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.

Bone marrow (or medulla ossea) is the soft tissue found in the hollow interior of bones. In adults, marrow in large bones produces new blood cells.

Marrow types

There are two types of bone marrow: red marrow (also known as myeloid tissue) and yellow marrow. Red blood cells, platelets and most white blood cells arise in red marrow; some white blood cells develop in yellow marrow. The color of yellow marrow is due to the much higher number of fat cells. Both types of bone marrow contain numerous blood vessels and capillaries.

At birth, all bone marrow is red. With age, more and more of it is converted to the yellow type. Adults have on average about 2.6kg (5.7lbs) of bone marrow, with about half of it being red. Red marrow is found mainly in the flat bones such as hip bone, breast bone, skull, ribs, vertebrae and shoulder blades, and in the cancellous ("spongy") material at the proximal ends of the long bones femur and humerus. Pink Marrow is found in the hollow interior of the middle portion of long bones.

In cases of severe blood loss, the body can convert yellow marrow back to red marrow in order to increase blood cell production.

Types of stem cells

Bone marrow contains two types of stem cells:

• Hematopoietic stem cells give rise to the three classes of blood cell that are found in the circulation: white blood cells (leukocytes), red blood cells (erythrocytes), and platelets (thrombocytes).

• Mesenchymal stem cells are found arrayed around the central sinus in the bone marrow. They have the capability to differentiate into osteoblasts, chondrocytes, myocytes, and many other types of cells. They also function as "gatekeeper" cells of the bone marrow.

Diseases involving the bone marrow

The normal bone marrow architecture can be displaced by malignancies or infections such as tuberculosis, leading to a decrease in the production of blood cells and blood platelets. In addition, cancers of the hematologic progenitor cells in the bone marrow can arise; these are the leukemias.

To diagnose diseases involving the bone marrow, a bone marrow aspiration is sometimes performed. This typically involves using a hollow needle to acquire a sample of red bone marrow from the crest of the ilium under general or local anesthesia.

Exposure to radiation or chemotherapy will kill many of the rapidly dividing cells of the bone marrow and will therefore result in a depressed immune system. Many of the symptoms of radiation sickness are due to damage to the bone marrow cells.

Donation and transplantation of bone marrow

Main article: Hematopoietic stem cell transplantation

It is possible to take hematopoietic stem cells from one person and then infuse them into another person (Allogenic), or into the same person at a later time (Autologous). If donor and recipient are compatible, these infused cells will then travel to the bone marrow and initiate blood cell production.

The stem cells are harvested directly from the red marrow in the crest of the ilium, usually under general anesthesia. Contrary to popular belief, it is a minimally invasive outpatient procedure with only minor discomfort. Another option is to administer certain drugs that stimulate the release of stem cells from the bone marrow into circulating blood. An IV is inserted into the donor's arm and the stem cells are filtered out of the blood. The procedure is similar to donating blood or platelets.

Transplantation from one person to another is performed in severe cases of disease of the bone marrow: the patient's marrow is first killed off with drugs or radiation, and then the new stem cells are introduced.

Before radiation therapy or chemotherapy in cases of cancer, some of the patient's hematopoietic stem cells are sometimes harvested, and later infused back when the therapy is finished to restore the immune system.

Bone marrow as a food

Though once used in various preparations, including pemmican, bone marrow for human consumption in America has recently fallen out of favor as a food. Now, it is commonly used only as a flavoring for soups and sauces, although dishes with intact bone marrow can still be found in some European restaurants. Bone marrow is a source of protein and high in monounsaturated fats. These fats are known to decrease LDL cholesterol levels resulting in a reduced risk of cardiovascular disease, prompting some to make bone marrow a dietary staple. The actual health effects of the addition of bone marrow to the diet remain unclear.

See also

• Leukemia
• Bone marrow transplant
• Bone marrow examination
• Aplastic anemia
• Scotch Broth
• Osso buco

External links

• Bioweb at UWLAX Microscopic slides of bone marrow, with explanations
• Marrow Balls
• Tostadas de tuetano

Immune system: Lymphatic system (Lymph, Lymphocytes)
Primary	Bone marrow - Thymus (Hassall's corpuscles)
Secondary: Spleen (blood)	Hilum - Trabeculae - Red pulp (Cords of Billroth, Marginal zone) - White pulp (Periarteriolar lymphoid sheaths)
Secondary: Lymph nodes (extracellular fluid)	Subcapsular sinus - Paracortex - Lymph vessels - High endothelial venules
Secondary: MALT (mucosa)	GALT - Peyer's patches

Bone and cartilage
Cartilage	chondroblast, chondrocyte, perichondrium, types (hyaline, elastic, fibrous), fibrocartilage callus, metaphysis
Bone	ossification (intramembranous, endochondral, epiphyseal plate), cycle (osteoblast, osteoid, osteocyte, osteoclast), types (cancellous, cortical), regions (epiphysis, metaphysis, diaphysis), structure (osteon/Haversian system, Haversian canals, endosteum, periosteum, Sharpey's fibres, enthesis, lacunae, canaliculi, trabeculae, medullary cavity, bone marrow), shapes (long, short, flat, irregular, sesamoid)

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