| Abstract: |
The origin and potentiality of the hematopoietic stem cell has been debated with two main concepts: the monophyletic hypothesis recognizing a common stem cell for all lympho-myeloid lineages; and the polyphyletic hypothesis recognizing a variety of distinct stem cells. The current consensus recognizes a pluripotential lympho-myeloid stem cell, and a hierarchy of progressively lineage-restricted progenitor cells with a major bifurcation at the level of the common lymphoid and common myeloid progenitor. In mammalian systems, hematopoiesis is clearly initiated in the YS, and subsequently in the aorta-gonad-mesonephros (AGM) region in association with the dorsal aorta and vitelline veins where the first stem cells arise with the capacity to engraft adult recipients. There has been extensive debate as to the role of the yolk sac (YS) in mammalian hematopoietic ontogeny with the site viewed as the initial source of stem cells that then migrated into the embryo, and, the other extreme that views the YS as a transient source of primitive erythroid progenitors, with definitive generation stem cells arising exclusively in the AGM region. There is increasing evidence that stem cells arise independently in both sites, and that both contribute to the subsequent colonization of the fetal liver. A number of studies have revived the concept of a YS origin of intraembryonic hematopoiesis in murine development. In murine ontogeny, primitive macrophage progenitors appear in the proximal region of the egg cylinder associated. © 2009 Copyright © 2009 Elsevier Inc. All rights reserved. |
