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 called 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, his or her bone marrow is destroyed by chemotherapy or radiation. 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.