BONE MARROW

also called Myeloid Tissue, soft, gelatinous tissue that fills the cavities of the bones. Bone marrow is either red or yellow, depending upon the preponderance of vascular (red) or fatty (yellow) tissue. In humans, the red bone marrow forms all of the blood cells with the exception of the lymphocytes, which are produced in the marrow and reach their mature form in the lymphoid organs. Red bone marrow also contributes, along with the liver and spleen, to the destruction of old red blood cells. Yellow bone marrow serves primarily as a storehouse for fats but may be converted to red marrow under certain conditions, such as severe blood loss or fever. At birth and until about the age of seven, all human marrow is red, as the need for new blood formation is high. Thereafter, fat tissue gradually replaces the red marrow, which in adults is found only in the vertebrae, hips, breastbone, ribs, and skull and at the ends of the long bones of the arm and leg; other cancellous, or spongy, bones and the central cavities of the long bones are filled with yellow marrow. Red marrow consists of a delicate, highly vascular fibrous tissue containing reticular cells, which differentiate into various blood cells. Reticular cells first become precursors or stem cells of various kinds; rubiblasts give rise to the red blood cells (erythrocytes), and myeloblasts become the granulocytes, a type of white blood cell. Platelets, small blood cells involved in clotting, form from giant marrow cells called megakaryoblasts. The new blood cells are released into the sinusoids, large thin-walled vessels that drain into the veins of the bone. In mammals, blood formation in adults takes place predominantly in the marrow. In lower vertebrates a number of other tissues may also produce blood cells, while in reptiles and amphibians some blood cells are formed in the bloodstream itself. In humans the red blood cells and the platelets undergo no further development or growth after leaving the marrow. See also blood cell formation. Because the white blood cells produced in the bone marrow are involved in the body's immune defenses, marrow transplants have been used to treat certain types of immune deficiency. The sensitivity of marrow to damage by radiation and some anti-cancer drugs accounts for the tendency of these treatments to impair immunity. Examination of the bone marrow is helpful in diagnosing certain diseases, especially those related to blood and blood-forming organs, because it provides information on iron stores and the site of blood production. Bone marrow aspiration, the direct removal of a small amount (about 1 ml) of bone marrow, is accomplished by suction through a hollow needle. The needle is usually inserted into the sternum, or breastbone, in adults, and into the upper part of the tibia, the larger bone of the lower leg, in children. The necessity for a bone marrow aspiration is ordinarily based on previous blood studies and is particularly useful in providing information on various stages of immature blood cells. Disorders in which bone marrow examination is of special diagnostic value include leukemia, multiple myeloma, Gaucher's disease, unusual cases of anemia, and other hematological diseases.