After birth and throughout life haemopoiesis takes place in the bone marrow. In the early embryo, blood cells, mainly erythrocytes, arise from blood islands in the yolk sac before more varied cells, including lymphoid and myeloid stem cells and precursors, are derived from the aorto-gonad-mesonephron of the para-aortic splanchnopleure. Fetal haemopoiesis occurs mainly in the liver.
The haemopoietic stem cell (HSC) is the pluripotent progenitor cell from which the cells of the blood and lymphoid systems are ultimately derived. They are capable of self-renewal as well as proliferation and differentiation. Their proper function depends on the microenvironment of the haemopoietic organ in which they develop – the haemopoietic niche. They can migrate to and circulate in the blood, and home into and repopulate the bone marrow. HSCs give rise to lymphoid and myeloid precursors. The myeloid precursors differentiate further into the erythrocyte, granulocyte and thrombocyte lineages that deliver red cells, granulocytes, monocytes and platelets to the circulation. Cell production is tightly controlled through cytokine and humoral loops and can be increased rapidly in response to demand.