Bone marrow

The soft fatty tissue that is found in bone cavities; it may be red or yellow. Red bone marrow is present in all bones at birth and is the factory for most of the blood cells. During the teens, red bone marrow is gradually replaced in some bones by less active yellow marrow. In adults, red marrow is confined chiefly to the spine, sternum, (breastbone), ribs, pelvis (hip-bones), scapulae (shoulderblades), clavicles (collarbones), and bones of the skull. Stem cells within the red marrow are stimulated to form blood cells by the hormone erythropoietin. Yellow marrow is composed mainly of connective tissue and fat. If the body needs to increase its rate of blood formation, some of the yellow marrow will be replaced by red. Sometimes marrow fails to produce sufficient numbers of normal blood cells, as occurs in aplastic anaemia (see anaemia, aplastic) or when marrow has been displaced by tumour cells. In other cases, marrow may overproduce certain blood cells, as occurs in polycythaemia and leukaemia.