What Is Bone Marrow?

The bone marrow is the spongy part of your bones where oxygen-carrying red bloods, infection-fighting white blood cells, and clot-forming platelets are made. The bone marrow is a soft tissue with many cavities located at the center of bones. It serves as the primary stem cell manufacturer of the body and participates in fat storage and bone remodeling.

Healthy bone marrow is essential to living. Sickle cell anemia, leukemia, lymphoma, and aplastic anemia are some life-threatening diseases that occur when the bone marrow fails to function properly.

Anatomy of the Bone Marrow

The bone marrow consists of bone cavities, a complex network of blood vessels, and two types of marrow—red marrow and yellow marrow.

Red marrow contains blood stem cells that can become red blood cells, white blood cells, or platelets. These hematopoietic (blood-cell forming) stem cells are pluripotent cells—meaning they're able to produce several types of cells). They are responsible for the production and maintenance of your blood cells.

Yellow marrow is made mostly of fat. It mainly consists of mesenchymal stem cells—multipotent cells found in the supporting tissue surrounding the bone marrow called the stroma. Mesenchymal stem cells have the potential to develop into a number of tissues, such as bone, cartilage, and fat.

Function

The primary function of the bone marrow is to manufacture blood cells—red cells, white cells, and platelets. Red blood cells carry oxygen throughout the body, whereas white cells help to fight infections and platelets are necessary for blood to clot.

The cells produced by the bone marrow are:

• Lymphocytes, neutrophils, and monocytes which fight against invading bacteria, viruses, and fungi
• Eosinophils and basophils, which respond to parasitic infections and participate in allergic reactions
• B cells, which form antibodies that fight infection
• T cells, involved in cell signaling and fighting infections
• Natural killer T cells that directly attack cells infected by viruses
• Osteoblasts and osteoclasts, which participate in bone remodeling
• Red blood cells, which carry oxygen
• Platelets involved in blood clotting
• Adipocytes or fat cells

Most of the immune and blood cells in the body are made in the red bone marrow, whereas cells that lead to the formation of bone, cartilage, and fat are made in the yellow marrow. Bone marrow also participates in bone remodeling, the removal of abnormal cells, and the recycling of cell parts used in the formation of new cells.

Your bone marrow makes more than 200 billion new blood cells every day. As you can imagine, it is vitally important to have a properly functioning bone marrow to keep up with the body's demands.

Blood and immune cells constantly undergo processes of renewal and regeneration. For example, red blood cells have a lifespan of about 120 days, so they need to be constantly replaced.

Diseases of the Bone Marrow 

The bone marrow may become damaged or malfunction due to:

• Leukemia, lymphoma, multiple myeloma, or other types of cancer
• Aplastic anemia
• Radiation, chemotherapy, or other medications
• Inherited abnormalities of red blood cells, such as sickle cell disease and thalassemia
• Inherited abnormalities of white blood cells that cause an immune deficiency
• Inherited or acquired abnormalities of platelets that lead to bleeding problems

Many diseases that affect the bone marrow also affect the bone itself, as the bone and marrow work together to regulate bone remodeling.

Bone Marrow Transplants

Bone marrow transplant is the leading treatment, and oftentimes the only curative treatment, for conditions that threaten bone marrow’s ability to function properly. Bone marrow transplants can help jumpstart or regenerate an immune system by increasing the body’s capacity to produce healthy blood cells.

Because of a person’s unique genetic makeup, a matching donor—usually a family member—needs to be found. If a genetic match is confirmed, then bone marrow is harvested and readied for transplant.

A bone marrow transplant may be used to:

• Treat diseases that have caused bone marrow to stop functioning or function abnormally. Blood disorders such as sickle cell anemia; blood cancers such as leukemia and lymphoma; and genetic conditions that limit or stop the bone marrow from making healthy or functional cells may require a transplant. 
• Replace damaged bone marrow. High doses of chemotherapy or radiation can damage the bone marrow. The marrow may also be damaged by viral illnesses or possibly by autoimmune disorders. Healthy bone marrow can replace the unhealthy bone marrow and help jumpstart the immune system. 
• Replace genetically altered bone marrow. Healthy marrow can replace unhealthy, nonfunctional, or largely absent bone marrow in genetic diseases such as Hurler syndrome and adrenoleukodystrophy.

Bone marrow is most commonly harvested from the long bones of the hips and thigh bones, although active marrow can be found inside the spine, shoulder bones, ribs, breastbone, and skull.

When Is a Bone Marrow Transplant Needed?

Malfunctioning bone marrow may present with nonspecific symptoms such as fatigue, easy bruising, unexplained weight loss, or fever. A person may also have recurrent infections.

When symptoms suggest the bone marrow isn't working properly, doctors conduct a complete medical history and physical exam, which may be followed by a series of tests, including a complete blood cell count, which assesses red cells, white cells, and platelets.

If these tests indicate problems, a bone marrow biopsy, genetic screening, and other tests may be performed to further explore a diagnosis.

If cancer or another blood disorder is found to be the cause of malfunctioning bone marrow, chemotherapy, radiation, or both may be used to eliminate the disease. Once the disease is eradicated, a bone marrow transplant may be considered.

Unfortunately, chemotherapy and radiation can sometimes damage the bone marrow and suppress the immune system, simultaneously heightening the potential need for a bone marrow transplant and the risk of its rejection.

Alternative Sources of Stem Cells

There are three other main sources of stem cells that may be used to repopulate the bone marrow.

Peripheral blood stem cells (PBSCs): These stem cells circulate in the veins and arteries of all healthy people. Patients recovering from chemotherapy and healthy people who are treated with certain drugs that stimulate the growth of the bone marrow have relatively large numbers of PBSCs in their blood. PBSCs can be collected and used in certain situations as a source of stem cells for transplantation.

Umbilical cord blood: Stem cells can be found in the placenta of newborn babies once the umbilical cord is cut. These stem cells have been more frequently used in stem cell transplantation.

Embryos: The use of stem cells is controversial. Ethical reasons have therefore limited their use, although more research is being done to explore the future potential of this treatment.