Gray's Anatomy illustration of cells in bone marrow.

Anatomy

Marrow types

Section through the femur head, showing the cortex, the red bone marrow and a spot of yellow bone marrow. The white bar represents 1 centimeter.

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. About half of the bone marrow is red. ^[1] Red marrow is found mainly in the flat bones, such as the 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.

Stroma

The stroma of the bone marrow is all tissue that isn't directly involved in the primary function of hematopoiesis. The yellow bone marrow belongs here, and makes the majority of the bone marrow stroma, in addition to stromal cells located in the red bone marrow.

Still, the stroma is indirectly involved in hematopoiesis, since it provides the hematopoietic microenvironment that facilitates hematopoiesis by the parenchymal cells. For instance, they generate colony stimulating factors, affecting hematopoiesis.

Cells that constitute the bone marrow stroma are:

• fibroblasts (reticular connective tissue)
• macrophages
• adipocytes
• osteoblasts
• blood vessels (sinusoid)

Macrophages contribute especially to red blood cell production. They deliver iron for hemoglobin-production.

Bone marrow barrier

The blood vessels constitute a barrier, inhibiting immature blood cells from leaving the bone marrow. Only mature blood cells contain the membrane proteins required to attach to and pass the blood vessel endothelium.

Hematopoietic stem cells may also cross the bone marrow barrier, and may thus be harvested from blood.

Stem cells

The bone marrow stroma contain mesenchymal stem cells (also called marrow stromal cells). These cells are multipotent stem cells that can differentiate into a variety of cell types. Cell types that MSCs have been shown to differentiate into in vitro or in vivo include osteoblasts, chondrocytes, myocytes, adipocytes, and, as described lately, beta-pancreatic islets cells. They can also transdifferentiate into neuronal cells.

Compartmentalization

There is biologic compartmentalization in the bone marrow, in that certain cell types tend to aggregate in specific areas. For instance, erythrocytes, macrophages and their precursors tend to gather around blood vessels, while granulocytes gather at the borders of the bone marrow.

Types of stem cells

Bone marrow contains three types of stem cells:^[2]

• Hematopoietic stem cells give rise to the three classes of blood cells 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.
• Endothelial stem cells

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. The average number of cells in a leg bone is about 440,000,000,000.

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.

Examination

A Wright's stained bone marrow aspirate smear from a patient with leukemia.

Donation and transplantation of bone marrow

Bone marrow harvest

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.

Harvesting

The stem cells are harvested directly from the red marrow in the crest of the ilium, often under general anesthesia. The procedure is minimally invasive and do not require stitches afterwards. Depending on the donor health and reaction to the procedure, the actual harvesting can be an outpatient procedure or requiring 1-2 days of recovery in the hospital. ^[3]. Another option is to administer certain drugs that stimulate the release of stem cells from the bone marrow into circulating blood. ^[4] 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.

It may also be taken from the sternum. The tibia may seem a good source, since it is very superficial. However, except in children, this bone marrow doesn't contain any substantial amount of red bone marrow, but rather only yellow bone marrow. ^[1]

In newborns, stem cells may be retrieved from the umbilical cord.

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; however, Alaskan Natives do continue to eat the bone marrow of caribou and moose. 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 decreased 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.

References

1. ↑ ^{a b c} Semester 4 medical lectures at Uppsala University 2008 by Leif Jansson
2. ↑
3. ↑ National Marrow Donor Program Donor Guide
4. ↑ Mayo Clinic: Bone marrow donation: What to expect when you donate

• Weiss, Leon (1976) "The hematopoietic microenvironment of the bone marrow: An ultrastructural study of the stroma in rats" Anatomical Record 186 (2): 161-184

See also

• Leukemia
• Bone marrow transplant
• Bone marrow examination
• Aplastic anemia
• Osso buco
• John Raymond Hobbs (Professor)

External links

• Marrow Balls
• Tostadas de tuetano
• Asian American Donor Program
• Organ Donation Blog

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