What is Hematopoiesis?

Definition of Hematopoiesis

Hematopoiesis. © McGill Molson Medical Informatics Project. Used With Permission.

How Your Blood Cells Are Formed

Hematopoiesis (hē-mə-tō pȯi-ˈē-səs) is the process by which all of your blood cells are formed, develop and mature into their final adult types. The term refers to the pathways or tracks of blood cell development, beginning with what’s known as a hematopoietic stem cell, going through a series of steps to arrive at the final product—a mature blood cell, whether it’s a red blood cell, a white blood cell such as a lymphocyte, or some other type of blood cell.

Other terms for this process of blood cell formation include hematogenesis, hemogenesis, and hemopoiesis.

Where Does This Happen?

The sites of blood cell production depend on whether you are talking about a baby, still in its mother’s womb or after birth, in infancy and throughout adulthood. After birth, the main site of hematopoiesis is the bone marrow. In utero, however, a developing child uses a variety of sites for hematopoiesis, including the liver, spleen, thymus, lymph nodes, as well as the bone marrow.

What is Hematopoietic?

You might come across the word hematopoiesis in a number of different scenarios. It can be used as an adjective, as in “hematopoietic stem cell transplantation” or “hematopoietic malignancies.” In the former, donated blood-forming cells are given to those in need, so that they can make new blood cells. In the latter, hematopoietic malignancy refers to cancer of the blood-forming cells.

Hematopoietic malignancies include leukemia, lymphoma and myeloma. “Extramedullary hematopoiesis” is the formation of blood cells outside the bone marrow, in cites such as the spleen, liver, and lymph nodes; at birth, the site of new blood cell formation has shifted to the bone marrow, so from infancy to old age, extramedullary hematopoiesis can be a sign of disease.

What Happens in Hematopoiesis?

Our bodies produce blood cells continuously from the womb to old age. Millions of blood cells are replaced each day as they live out their lifespans. Different types of cells have different life spans. For instance, in healthy adults, red blood cells normally live about 100 to 120 days. Your blood system has more than 10 different kinds of blood cell types, each performing an essential task. Within the bone marrow there are hematopoietic stem cells (HSC) also called pluripotent hematopoietic stem cells (PHSC) that give rise to all of the different types of blood cells.

While some of these cells remain undifferentiated, others differentiate to be the progenitors or precursors of different cell lines. There are two main branches of the family tree: myeloid, which refers to red cells and certain kinds of white cells; and lymphoid, which refers to certain other kinds of white blood cells.

Circulating blood is composed of cells, cell products, and fluids. The cell-based substance of your blood can be described as follows:

·       White blood cells (leukocytes): These include lymphocytes, monocytes and polymorphonuclear white cells that provide our bodies with protection from infection. White blood cells are key components of our immune system that help destroy invaders using a variety of tactics, including the production of antibodies that stick to the invaders. Problems with white blood cells can lead to infection.

·       Red blood cells: These cells contain the hemoglobin that gives your blood its red color and carries oxygen to the cells and tissues in your body. Deficiency of red blood cells can lead to anemia, with symptoms such as fatigue, weakness and intolerance to exercise.

·       Platelets: Megakaryocytes in the bone marrow are the monster cells that produce little packages of cellular material (platelets) that help control bleeding after injury. Platelet deficiencies can lead to easy bruising and trouble with bleeding.

HSCs have the ability to divide and create other HSCs, or to commit into one of several differentiation pathways—or different branches of the blood cell family tree. These pathways eventually result in the production of a single type of blood cell. If the HSC commits to producing mature blood cells, it will undergo several (usually five or more) cell divisions before becoming that cell. Every time the cell divides, it takes on more and more of the characteristics of the adult cell it will become. In other words, it becomes more differentiated or specialized.

Stimulating the body to produce more new blood cells—a sort of artificial hematopoiesis—can be helpful in certain situations. For instance, sometimes the bone marrow is stimulated in advance of a planned cancer therapy, when profound suppression of the blood-forming cells in the marrow is expected.

Which Cancers Arise from Blood-forming Cells?

Like any cell, the HSCs can undergo a mutation that leads to dysfunctional or malignant cells being produced rather than healthy cells. Depending on what stage of differentiation the cell is in when it makes this transformation, it gives rise to different types of disorders: myeloproliferative disorders, leukemias, lymphomas and myelomas.

An abnormal younger cell type may be referred to as a ‘blast.’ Blasts in patients with leukemia can suggest the cancerous transformation occurred in a blood-forming cell that was at the earlier stage of development. If the predominant cells in a leukemia or lymphoma are more mature types, this indicates the cancerous transformation happened to a more mature cell, or a cell that was closer to the final adult stage.

In lymphoma, there can be different lymphomas that reflect all different stages of lymphocyte development, including the developmental paths for B-cells and T-cells; thus, there are B-cell lymphomas, T-cell lymphomas, and even Natural Killer T-cell lymphomas.

Hematopoietic Cell Transplantation - Bone Marrow Transplants

The treatment of leukemia, lymphoma and other blood cancers may involve a transplant of hematopoietic stem cells. These can be your own cells, harvested from your bone marrow (autologous), or from a donor (allogenic). Techniques used to obtain healthy blood-forming cells from the donor vary, but the transplant itself is a simple transfusion as the hematopoietic stem cells migrate from the blood into the bone marrow.

Extramedullary Hematopoiesis

This is the term used for blood cell production that occurs outside of the bone marrow. It can be seen in chronic anemia, with production in the liver, spleen and sometimes in the lymph nodes. At other times, there are malignant hematopoietic cells located in areas outside of the bone marrow.

Sources

Williams, L. Comprehensive Review of Hematopoiesis and Immunology: Implications for Hematopoietic Stem Cell Transplant Recipients. In Ezzone,S. (2004) Hematopoietic Stem Cell Transplantation: A Manual for Nursing Practice. Oncology Nursing Society. Pittsburg, PA (pp.1-13).

Michael A. Rieger and Timm Schroeder Hematopoiesis. Cold Spring Harbor Perspective in Biology, 2012, Cold Spring Harbor Laboratory Press.

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