Bone marrow is the soft, spongy tissue found inside bones.
It is the medium for development and storage of about 95 percent of the body's blood cells. The blood cells that produce other blood cells are stem cells . It is the stem cells that are needed in bone marrow transplantation. When a patient's stem cells are defective, immature, or low in number because of life-threatening disease, then chemo or radiation will be used to destroy the bone marrow. Healthy marrow is then transplanted or transfused into the patient's blood stream where it migrates to the bone cavities to begin producing normal blood cells.