An accumulation of cells and supporting tissues found within the central cavity of all bones. Yellow marrow consists of fat cells, blood vessels, and a minimal framework of reticular cells and fibres. Red marrow consists of numerous blood cells of all kinds, as well as the substances from which these cells are formed. At birth all bones contain highly cellular red marrow; with increasing age the marrow becomes fattier, until in adults red marrow is present only in the vertebrae, sternum, ribs, flat bones of the skull and pelvis, and upper ends of the femur and humerus. The functions of red marrow are (1) the formation of red blood cells (erythrocytes), blood platelets, granulocytes, and to a lesser extent monocytes and lymphocytes, and (2) the destruction of old (c.120 days), worn-out erythrocytes.
Red and yellow marrow
There are two types of bone marrow: red marrow (also known as myeloid tissue) and yellow marrow. Red blood cells, platelets and most white blood cells arise in red marrow; Red marrow is found mainly in the flat bones such as hip bone, breast bone, skull, ribs, vertebrae and shoulder blades, and in the cancellous ("spongy") material at the proximal ends of the long bones femur and humerus.
In cases of severe blood loss, the body can convert yellow marrow back to red marrow in order to increase blood cell production.
Types of stem cells
Bone marrow contains two types of stem cells:
Hematopoietic stem cells give rise to the three classes of blood cell that are found in the circulation: white blood cells (leukocytes), red blood cells (erythrocytes), and platelets (thrombocytes). Mesenchymal stem cells are found arrayed around the central sinus in the bone marrow.Diseases involving the bone marrow
The normal bone marrow architecture can be displaced by malignancies or infections such as tuberculosis, leading to a decrease in the production of blood cells.
To diagnose diseases involving the bone marrow, a bone marrow biopsy is sometimes performed.
Exposure to radiation or chemotherapy will kill many of the rapidly dividing cells of the bone marrow and will therefore result in a depressed immune system. Many of the symptoms of radiation sickness are due to damage to the bone marrow cells.
Donation and transplantation of bone marrow
It is possible to take hematopoietic stem cells from one person and then infuse them into another person, or into the same person at a later time. If donor and recipient are compatible, these infused cells will then travel to the bone marrow and initiate blood production. The stem cells can either be harvested directly from the red marrow in the crest of the ilium, usually under general anesthesia because the procedure involves numerous puncturings, or by administering certain drugs that stimulate the release of stem cells from the bone marrow into circulating blood and then filtering the stem cells out of circulating blood.
Transplantation from one person to another is performed in severe cases of disease of the bone marrow: the patient's marrow is first killed off with drugs or radiation, and then the new stem cells are introduced.
Bone marrow as a food
Though once used in various preparations, including pemmican, bone marrow has recently fallen out of favor as a food in some parts of the world.
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