The Essential Functions of Bone Marrow

It is a soft, gelatinous tissue inside the bones. The two types of bone marrow are myeloid tissue or red bone marrow and fatty tissue or yellow bone marrow. Capillaries and other blood vessels fill both types. Red marrow is usually present in the breast bone, skull, ribs, vertebrae, hip bones, shoulder blades, and the ends of long bones. Yellow marrow exists in the cavities of long bones.

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Red bone marrow contains hematopoietic stem cells, which produce blood cells. This type of marrow is one of the major tissues of the lymphatic system -- immature progenitor cells in marrow develop into the various types of white blood cells. Lymphocytes form in red marrow, then travel to the lymphoid organs to mature. The liver, spleen and red marrow destroy old red blood cells.

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Yellow bone marrow contains mesenchymal stem cells or marrow stromal cells, which are responsible for producing fat, cartilage, and bone, and producing some white blood cells. The most common cells in yellow marrow are adipocytes. Yellow-tinged adipocytes are specialized to store fat and their production increases with age.

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The number and type of cells produced by bone marrow vary depending on many factors. Blood loss or anemia may reduce oxygen in the body's tissues. The kidneys counteract decreased oxygen by releasing a hormone to stimulate red blood cell production in red marrow. An infection stimulates increased white blood cell production, while the marrow will produce more platelets in response to bleeding. In extreme cases of blood loss, yellow bone marrow may change to red marrow to create additional red blood cells and platelets.

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Blood vessels within bones act as a barrier to prevent immature blood cells from leaving marrow. Membrane proteins on blood cells attach to the inner walls of blood vessels to enter the bloodstream. These proteins are only present on mature blood cells. Specific types of cells tend to form groups in certain areas of the marrow, the process of compartmentalization. Macrophages, a type of white blood cell, and red blood cells move towards blood vessels, while other types of white blood cells, granulocytes, clump around the borders.

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Bone marrow disease usually relates to problems in stem cell development. Aplastic anemia occurs when red marrow does not make red blood cells. Myeloproliferative disorders occur following overproduction of white blood cells. Inherited bone marrow failure syndromes, IBMFS, are rare genetic blood disorders characterized by a severe inability of bone marrow to produce enough blood cells. Common symptoms of marrow disorders are joint and bone pain, swelling of internal organs, and fatigue or generalized weakness.

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Both radiation and chemotherapy are treatments for cancer and some other illnesses, but side effects can be severe and include a weak immune system, hair loss, nausea, vomiting, and fatigue. Cells divide rapidly to produce billions of cells per day continuously, but cancer cells also divide quickly. Cancer treatment destroys rapidly dividing cells, whether they are bone marrow or tumors.

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Bone marrow examination helps diagnose diseases of the blood or blood-producing organs through evaluation of iron stores and blood production. Blood work is the first step to determine appropriate diagnostic tests. Bone marrow aspiration is the process of removing a small amount of marrow with a hollow needle and suction. Doctors remove marrow from the hip or sternum in adults and the tibia in children. Aspiration provides vital information on immature blood cells and developmental stages.

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Leukemia is the term for various cancers in bone marrow and the lymphatic system. The disease damages hematologic progenitor cells, which are responsible for white blood cell formation in bone marrow. People with leukemia are susceptible to illness because their white blood cells don't form or form incorrectly. Symptoms include fever or chills, weight loss, swollen lymph nodes, an enlarged liver or spleen, excessive bleeding or bruising, and bone pain.

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Bone marrow transplants can treat some immune deficiencies and blood disorders, especially leukemia, because marrow produces the white blood cells vital to the immune system. The treatment removes stem cells from marrow or blood then filters them and gives them back to the donor or a new recipient. This process is also referred to as a stem cell transplant because stem cells usually come from blood instead of marrow. The most dangerous part of a bone marrow transplant is the period before the intravenous return of filtered cells: the recipient's bone marrow must be destroyed before the transfusion so healthy stem cells can replace the progenitor cells responsible for the abnormal white blood cells.

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