Origin of erythropoietic stem cells in avian development: shift from the yolk sac to an intraembryonic site.

Haemopoietic stem cells are currently considered as deriving exclusively from the embryonic yolk sac in birds and mammals. A new experimental approach has been devised: quail embryonic bodies are grafted on to chick yolk sacs before establishment of circulation, and allowed to develop. The proportions of quail and chick erythrocytes in the circulating blood are assayed through a differential immunohaemolysis technique. Up to 10 days of incubation, grafted quails have 80 to 95% chick erythrocytes. Between 10 and 13 days, quail erythrocytes appear in increasing proportions making up as much as 80% of the red blood cells in some embryos. Morphological analysis reveals that chick cells colonize transiently the spleen of grafted embryos between 10 and 12 days, and undergo erythropoiesis in that organ. However at 13 days chick cells have disappeared from the spleen of grafted embryos. It is concluded that yolk sac stem cells are capable of colonizing intraembryonic organs, but do not do permanently and that there is an intraembryonic source of definitive erythropoietic stem cells.