ACS :: Sources of Stem Cells for Transplants

There are 3 possible sources of stem cells to use for transplants: bone marrow, the bloodstream (peripheral or circulating blood), and umbilical cord blood from newborns.

Bone marrow is the spongy tissue in the center of bones. Its main job is to make blood cells that circulate in your body and immune cells that fight infection.

Bone marrow was the first source used for stem cell transplants because it has a rich supply of stem cells. The bones of the pelvis (hip) contain the most marrow and thus have large numbers of stem cells. For this reason, cells from the pelvic bone are used most often for a bone marrow transplant. Enough marrow must be removed to collect a large number of healthy stem cells.

For a bone marrow transplant, the donor gets general anesthesia (drugs are used to put the patient into a deep sleep). Several large needle sticks are made through the skin into the back of the pelvic bone (an area called the iliac crest) to remove marrow. The thick, liquid marrow is pulled out through the needle. Aside from the risks of general anesthesia, the main side effect is that the donor is sore for a few days afterward.

The harvested marrow is filtered, stored in a special solution in bags, and then frozen in liquid nitrogen. When the marrow is to be used, it is thawed and then given just like a blood transfusion. The stem cells travel to the recipient's bone marrow. They engraft or "take" there over time and begin to make blood cells. Signs of the new blood cells usually can be measured in the patient's blood tests in about 2 to 4 weeks.

Normally, very few stem cells are found in the blood. But giving hormone-like substances called growth factors to stem cell donors a few days before the harvest causes their stem cells to grow faster and move from the bone marrow into the blood.

For a peripheral blood stem cell transplant, the stem cells are removed or harvested from the bloodstream through a process called apheresis. This takes several hours. A catheter (a very thin flexible tube) is put into a vein in the donor's arm and attached to tubing that goes to a special machine. The donor's blood is run through this machine which separates and keeps only the stem cells. The rest of the blood is returned to the donor. This process is sometimes repeated for a few days until enough stem cells have been collected. The harvested stem cells are filtered, stored in a special solution in bags, and frozen until the patient is ready for them.

After the recipient gets treatment, the stem cells are given in an infusion much like a blood transfusion. The stem cells travel to the bone marrow, engraft, and then grow and make new, normal blood cells. The new cells are usually found in the patient's blood a few days sooner than when bone marrow stem cells are used, usually in about 10 to 20 days.

Not everyone who needs an allogeneic stem cell transplant can find a well-matched donor among the people who have signed up to donate. For these patients, umbilical cord blood may be another potential source of stem cells.

A large number of stem cells are normally found in the blood of newborn babies. After birth, the blood that is left behind in the placenta and umbilical cord (known as cord blood) can be taken and stored for later use in a stem cell transplant. After the umbilical cord is clamped and cut, the placenta and umbilical cord are cleaned and cord blood is put into a sterile container. The cord blood is mixed with a preservative solution and frozen until needed.

Cord blood transplant uses blood that would otherwise be thrown away. It is fairly easy to collect and does not pose a risk to the donor. The first cord blood transplant was done in 1988, but it is still a fairly new technique.

A possible drawback is the smaller number of stem cells present. But this may be balanced by the fact that each cord blood stem cell can form more blood cells than a stem cell from adult bone marrow. To be safe, most cord blood transplants done so far have been in children and smaller adults. Researchers are now looking for ways to increase the numbers of these cells in the lab before the transplant so that this source can be used in larger adults, too.

All 3 sources of stem cells can be used for the same goal: to give the patient healthy stem cells that will mature into healthy blood cells. There may be some minor pros and cons to each source, but all are usually able to provide the needed number of stem cells (with the exception noted above in umbilical cord blood).

At first, all stem cell transplants done were bone marrow transplants. But today peripheral blood stem cell transplants are more common. Often, doctors are able to harvest more stem cells from peripheral blood than from bone marrow. It's also easier to donate peripheral blood stem cells than bone marrow. Another plus for peripheral blood stem cell transplant is that the recipient's blood count often recovers faster than with a bone marrow transplant.

Cord blood transplant may be an option if a good match can't be found among volunteer stem cell donors. Even though well-matched cord blood is best, studies suggest that cord blood may not have to be as closely matched as bone marrow or peripheral blood. This may be an advantage for patients with rare tissue types. This type of transplant also does not require a separate donation procedure and may reduce the severity of graft-versus-host disease (described in the "Problems in the post-transplant period" section). But it is still a new technique, and it is not known if cord blood has enough stem cells for larger adults. Cord blood cells also usually take longer to engraft, leaving the patient at a high risk for infection longer than transplanted marrow or peripheral blood stem cells. Another drawback is that, unlike bone marrow transplant or peripheral blood stem cell transplant, the donor cannot be called back to give more if needed after the cord blood stem cells are used.

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