INTRAEMBRYONIC HEMATOPOIETIC STEM-CELLS

Intraembryonic hematopoietic stem cells (HSC) were first detected in a vian chimeras associating an embryo with a yolk sac (YS). Cell markers were used to construct chimeras. The results showed that YS blood pre cursors undergo primitive erythropoiesis and become extinct, whereas i ntraembryonic precursors colonize rudiments of blood-forming organs an d settle in the bone marrow as self-renewable HSC. The model is valid in the mouse as shown by in vitro cultures of cells obtained from embr yo structures or YS separated prior to circulation. This approach, as well as restoration of irradiated adults, demonstrates that YS precurs ors have a limited potential compared with embryo precursors. The emer gence of hematopoietic precursors in both YS and embryos is closely li nked to the emergence of the endothelial network and is restricted to the mesoderm layer associated with endoderm.