What are Hematopoietic Stem Cells?

Hematopoietic stem cells (HSCs) are cells that can produce all blood type cells. To accomplish this HSCs generate lymphocytes, granulocytes, and red blood cells. Lymphocytes and granulocytes are types of white blood cells.

Hematopoiesis is the process through which blood is continually replenished, as it is needed. Hematopoietic stem cells create the blood cells needed in this process. HSCs also make new stem cells.

HSCs are typically located in bones that contain marrow. They are also found, in very small numbers, in the blood and, in large numbers, in umbilical cord blood. Stem cells, especially HSCs, have the potential to replace damaged cells, such as damaged bone marrow in the case of leukemia. No other type of stem cell is as regularly used for therapy and transplants as HSCs.

Blood cells are produced by blood-producing organs in the bone marrow of certain bones, like the femur, hip, ribs, and sternum. Marrow is responsible for producing up to 70% of all white blood cells, as well as all platelets. Platelets are disc-shaped structures that enable the blood to clot.

Hematopoietic stem cells have the ability to activate while in the bone marrow. They can also enter the blood stream. HSCs are also characterized by the ability to enable their own cell death, or apoptosis, if the cell is damaged or unneeded.

HSCs have been heavily studied since the 1950s and have been used in therapy for patients with immune system disorders, blood disorders, and cancer since the 1960s. These cells can be implanted into other tissues and become other cells, like neurons. While there is great potential for these cells to be used in therapies for degenerative diseases, there are some problems with utilizing them.

Stem cells, HSCs in particular, cannot replicate and differentiate on their own in an artificial environment, like a culture dish or test tube. There is also no way to accurately identify a stem cell from other white blood cells. Hematopoietic stem cells tend to have the same size and shape, as well as the same behavior, as white blood cells.

Identification of these cells is further hampered because there are two types of hematopoietic stem cells, long-term and precursor cells. Long-term cells can renew themselves indefinitely and are found in bone marrow. Precursor cells, also called progenitor cells, can be found in the bone marrow as well but cannot replicate endlessly. For therapeutic purposes, long-term stem cells are far more beneficial.

When a patient has leukemia — a cancer of the bone marrow or other blood-forming organs — he is often given an allogeneic hematopoietic stem cell transplant as treatment. The first step in such a procedure requires the patient to undergo radiation therapy to eradicate his own bone marrow and HSCs. Donor bone marrow is then transplanted into the leukemia patient. After it is implanted, the new HSCs will multiply and differentiate, to replace the cells that were destroyed by radiation.