There are 3 possible sources of stem cells to use for transplants: bone marrow, the bloodstream (peripheral blood), and umbilical cord blood from newborns. Although bone marrow was the first source used in stem cell transplant, peripheral blood is used most often today.

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 have large numbers of stem cells in them. 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 so they dont feel pain). A large needle is put through the skin and into the back of the hip bone. The thick, liquid marrow is pulled out through the needle. This is repeated several times until enough marrow has been taken out (harvested). (For more on this, see the section called Whats it like to donate stem cells?)

The harvested marrow is filtered, stored in a special solution in bags, and then frozen. When the marrow is to be used, its thawed and then given just like a blood transfusion. The stem cells travel to the recipients bone marrow. There over time, they engraft or take and begin to make blood cells. Signs of the new blood cells usually can be measured in the patients blood tests in about 2 to 4 weeks.

Normally, 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 taken from blood. A very thin flexible tube (called a catheter) is put into one of the donors veins and attached to tubing that carries the blood to a special machine. The machine separates the blood, and keeps only the stem cells. The rest of the blood goes back to the donor. This takes several hours, and may need to be repeated for a few days to get enough stem cells. The stem cells are filtered, stored in bags, and frozen until the patient is ready for them. (For more on this, see the section called Whats it like to donate stem cells?)

After the patient is treated with chemo and/or radiation, 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 patients 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 family members or among the people who have signed up to donate. For these patients, umbilical cord blood may be a source of stem cells. Around 30% of unrelated hematopoietic stem cell transplants are done with cord blood.

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. The cord blood is frozen until needed.

More:
Sources of stem cells for transplant - American Cancer Society