Ontogeny of the haematopoietic system
Embryonic blood development. Haematopoiesis in mammalian embryos takes place in several tissues where blood cells are generated and/or undergo maturation. The first haematopoietic progenitors are found extra-embryonically around E8.5, in the yolk sac blood islands in close proximity with emerging endothelial cells. Once circulation is established, blood cells colonise other developing haematopoietic organs. Around E10.5, the AGM, placenta, umbilical artery and vitelline artery initiate the generation of blood precursors that, together with yolk sac cells, migrate to the foetal liver rudiment. The foetal liver is the major haematopoietic site where blood progenitors expand and/or mature at E11.5. Finally, the bone marrow is colonised by precursors from the foetal liver before birth and remains the main haematopoietic niche throughout adult life. The true HSC nature of the progenitors generated in each site and their relative contribution to the adult pool is a matter of controversy. UA, umbilical artery; VA, vitelline artery.