Bone marrow

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Bone marrow (or medulla ossea) is the soft tissue found in the hollow interior of bones. It is the place where most new blood cells are produced.

1 Red and yellow marrow
2 Types of stem cells
3 Diseases involving the bone marrow
4 Donation and transplantation of bone marrow
5 Bone marrow as a food
6 See also
7 External links

[edit] Red and yellow marrow

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 2.6 kg 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. Yellow 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.

[edit] 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 cell. They also function as "gatekeeper" cells of the bone marrow.

[edit] 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. 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 biopsy 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 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.

[edit] Donation and transplantation of bone marrow

Main article: Bone marrow transplant

Image:Bone marrow biopsy.jpg

It is possible to take hematopoietic stem cells from one person and then infuse them into another person, or into the same person at a later time. If donor and recipient are compatible, these infused cells will then travel to the bone marrow and initiate blood production. The stem cells can either be harvested directly from the red marrow in the crest of the ilium, usually under general anesthesia because the procedure involves numerous puncturings, or by administering certain drugs that stimulate the release of stem cells from the bone marrow into circulating blood and then filtering the stem cells out of circulating blood.

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.

[edit] Bone marrow as a food

Though once used in various preparations, including pemmican, bone marrow has recently fallen out of favor as a food in some parts of the world. 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 unknown.