Development of the definitive hematopoietic hierarchy in the mouse

Recent research on the ontogeny of the hematopoietic system in mammals has shown that a simple textbook steady-state hematopoietic hierarchy can not be strictly applied to the hematopoietic cells found within the embryo. During embryonic development, hematopoietic cells originate, migrate and differentiate in a number of distinct anatomical sites such as the yolk sac, AGM region and liver and thus represent various classes of cells within diverse microenvironments. In this manuscript we review both cellular and molecular aspects of developmental hematopoiesis and present our current views on the numerous complex mechanisms underlying the establishment of definitive hematopoiesis.