Temporal xuất hiện các loại khác nhau của các tế bào tạo máu assayable: thuộc địa hình thành các đơn vị-văn hóa (CFU-C), tiền thân multipotent, thuộc địa hình thành đơn vị lá lách (CFU-S), và các tế bào gốc tạo máu (HSC) trong túi noãn hoàng (hàng đầu ), | 382 HEMATOPOIESIS Primitive Multipotential CFU-S Erythrocytes Progenitors CFU-C Multipotential CFU-S HSC Progenitors Primitive Erythrocytes CFU-C HSC CFU-S HSC E9 E10 E11 FIGURE 50 Temporal appearance of different types of assayable hematopoietic cells colony forming units-culture CFU-C multipotent progenitors colony forming units-spleen CFU-S and hematopoietic stem cells HSC in the yolk sac top para-aortic splanchnopleure aorta-gonad-mesonephros PAS AGM region middle and fetal liver bottom . Note that the more-differentiated progenitors can be detected at earlier stages than the hematopoietic stem cells. The lineage relationships between these cells remains to be established. Also note that multipotential progenitors and HSCs are detected 1 day earlier in the PAS AGM than in the yolk sac. From Dzerziak E. and Oostendorp R. Hematopoietic stem cell development in mammals in Hematopoiesis A Developmental Approach Zon . Ed. Oxford University Press Oxford 2001 p. 211. With permission. adult recipients are seen both in the yolk sac and in the liver. For many years investigators have believed that yolk sac blood islands contained HSCs capable of primitive hematopoiesis and of migration to the developing liver to initiate definitive hematopoiesis. Challenging the idea of a singular origin of hematopoiesis in the yolk sac it has been proposed that there is a more potent intraembryonic HSC site in the AGM region. HSCs arise for the first time in the AGM region and migrate to the yolk sac and fetal liver the main source of hematopoietic cells in fetal life. Around the time of birth HSCs migrate from the liver to the bone marrow to be responsible for adult hematopoiesis see Figure 52 . However this sequential migration has not been directly demonstrated in vivo in mammalian species. Instead evidence for this view is based on the quantitative temporal measurement of hematopoietic activity in the yolk sac and liver using adult recipients following establishment in
