bone marrow, soft tissue filling the spongy interiors of animal bones. Red marrow is the principal organ that forms blood cells in mammals, including humans (see blood). In children, the bones contain only red marrow. As the skeleton matures, fat-storing yellow marrow displaces red marrow in the shafts of the long bones of the limbs. In adults red marrow remains chiefly in the ribs, the vertebrae, the pelvic bones, and the skull. Erythrocytes (red blood cells), platelets, and all but one kind of leukocyte (white blood cell) are manufactured in human red marrow. The marrow releases about 10 million to 15 million new erythrocytes every second, while an equivalent number are destroyed by the spleen.
Diseases of the marrow, such as leukemia or multiple myeloma, or injury to it from metallic poisons can interfere with the production of erythrocytes, causing anemia. A bone marrow biopsy, in which a small sample of bone marrow is obtained by aspiration through a thin needle, may be used to aid in the diagnosis of leukemia, anemia, and other blood disorders, as well as to gain insight on the normal functioning of the cells of the bone marrow.
Bone marrow transplantation is a technique that infuses healthy bone marrow into a patient whose bone marrow is defective. The transplant can be autologous, consisting of bone marrow removed from the patient, treated, and then reinserted, or it can be allogeneic, consisting of healthy bone marrow obtained from a closely related donor, such as a sibling (see transplantation, medical). Bone marrow transplants are most frequently undergone for leukemia, severe forms of anemia, and disorders of the immune system. The major complications are graft-versus-host disease (as a result of allogeneic transplantation) and infections that occur before the transplanted marrow begins to produce leukocytes.