| During vertebrate embryogenesis, blood cell formation (hematopoiesis) and vasculogenesis are intertwined temporally and spatially to satisfy the metabolic requirements of the developing embryo. This intimate relationship led to the postulation of the hemangioblast, a common precursor able to generate stem cells for blood cell lineage and endothelial cell lineage, respectively. Both hematopoiesis and vasculogenesis occur in two distinct, successive waves, the first wave appears in the extraembryonic yolk sac followed by the second wave in the embryo proper. The focus of this thesis is to study the roles of erythropoietin (EPO), the principle hormone in regulating red blood cell formation, and its cognate receptor (EPOR), in regulating the two waves of erythropoiesis and vasculogenesis.; Using a combination of molecular genetics and cell biology approaches, we demonstrated that while EPO/EPOR are absolutely required for the second, definitive wave of erythropoiesis as well as the second wave of vasculogenesis, they are dispensable for the first waves of blood cell and vessel formation in the yolk sac. Further studies indicate that EpoR does express extraembryonically in the cell compartments where the first wave of erythropoiesis and vasculogenesis take place, but Epo is absent. Thus, these segregated Epo and EpoR expression patterns may provide a cellular mechanism as to why the first waves of erythropoiesis and vasculogenesis proceed with their respective differentiation programs in the yolk sac in dependent of EPO/EPOR. After the commencement of blood circulation which links the extraembryonic and embryonic compartments, Epo expression begins intraembryonically in parallel with EpoR, thereby allowing EPO-dependent definitive erythropoiesis and angiogenesis to proceed in the embryo proper. Therefore, the onset of Epo expression serves as a switch to turn on the second wave of erythropoiesis and vasculogenesis. Finally, EPO/EPOR regulate VEGF and Ang-1 transcriptionally under normoxic conditions; and acute EPO administration leads to increased vessel leakiness. Taken together, evidence contained in this dissertation suggests that EPO/EPOR regulate the timing and location of definitive erythropoiesis and angiogenesis, as well as the expression of crucial angiogenic growth factors. This study also raised precautions against potential therapeutic uses of human recombinant EPO. |
