Inside the cavities of cancellous bone is soft, fatty tissue comprised of an irregular network of blood vessels and cell types. This is called bone marrow. There are two types of marrow: red and yellow.
Red marrow contains stem cells, unspecialized cells that can grow into different types of specialized cells. They're responsible for replenishing and replacing cells in the body that have been damaged or lost. (For the whole story on stem cells, give How Stem Cells Work a read.) There are two types of stem cells found in red marrow:
- Hematopoietic stem cells (HSCs). This type of stem cell is responsible for creating billions of new blood cells daily, at a rate of about 8 million every second [source: Houston Museum of Natural Science]. HSCs create every type of blood cell: red blood cells (which carry oxygen throughout the body), white blood cells (which fight infections and kill bacteria) and platelets (which help your blood clot). Marrow stem cells can even produce more marrow stem cells. HSCs can leave the marrow and enter the bloodstream, where the ratio of blood cells to stem cells is about 100,000-to-1 [source: National Institutes of Health].
- Stromal stem cells. This type of stem cell generates bone cells, cartilage, fat cells and connective tissue. Stromal stem cells are being studied for their possible use in repairing spinal cord damage and healing disorders of the lymphatic system.
Yellow marrow is mostly fat, and as we age, it can be found in places where red marrow once resided -- some of the bones in our arms, legs, fingers and toes, for instance. If the body needs more blood cells, yellow marrow can transform back into red marrow and produce them. Some bones have a lot more red marrow than others -- the pelvic bone, the spine's vertebrae and our ribs are all rich with it. The body also stores iron in bone marrow.
Bone marrow can become diseased. Myeloproliferative disorders (MPDs) cause the overproduction of immature cells from the marrow. Disorders such as aplastic anemia and myelodysplastic syndromes (MDS) hinder the marrow's ability to produce enough blood cells.
Several marrow diseases can be treated through stem cell transplants, which introduce healthy stem cells to the patient's body to replace the diseased cells. The traditional way to transplant these stem cells is to extract bone marrow from the donor's hip bone with a syringe and introduce the material into the recipient's body. You don't have to actually experience someone penetrating the process to imagine how unpleasant it is. Increasingly, doctors are harvesting the marrow stem cells from the bloodstream instead, resulting in better stem cell samples for the recipient and less pain and discomfort for the donor.
In the next section, we'll examine some of the bones that help prevent your brain and lungs from sliding down into your socks -- the axial bones.