Reproductive Capability of Yolk Sac Stem Cells

To the Editor: As it is known, until recently, most people believed that extraembryonic areas (yolk sac, allantois, body stalk) are the sites of origin even for definitive hemopoiesis, and the role of stem cell migration was widely accepted [1, 2]. In the last years, excellent authors working at the Embryological Institute in Nogent-sur-Marne, Paris, France, concluded that stem cells for definitive hemopoiesis arise, independently from extraembryonic sources, inside the animal embryo. Intraembryonic stem cells were identified as dense clusters developing in close association with the abdominal, ventral aortic endothelium, at first on its intravascular aspect, and later on in the surrounding paraaortic mesenchymal tissue, the splanchnopleural meoderm. It was suggested that the discovery is valid even for humans [3-4]. One of the difficulties to definitely prove the validity of these milestone-forming observations lies in the fact that extraembryonic and intraembryonic circulatory pathways fuse before the onset of abdominal aortic/paraaortic stem cell clusters, and so, aortic/paraaortic stem cells may originate from migrating extraembryonic cells, including primordial germ cells (see below). Quite recently, Dieterlen-Lièvre et al. traced back the origin of these cells to the presomite (animal) embryo, i.e., before blood circulation [5].