Bone marrow is a distinctive class of tissue that fills the cores of larger bones in humans and other animals. Unlike the hard, or compact, tissue that forms the outer shells of bones, the marrow has a malleable, sponge-like texture. It serves an active function in the body by producing all three types of blood cells, as well as lymphocytes, which support the immune system. Transplants are frequently performed in patients whose own marrow has become diseased. Additionally, marrow is a major source of stem cells, which can be harvested for certain medical treatments. Rich in nutrients, bone marrow even makes a highly desirable food source for animals as well as for humans in numerous cultures.
Bones that Contain Marrow
In humans, marrow is found in the interior of most major bones of the body. These include flat bones such as the sternum, skull, and pelvis, as well as most long bones, such as the humerus and the femur. Other smaller bones, by contrast, like those in the spine and lower jaw, contain little or no marrow. Marrow is referred to as a spongy bone. It's infused with blood vessels that supply oxygen and carry away newly created cells.
Types of Marrow
There are two categories of bone marrow: yellow and red. The yellow type mostly contains fat, and serves to provide sustenance and maintain the correct environment for the bone to function. It tends to be located in the center-most cavities of long bones, and is generally surrounded by a layer of red marrow. Red marrow is directly involved in cell production. As a body ages, the quantity of red marrow tends to shrink while the amount of yellow marrow increases, but it tends to have the strongest concentrations in flat bones such as the sternum or ilium.
Function
Immature stem cells, along with extra iron, can be found inside bone marrow. These stem cells wait until weak, unhealthy, or damaged cells need to be replaced, and then differentiate, or become specialized. An undifferentiated stem cell can, for example, turn into a red or white blood cell or a platelet. Likewise, lymphocytes, part of the lymphatic system, are formed in this way. This is how such cells get replaced to keep the body healthy, making healthy bone marrow tissue crucial in fighting pathogens like fungi, bacteria, and viruses.
A number of diseases, often incurable, pose a threat to bone marrow. Put simply, they prevent it from turning stem cells into essential cells. Leukemia, Hodgkin's Disease, and other lymphoma cancers are known to damage the marrow's productive ability and destroy stem cells.
Bone Marrow Transplants and Stem Cell Harvesting
The leading treatment for conditions that threaten the marrow's ability to function is a bone marrow transplant. This procedure typically begins with chemotherapy to eliminate the compromised marrow. A matching donor must then be found; in most cases, this is a close family member. A needle is usually used to extract the donor's red marrow, often from one of the pelvic bones. The red marrow is then injected into the patient's bloodstream. With luck, the donation will "take," and make its way into the central shaft of larger bones to restore stem cell function.
Through a similar procedure, stem cells themselves can be harvested for certain cancer treatments, as well as for ongoing medical research into other potential medical uses. Stem cells may either be directly extracted, in the same way as they are for bone marrow transplants, or medication can be given that stimulates the marrow to release the cells into the bloodstream. In the latter case, after blood is drawn from the donor, the stem cells are then filtered out.
As a Food Source
The high concentration of fat, as well as minerals including iron, make bone marrow an eagerly sought food source. Many species of animals chew on large bones of their prey, even after the meat is gone, in order to crack them open and gain access to the treat held within. Likewise, it is considered a delicacy in a variety of human cultures around the world, ranging from parts of Europe to Asia and the Americas.
