Stem cells are unspecialized cells from which other more specialized cells can be derived. Stem cells obtained from embryos and fetuses are thought to be best because they are more likely to survive transplantation than those obtained from children or adults. However, adults also have some kinds of stem cells. Stem cells for different kinds of blood cells can be obtained from the bone marrow (bone marrow transplantation) or, in small numbers, from the blood.

What Are Stem Cells?
Stem cells are undifferentiated cells that have the potential to become one of 200 types of cells in the body, including blood, nerve, muscle, heart, glandular, and skin cells. Some stem cells can be triggered to become any kind of cell in the body. Others are already partially differentiated and can only become, for example, any kind of nerve cell. Stem cells divide, producing more stem cells, until they are triggered to specialize. Then as they continue to divide, they become more and more specialized until they lose the ability to be anything but one kind of cell. Researchers hope to use stem cells to repair or replace cells or tissues damaged or destroyed by such disorders as Alzheimer's disease, Parkinson's disease, diabetes, and spinal injuries by triggering the genes that cause the stem cells to specialize. Researchers are so far able to obtain stem cells from four sources: Embryos: During in vitro fertilization, sperm from the man and several eggs from the woman are placed in a culture dish. The sperm fertilizes the egg and the resulting cell divides, forming an embryo. Several of the healthiest-looking embryos are placed in the woman's uterus. The rest are discarded or frozen to be used later if needed. Stem cells can be obtained from the embryos that are not used. Because the embryos then lose the ability to grow into a complete human being, the use of stem cells from embryos is controversial, but researchers think that these stem cells have the most potential for producing different kinds of cells and for surviving after transplantation. Fetuses: After 8 weeks of development, an embryo is called a fetus. Stem cells can be obtained from fetuses that have been miscarried or aborted. Umbilical Cord: Stem cells can be obtained from the blood in the umbilical cord or placenta after a baby is born. These stem cells can produce only blood cells and have been used for transplantation only in recent years. Children and Adults: The bone marrow and blood of children and adults contain stem cells. These stem cells can produce only blood cells. These stem cells are most often used for transplantation.

Stem cell transplantation can be used as part of the treatment for blood disorders such as leukemia, certain types of lymphoma (including Hodgkin lymphoma), aplastic anemia, thalassemia, sickle cell anemia, and some congenital metabolic or immunodeficiency disorders (such as chronic granulomatous disease). Certain types of stem cells can also be used as transplants for people whose bone marrow has been destroyed by the high doses of chemotherapy or radiation therapy used to treat some cancers. Stem cell transplantation may some day become useful for treating other disorders, such as Parkinson's disease and Alzheimer's disease, in which the transplanted stem cells can become brain cells.

Stem cells may be the person's own cells (autologous transplantation) or those of a donor (allogeneic transplantation). When the person's own stem cells are used, they are collected before chemotherapy or radiation therapy because these treatments can damage stem cells. They are injected back into the body after the treatment.

For bone marrow transplantation, the donor is usually given a general anesthetic. Doctors then remove marrow from the donor's hip bone with a syringe. Removal of bone marrow takes about 1 hour.

Sometimes stem cells from adults are obtained from blood during an outpatient procedure. First, the donor is given a drug that causes the bone marrow to release more stem cells into the bloodstream. Then blood is removed through a catheter inserted in one arm and is circulated through a machine that removes stem cells. The rest of the blood is returned to the person through a catheter inserted in the other arm. Usually, about six 2- to 4-hour sessions during a period of 1 to 2 weeks are required to obtain enough stem cells. Stem cells can be preserved for later use by freezing them.

Stem cells are injected into the recipient's vein. The injected stem cells migrate to and begin to multiply in the recipient's bones and produce blood cells.

Stem cell transplantation is risky because the recipient's white blood cells have been destroyed or reduced in number by chemotherapy or radiation therapy. As a result, the risk of infection is very high for about 2 to 3 weeks—until the donated stem cells can produce enough white blood cells to protect against infections.

Another problem is that the new bone marrow obtained from another person may produce cells that attack the recipient's cells, causing graft-versus-host disease (see Blood Transfusion: Graft-versus-host disease). Furthermore, the original disorder may recur.

The risk of infection can be reduced by keeping the recipient in isolation for a period of time (until the transplanted cells begin to produce white blood cells). During this time, everyone entering the room must wear masks and gowns and wash their hands thoroughly. Antibodies isolated from the donor's blood may be given intravenously to the recipient to help protect against infection. Growth factors, which stimulate the production of blood cells, can help reduce the risk of infection and graft-versus-host disease.

Recipients of a stem cell transplant usually remain in the hospital for 1 to 2 months. After discharge from the hospital, follow-up visits are necessary at regular intervals. Most people need at least 1 year to recover.

Last full review/revision September 2008 by Martin Hertl, MD, PhD; James F. Markmann, MD, PhD; Paul S. Russell, MD; Heidi Yeh, MD