As hematopoiesis transitions from the hepatic phase into the bone marrow, it becomes less evident within the fetal spleen. The spleen is not normally a site of hematopoiesis in postnatal life in humans, but it is a rich source of hematopoietic stem cells and can support hematopoiesis in a number of pathologic states. 1. First blood cells are primitive erythroblasts formed in the embryonic yolk sac in a process of extraembryonic hematopoiesis 2. Intraembryonic hematopoiesis then begins in a region along the developing aorta called the aorta-gonads-mesonephros (AGM). Normal Haematopoiesis The bone marrow is the only site of blood production in extrauterine life. The yolk sac and the liver produce blood in the intrauterine life. The two sites have distinct precursors. The yolk sac haematopoiesis, known as primitive haematopoiesis, is transient haematopoiesis that serves the purpose of rapidly providing blood to the growing embryo..
Establishment and maintenance of the blood system relies on self-renewing hematopoietic stem cells HSCs that normally reside in small numbers in the bone marrow niche of adult mammals. This Review describes the developmental origins of HSCs and the site of hematopoiesis in the adult
mechanisms that regulate lineage-specific differentiation. Studies of hematopoiesis provide critical insights of general relevance to other areas of stem cell biology including the role of cellular interactions in development and tissue homeostasis, lineage programming and reprogramming by transcription factors, and stage- and age-specific differences in cellular phenotypes. The blood system serves as a paradigm for understanding tissue stem cells, their biology, and involvement in aging, disease, and oncogenesis. Because mature blood cells are predominantly short lived, stem cells are required throughout life to site of hematopoiesis in the adult
multilineage progenitors and the precursors committed to individual hematopoietic lineages.