Development of the Hematopoietic Tissue
In the mouse at 7.5 day post-coitum (DPC), before circulation is established between the embryo
and the yolk sac, multipotent stem cells capable of lymphoid and myeloid differentiation are
found in the Aorta-Gonad-Mesonephros (AGM) region. Isolated AGM cultured in vitro demonstrated
that this region is a source of hematopoietic stem cells, which arise autonomously and
independently from the yolk sac (Dzierzak and Medvinsky, 1995).
In the human embryo, pluripotent hematopoietic progenitors arise from non-hepatic
intraembryonic tissues between 25 and 35 DPC. Nucleated erythroid cells arise in the
yolk sac and circulate through the embryo supplying oxygen and nutrients to the developing
tissues. As the embryo develops this circulation is replaced by the multilineage hematopoiesis
in the fetal liver. Around the 20th week of gestation hematopoiesis is established in the bone
marrow. Progressively, hepatic hematopoiesis decreases and the bone marrow becomes the main
site for formation of the blood cells. After birth, bone marrow is the only site of blood
formation, and liver and spleen may present hematopoietic activity only under pathologic
conditions (metaplasia) (Marshall and Thrasher, 2001; Evans, 1997).
Selected References
Dzierzak, E., et al. (1995) Mouse embryonic hematopoiesis. Trends
Genet. 11, 359-66.
Evans, T. (1997) Developmental biology of hematopoiesis. Hematol.
Oncol. Clin. North Am. 11, 1115-47.
Marshall, C.J., et al (2001) The embryonic origins of human
haematopoiesis. Br. J. Haematol. 112, 838-50.
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