The agm region: Initiation and characterization of the first definitive hematokäetic stem cells in the mouse embryo

Two pre-liver compartments within die mammalian embryo contribute to the generation of the hematopoietic system: the yolk sac which produces primitive hematopoietic cells early in mouse gestation and the aorta-gonads-mesonephros (AGM) region which slightly later in embryonic developmental time generates definitive hematopoietic stem cells. Since clinical transplantation therapies for leukemias and genetic diseases of the blood system are based on the complete, long term hematopoietic repopulation by stem cells, we have examined the mouse embryo for the temporal and spatial onset of the definitive hematopoietic system. Transplantation assays performed on yolk sac and AGM region cells have demonstrated that the first definitive hematopoietic stem cells appear in the AGM region at 10.5 dpc. Furthermore, by performing an embryo expiant I organ culture step to prevent cells from circulating or migrating between the yolk sac and AGM region, we have found that the AGM region can autonomously and exclusively initiate the production of hematopoietic stem cells at 10.5 dpc in the absence of other embryonic tissues. Within the 3 day in vitro organ culture period, hematopoietic stem cells increase by a factor of 15. Phenotypic characterization of these AGM hematopoietic stem cells has revealed the expression of markers such as c-kit, CD34 and Sca1 which are known to be expressed on fetal liver and bone marrow stem cells. Thus, the AGM region induces and expands definitive hematopoietic stem cells and may provide insight into the in vitro manipulation of adult stem cells for therapeutic purposes.