The process of blood cell formation is called hematopoiesis. Hematopoiesis starts early during development, on the seventh day of embryogenesis. This phase of hematopoiesis is called the primitive wave, wherein the extraembryonic yolk sac allows the production of erythroid cells and endothelial cells from a common precursor called hemangioblast. The erythroid cells provide oxygen to support the growth of the rapidly dividing embryo. Hemangioblasts later develop into hematopoietic stem cells or HSCs that can produce all types of blood cells due to their multipotent nature. The HSCs then migrate to the liver during the fetal development stage. In adults, hematopoiesis occurs in the bone marrow of organisms.
Inside the bone marrow, there are two populations of HSCs: dormant or quiescent HSCs and the active or primed HSCs. Dormant HSCs are non-proliferative and remain attached to the inner surface of the bone. On the other hand, active HSCs rapidly proliferate in response to vascular endothelial growth factors and are present in the central marrow region.
HSCs migrate from the bone marrow into the circulating blood to reach the different target tissues. During this migration, HSCs lose cell adherence molecules and chemokine receptor CXCR4. HSCs may return to the bone marrow or reach the target tissue by the process called HSC homing. Homing to bone marrow is essential to regulate HSC homeostasis and maintain the HSC pool through proliferation. During homing, HSCs gain cell adherence molecules and chemokine receptor CXCR4 that help them bind and anchor the target tissue following chemokine gradients.
