Initiation of hematopoiesis in the mouse embryo.

CD41, a candidate target gene of SCL, marks the initiation of definitive hematopoiesis during embryoid body development in vitro and in the yolk sac in vivo. Expression of the panhematopoietic marker CD45 appears later in a subpopulation of hematopoietic progenitors in the yolk sac. By the fetal liver stage, the expression of CD41 is down-regulated, whereas CD45 is expressed in all definitive hematopoietic progenitors.