Delineation of niches which support hematopoiesis

[Extract] Hematopoiesis is the process of blood cell formation from self-renewing, multipotent hematopoietic stem cells (HSC) crucial for blood homeostasis of all living organisms [1]. This developmental process has two distinct phases, primitive and definitive hematopoiesis. In mice, the primitive phase of hematopoiesis occurs in the yolk sac at embryonic (E) day 7.5 [2-5]. It results in the production of primitive hematopoietic cells including mainly large, nucleated erythroblasts, some megakaryocytes, and primitive macrophages needed for embryonic growth [2-5]. The definitive phase of hematopoiesis in mice begins in the extra-embryonic yolk sac at E8.25 [2, 6]. Development in this phase is a hierarchical process, with two main lymphoid and myeloid lineages forming from which all blood cells develop. The lymphoid lineage comprises T and B lymphocytes and natural killer cells that arise from a common lymphoid progenitor (CLP). In contrast, the myeloid lineage comprises granulocytes (neutrophils, eosinophils, mast cells, and basophils), monocytes, erythrocytes and dendritic cells (DC), all developing from a common myeloid progenitor (CMP). The multipotent HSC resides at the apex of this hematopoietic hierarchy.