Stem cells are unspecialized cells from which all specialized cells are derived. Adults, as well as embryos, have stem cells. Stem cells for different kinds of blood cells can be obtained from the bone marrow or, in small numbers, from the blood. Stem cells obtained from fetuses are thought to be best because they are more likely to survive transplantation than those obtained from children or adults. Bone marrow transplantation is one type of stem cell transplantation, because bone marrow contains stem cells that produce more blood cells.

Stem cell transplantation can be used as part of the treatment of leukemia, certain types of lymphoma (including Hodgkin's disease), and aplastic anemia. It can also be used to treat children with certain genetic disorders, including 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 is destroyed by high doses of chemotherapy or radiation therapy used to treat cancers such as breast cancer. Stem cell transplantation may 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 (a procedure called autologous transplantation) or those of a donor (a procedure called 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. Then a doctor removes 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 in 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.

The doctor injects the stem cells 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: Precautions and Adverse Reactions). 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 blood cells). During this time, staff members and visitors must wear masks and gowns and wash their hands thoroughly before entering the room. 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.

What Are Stem Cells? Stem cells are undifferentiated cells. That means that they have the potential to become one of many different kinds of specialized cells. Some stem cells can be triggered to become any kind of cell in the body. Others are already partially differentiated; these stem cells can become, for example, any kind of nerve or glandular 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. Stem cells produce all the cells in the body—over 200 types of cells, including blood, nerve, muscle, heart, glandular, and skin cells. Researchers think that stem cells can be directed to repair or replace cells or tissues damaged or destroyed by such disorders as Alzheimer's disease, Parkinson's disease, diabetes, and spinal injuries. Stem cells can be directed by stimulating the genetic code that causes them to specialize. Stem cells can be obtained from four sources (but other sources may soon be discovered): Embryos: Stem cells are taken from embryos produced in fertility clinics by test tube (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. In the process, the embryos are destroyed. For this reason, the use of stem cells from embryos is controversial. Researchers think that these stem cells have the most potential for producing different kinds of cells and for surviving after being transplanted. Fetuses: After 8 weeks of development, the 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. Children and Adults: The bone marrow and blood of children and adults contain stem cells. These stem cells can produce only blood cells. Currently, the only stem cells used for transplantation are these cells.

Last full review/revision February 2003