Hematopoietic stem cells (HSCs) are the blood cells that give rise to all the other blood cells.

They give rise to the myeloid (monocytes and macrophages, neutrophils, basophils, eosinophils, erythrocytes, megakaryocytes/platelets, dendritic cells), and lymphoid lineages (T-cells, B-cells, NK-cells). The definition of hematopoietic stem cells has changed in the last two decades. The hematopoietic tissue contains cells with long-term and short-term regeneration capacities and committed multipotent, oligopotent, and unipotent progenitors. HSCs constitute 1:10.000 of cells in myeloid tissue.

HSCs are a heterogeneous population. Three classes of stem cells exist, distinguished by their ratio of lymphoid to myeloid progeny (L/M) in blood. Myeloid-biased (My-bi) HSC have low L/M ratio (>0, <3), whereas lymphoid-biased (Ly-bi) HSC show a large ratio (>10). The third category consists of the balanced (Bala) HSC for which 3 L/M 10. Only the myeloid-biased and -balanced HSCs have durable self-renewal properties. In addition, serial transplantation experiments have shown that each subtype preferentially re-creates its blood cell type distribution, suggesting an inherited epigenetic program for each subtype.

HSC studies through most of the past half century and have led to a much deeper understanding. More recent advances have resulted in the use of HSC transplants in the treatment of cancers and other immune system disorders.[1]

HSCs are found in the bone marrow of adults, with large quantities in the pelvis, femur, and sternum. They are also found in umbilical cord blood and, in small numbers, in peripheral blood.[citation needed]

Stem and progenitor cells can be taken from the pelvis, at the iliac crest, using a needle and syringe.[citation needed] The cells can be removed a liquid (to perform a smear to look at the cell morphology) or they can be removed via a core biopsy (to maintain the architecture or relationship of the cells to each other and to the bone).[citation needed]

In order to harvest stem cells from the circulating peripheral, blood donors are injected with a cytokine, such as granulocyte-colony stimulating factor (G-CSF), that induce cells to leave the bone marrow and circulate in the blood vessels.[citation needed]

In mammalian embryology, the first definitive HSCs are detected in the AGM (Aorta-gonad-mesonephros), and then massively expanded in the Fetal Liver prior to colonising the bone marrow before birth.[2]

As stem cells, HSC are defined by their ability to replenish all blood cell types (Multipotency) and their ability to self-renew.

It is known that a small number of HSCs can expand to generate a very large number of daughter HSCs. This phenomenon is used in bone marrow transplantation, when a small number of HSCs reconstitute the hematopoietic system. This process indicates that, subsequent to bone marrow transplantation, symmetrical cell divisions into two daughter HSCs must occur.

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