precursors to platelets, small blood cells that play an essential role in stabilizing and assisting in the repair of damaged tissue following injury. In the adult bone marrow, megakaryocytes and erythroid cells (red blood cells) are derived from common bipotential megakaryocyte-erythroid progenitors (MEP) that are derived from hematopoietic stem cells (HSC). Both megakaryocytes and erythroid cells have been described in early mouse and human embryos before detectable HSC are evident. However, little is known about their developmental origin, whether embryonic megakaryocyte and erythroid lineages share a common precursor, or time of their maturation. In this thesis, I examine the entire hierarchy of megakaryopoiesis in the mouse embryo from the emergence of the first megakaryocyte precursors to the identification of the first, unique platelets in the embryonic circulation. Previously unrecognized bipotential primitive megakaryocyte-erythroid progenitors (pMEP) support the concept that the first embryonic megakaryocyte progenitors arise from hemangioblast precursors and are closely associated with the primitive erythroid lineage, the predominant hematopoietic lineage during early embryogenesis. pMEP and megakaryocyte progenitors (Meg-CFC) produced during primitive hematopoiesis likely produce the first mature megakaryocytes seen in the yolk sac and the first, large highly reticulated platelets that enter the embryonic circulation. We also show evidence that adult-like definitive-MEP (dMEP) are later produced in the yolk sac then migrate to the fetal liver, demonstrating that throughout development the erythroid and megakaryocyte lineages remain closely associated. Our hypothesis that there are two distinct megakaryocyte lineages is supported by the inability of c-myb-knockout mice and embryonic stem cells to fully contribute to definitive hematopoiesis. Using these models, we conclusively determined that primitive erythro- and megakaryopoiesis occurs independently of c-myb but is required after the onset of definitive hematopoiesis to produce a normal complement of Meg-CFC. Furthermore, these later staged Meg-CFC have severely reduced proliferative capacity suggesting c-myb is required for the expansion of definitive, but not primitive Meg-CFC. Thrombopoiesis also initiates normally in c-myb-null embryos, but this is not sustained, supporting our hypothesis that the first platelets are derived from primitive megakaryocytes.