2. Bone Marrow and Spleen

Where are blood cells produced?

The three main types of blood cells, red blood cells, white blood cells and platelets, are all produced in red bone marrow.

What is extramedullary hematopoiesis?

In individuals with diseases that destroy the bone marrow, some blood cell production can actually occur in the liver and spleen – this is called extramedullary hematopoiesis, since it occurs outside the medulla (middle) of the bone. The liver and spleen normally produce blood cells in a fetus, before bones are fully formed. Thus, if the bone marrow is compromised, some blood cell production can return to its fetal location.

What is bone marrow and how is it sampled?

Bone marrow is the spongy centre of bones.  Marrow that produces blood cells is called red marrow, and marrow that doesn’t is called yellow marrow, after the colour of the fat that it’s infiltrated with. At birth, the marrow of all bones is the active, ‘red’ marrow.  The red marrow is gradually replaced with yellow marrow such that by adulthood the only significant amounts of red marrow are in the pelvic bones, shoulder bones, ribs, sternum, vertebrae and lower skull. Even in those locations about half of the space of the red marrow is infiltrated by fat. Samples of red bone marrow are most often taken from the pelvis as it’s the largest, safest to access space. There is enough movement of cells between the red marrow of different bones that a sample from one location is typically considered representative of all the red marrow spaces.

How are blood cells made inside the bone marrow?

Inside the red marrow are cells that can become any one of many different blood cell types –  these are types of stem cell. Note though that the stem cells in bone marrow, although they have many potential fates, can’t become ANY cell type – they still have some restrictions. Which fate a cell takes and when it starts to take on that fate are dependent on signals the cell gets from its environment.  The process of a cell that could become many different things becoming a cell with fewer ‘options’ for what it could function as, is called differentiation. The actual process of differentiation involves changes in the way the cell uses the information encodes by its genes, such that different proteins are made in the cell, which in turn trigger a series of changes in the cells behavior. Stem cells can also produce more stem cells through cell division, maintaining a population of stem cells in the bone marrow.

Which stem cells produce which of the blood cells?

There are two main lineages of blood cells: myeloid and lymphoid. Lymphoid cells (B-cells, T-cells, and natural killer cells) are produced from a lymphoid stem cell, while myeloid cells (red blood cells, platelets, and all the white blood cells except for B and T-cells) are produced from a myeloid stem cell. Note that there are several ‘intermediate’ stem cells whose fate is more restricted than that of the cell they differentiated from, but that still have multiple fate options. There are also ‘immature’ cells that have only one fate they could become, but are not yet able to function as they will later on; these immature cells may be called ‘blasts’.

Where do red blood cells go?

Red blood cells circulate for about 120 days, during which time they accumulate damage from having to squeeze through the very small blood vessels. Damaged red blood cells are removed by the spleen.  Blood percolates through a cellular mesh inside of the spleen that’s filled with macrophages, cells that ‘eat’ damaged cells. The spleen also plays a role in the lymphatic system (see lymphocytes section).


Leukemia and Lymphoma Society of Canada. Disease and Support Information. July 11 2013. Available at http://www.llscanada.org/?gclid=COr5jbK34roCFQVgMgodfl4AMQ#/diseaseinformation/. Accessed Nov 2013.

Koury MJ, Lichtman MA. Chapter 4. Structure of the Marrow and the Hematopoietic Microenvironment. In: Prchal JT, Kaushansky K, Lichtman MA, Kipps TJ, Seligsohn U, eds. Williams Hematology. 8th ed. New York: McGraw-Hill; 2010. http://www.accessmedicine.com/content.aspx?aID=6108794. Accessed November 13, 2013.


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