Are developmental hemoglobin changes related to the origin of stem cells and site of erythropoiesis?

Hemoglobin ontogenic changes have been studied in chimeras composed of a quail embryo grafted on a chick yolk sac. From the blood of such chimeras, quail erythrocytes are separated from chick by means of a specific immune hemolytic technique, thus yielding two populations of red cells, one deriving from yolk sac stem cells and the other from intraembryonic stem cells. Both populations comprise in succession primitive and definitive erythrocytes. However, quail primitive erythrocytes are produced in very small or often undetectable quantities, whereas quail definitive erythrocyte inflow increases steadily; on the other hand, chick cells make up the bulk of the primitive series and taper off in the definitive series. Previous work by our group has shown that intraembryonic stem cells travel to the yolk sac during the second phase of erythropoiesis. The results mean that the sequence of erythrocytic series is determined by a time program rather than by the origin of stem cells or site of erythropoiesis. All areas of the blastoderm may give off erythroid stem cells with similar potentialities, but most yolk sac stem cells engage in erythropoiesis at the early phase, while most intraembryonic stem cells do so later.