| Abstrakt: |
Hematopoiesis starts in the yolk sac blood islands and in a region of dorsal aorta in the early embryo. Interest in the emergence of the hematopoietic stem cells in the embryo proper has increased in last years. In an avian model the primitive erythropoiesis derived from extraembryonic yolk sac is short lived and the evidence that adult hematopoiesis arises from intraembryonic sources has been extended from birds to mammals. Definitive lymphohematopoiesis initiates in the ventral wall of the dorsal aorta in all the vertebrate species. Second major issue in early hematopoiesis is the existence of hemangioblast, a bipotential cell, able to differentiate into a hematopoietic stem cell or a vascular endothelial cell. It has become clear that the emergence of hematopoietic stem cells and hemangioblasts is tightly linked. The rapid commitment of hematopoietic and endothelial lineages in the developing embryo indicates that the distinction between mesoderm and hemangioblast maybe difficult to define. It is, however, possible that the hemangioblast stage is very short lived. As the intraembryonic hemogenic sites start to seed the lymphoid organ rudiments, developmental programs of the homing progenitors are gradually restricted towards downstream lineages. Eventually, thymus, bone marrow, spleen and the avian bursa of Fabricius serve as permissive environments for further maturation, proliferative expansion and selection processes. During these complex events multiple both cell-intrinsic and -extrinsic mediators critically influence the emergence of lymphohemopoietic progeny. [ABSTRACT FROM AUTHOR] |
