Hematopoiesis in the human yolk sac: quantitation of erythroid and granulopoietic progenitors between 3.5 and 8 weeks of development.

In a first attempt to investigate the regulation of the early steps of human embryonic hematopoiesis, we measured the number of erythroid and granulopoietic progenitors in 38 human yolk sacs and 15 embryonic livers between 27 and 62 days of development. Both erythroid and granulopoietic progenitors were identified in the yolk sac as soon as 27 days, while in the embryonic liver significant numbers were not observed before 40 days. In the yolk sac, the number of granulopoietic and/or macrophagic progenitors was significantly negatively correlated with gestational age. Such a correlation was not observed for CFU-E and BFU-E, even though in the youngest (less than 30 days) and the oldest embryos studied (greater than 40 days), the number of BFU-E was clearly lower.