Bone Marrow

About the Bone Marrow

Bone marrow is the soft tissue filling the internal cavities of bones and is the main site of blood cell production, a process called hematopoiesis. Red marrow contains hematopoietic stem cells that give rise to red blood cells, the many types of white blood cells, and platelets, producing billions of new cells every day. In newborns nearly all marrow is red, but with age much of it in the long bones is replaced by yellow marrow, composed mainly of fat cells, so that in adults blood formation occurs mostly in the flat bones such as the pelvis, sternum, vertebrae, ribs, and skull. Marrow also contains mesenchymal stromal cells that can form bone, cartilage, and fat. B lymphocytes mature in the marrow, while T lymphocytes leave it to mature in the thymus. Diseases of the marrow include leukemias, myeloma, aplastic anemia, and myelodysplastic syndromes. Samples are obtained by aspiration and biopsy, usually from the back of the hip bone, and bone marrow transplantation is used to treat many blood cancers and marrow failure.