What Is Bone Marrow?

This section explains what bone marrow is, why it is important for life, and what happens when it is defective.  Additionally, this section introduces and defines keywords that doctors and scientists often use when talking about bone marrow.  These words are bolded in red and italicized, and a short glossary can be found at the end of the section.

What is bone marrow?

Bone marrow is a large and diverse collection of cells that compose a jelly-like tissue found on the inside of bones in the human body. In total, all of the bone marrow in the average adult weighs 5.7 pounds.  Many small blood vessels, called capillaries, run through this tissue and exchange cells between the bloodstream and the marrow surrounding it.  This exchange allows marrow to do one of its primary jobs: keeping the blood well-stocked with all of its necessary cells and supplies.

Bone marrow is made up of many different components, which are visible in this diagram.

There are two main types of marrow with very different functions: red marrow and yellow marrow.

Red marrow – Red marrow is found in many of the large, flat bones of the body, such as the hip, ribs, and skull, and is the predominant type of marrow that humans are born with.  It is involved only in the process of making new blood cells (called hematopoesis). Therefore, it contains cells that are at all stages of development.  The three main types of blood cells produced here are red blood cells (involved in the transportation of oxygen from the lungs to the rest of the body – it is why blood appears red-colored), white blood cells (involved in defending the body from outside intruders, infections and diseases – pus is mostly white blood cells), and platelets (involved in healing wounds that stop bleeding and repair the blood vessels when the vessels are broken – they are invisible to the naked eye).

Yellow marrow – Yellow marrow is found in the centers of many of the long bones of the body (such as the femur that runs through the thigh).  It is yellow because it is composed almost entirely of fat cells.  As humans grow older, the amount of yellow marrow in their bones increases, as their need to store fat increases.  If a person begins to starve, he or she can consume this supply of fat cells in order to stay alive for longer.  Additionally, in times of emergency, when the red marrow is not capable of producing enough new blood cells, yellow marrow can be converted back into red marrow.

In this cross-section of a bone, we can see the two types of bone marrow: red marrow and yellow marrow. The bar in the upper left corner provides a scale of 1 centimeter.

Scientists have been studying bone marrow for a very long time in order to find out how it is able to generate new blood cells.  They have found that red marrow contains a large number of cells that do not have a determined identity and can be converted into many different other types of cells.  These cells are called stem cells, and bone marrow contains three types: hematopoietic stem cells, mesenchymal stem cells, and endothelial stem cells.

Hematopoietic stem cells – these stem cells are multipotent, meaning that they can be converted into several, but not unlimited, different cell types.  These cells form many of the cells that can be found flowing through your blood vessels.  The majority of these cells need to be replenished frequently because they only survive for a few weeks.  Outside of the bone marrow, these stem cells can also be found the blood of the umbilical cord, and in small amounts in the adult blood stream.

Mesenchymal stem cells – these cells are found primarily in the centers of long bones, along with the yellow marrow.  These cells are also multipotent, and can turn into bone cells, cartilage cells, and fat cells.  In some cases, they can also be turned into brain cells (called neurons).

Endothelial stem cells – these multipotent stem cells are found in lower quantities throughout marrow and blood flow, and can turn into the cells that line the walls of blood vessels, allowing for the creation of new blood vessels by branching off of existing ones.

Bone marrow diseases

Because bone marrow is responsible for generating new blood cells and storing vital nutrients in case of emergency, a person cannot survive without functioning marrow.  There are numerous diseases in which the bone marrow does not function properly.  Some prominent examples include:

Leukemia – cancer in the white blood cells that grows and spreads rapidly.  The cancerous cells do not function properly, exposing a person to frequent and severe infection.  Additionally, the cancerous cells can prevent the production of all three types of blood cells, causing the person to have increased bleeding and insufficient oxygen delivery.

Aplastic anemia – condition in which the stem cells that are specified to produce red blood cells no longer function properly, causing a decrease in the number of red blood cells in circulation and an inability to deliver sufficient oxygen throughout the body.  This condition can be caused by physical injury, exposure to certain chemicals, or by rare genetic errors.  However, the cause of many cases is currently unknown.

Myeloproliferative disease – disease in which the blood-forming stem cells prefer to form into only one of the three types of blood cells, causing overproduction of one type of blood cell and underproduction of the other two types.  Each type of overproduction has different negative implications.

Relevance for sickle-cell disease

If sickle cell disease is not specifically a bone marrow disease, then why is bone marrow transplantation a potential treatment option?

When hematopoietic stem cells are turning into red blood cells, they follow directions that are encoded in individual genes, in their DNA.  For people who have sickle cell disease, there is an error in the directions, called a mutation, that encode the creation of hemoglobin.  Therefore, all of the red blood cells produced from the stem cells of a person with sickle cell disease will carry this error.  One method to enable a person to produce red blood cells that do not have the sickle cell hemoglobin defect is to replace their blood-producing stem cells with those from a person who does not have sickle cell disease.  For this reason, bone marrow transplantation is frequently called hematopoietic stem cell transplantation. 

Glossary

Hematopoiesis – the process of producing the three types of blood cells: red blood cells, white blood cells, and platelets.
Stem cells – cells that can self-renew and do not have a particular identity.  They can form a different range of specific cells depending on what type they are.
Multipotent – a characteristic of stem cells where they can only produce a small range of different cells.
Hematopoietic stem cells – stem cells found in bone marrow that produce the three types of blood cells.
Mesenchymal stem cells – stem cells found in bone marrow that produce bone cells, cartilage cells, fat cells, and neurons.
Endothelial stem cells – stem cells found in bone marrow that produce new blood vessels.
Hematopoietic stem cell transplantation – a type of bone marrow transplantation where only the blood-producing stem cells are targeted for replacement.  Increasingly common in sickle cell disease patients for whom the other bone marrow components are functioning normally.

More resources

http://www.nlm.nih.gov/medlineplus/bonemarrowdiseases.html The Medline Plus page for bone marrow diseases.  Medline Plus is an information service run by the US National Institute of Health (NIH).  The website provides basic information on bone marrow and its related diseases, as well as links to other pages explaining disease symptoms and treatment options, glossaries, scientific research articles, and clinical trial information.

http://www.marrow.org/ Website of the National Marrow Donor Program.  Provides information on bone marrow related diseases and treatments, how to become a bone marrow donor, information about the transplantation process, and support for patients (and families of patients) who require bone marrow transplantation.