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The Immune System / The Bone Marrow
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Bone marrow is the 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. More information can be found by reviewing the section on blood.
Erythropoiesis
Erythropoiesis, or the formation or red blood cells, constitutes 10-30% of hemopoietic cells in the bone marrow. The mature erythrocyte is derived from the stem cell, which differentiate to erythroid colony forming cells (BFU-E, CFU-E), and next to the proerythroblast, the first morphologically recognizable cell of the series. The proerythroblast matures to the basophilic normoblast, then the polichromatic normoblast, where synthesis of hemoglobin is started. At the end, the polychromatophilic normoblast matures to the orthochromatic normoblast. The orthochromatic normoblast loses its nucleus and developes into reticulocyte, which after 2-4 days develops into mature erythrocyte. The mature erythrocyte stays for ca. 4 months in the blood.
Granulopoiesis
Granulopoiesis constitutes 40-80% of the haemopoietic cells in the normal bone marrow. The myeloblast is the first recognizable cell of the granulopoiesis. The myeloblast enlarges and differientiates to the promyelocyte. At this stage specific granules begin to appear (eosinophilic granules in the eosinophilic leucocytes, basophilic granules in the basophilic granulocytes and neutrophilic granules in the neutrophilic leucocytes). The promyelocytes mature and become the myelocytes and then metamyelocytes. In the end of the maturation process metamyelocytes become band neutrophil and then segmented neutrophil. These cells move from the marrow to the blood. The majority of granulopoiesis in the normal marrow consists of neutrophilic leucocytes.The percentage of eosinophilic leucocytes and basophilic leucocytes should not exceed 5% and 1% respectively.
Monocytes
Monocytes and their precursors constitute a small percentage of the hemopoietic cells (less than 2%). Usually they appear as mature cells (monocytes), rarely precursors of the monocytes (monoblasts, promonocytes) are observed.
Lymphopoiesis
Lymphopoiesis constitute 5-20% of the bone marrow cells. The lymphocytes - as all hemopoietic cells are derived for the stem cell, which differentiates into cells common for the two lymphocytic series (B and T-cells). The cell divides into precursors pro-B and pre-T cells, which develop into the stage of the mature B- and T-lymphocyte. After antigen exposition the mature lymphocytes differantiate into intensive proliferative lymphocytes. They are capable of synthesis immunoglobulins and immunological memory cells.
Diseases of the Bone Marrow
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. 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.
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.
Related Links
1-Up Info
Discusses the anatomy of the soft tissue that fills up the spongy interiors of animal bones and tells about the diseases of the marrow. Read on bone marrow transplant.
Internet Atlas of Hematology
Includes over 200 photographs depicting microscopic images of cells in the peripheral blood and the bone marrow with detailed descriptions.
Bone & Marrow Information Network
Newsletter features articles about current research. It also sells educational materials geared to children.
Bone Marrow Donors Worldwide
The BMDW is a voluntary collaborative effort of bone marrow donor registries and cord blood banks with the goal to provide centralised and anonymous information on blood typing and other relevant data of unrelated bone marrow donors and cord blood units and make this information easily accessible to the physicians of patients in need of a hemopoietic stem cell transplant.
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