Bone Marrow Transplants: Sources of Stem Cells

by Food and Drug Administration (FDA)

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Alternative Sources of Stem Cells

Researchers realized in the late 1980s that stem cells are not only found in bone marrow, but also in the bloodstream. When stem cells are collected from the bloodstream, they are called peripheral blood stem cells. "The challenge is that stem cells don't generally circulate in large numbers in the peripheral blood," FDA's Litwin says. Growth factors — also known as mobilizing agents — are given to stimulate the bone marrow to produce more cells, which are then released into the bloodstream. To collect these cells, blood is circulated through a cell separator that removes peripheral stem cells and returns the rest of the blood — including hemoglobin-containing red cells — to the body. The process is called apheresis.

FDA and the National Marrow Donor Program are studying peripheral blood stem cells as an alternative to bone marrow for initial transplants. This procedure has been most widely used as a follow-up transplant to supplement the marrow transplant, Confer says. The research aims to document the safety of this collection method and compare how donors view bone marrow transplants vs. peripheral stem cell transplants. "As we learn more," Confer says, "I anticipate we'll find out that there are some diseases for which peripheral blood is superior and others for which bone marrow is superior."

In the last decade, doctors have also used stem cells collected from umbilical cord blood for bone marrow transplants. This new field is expected to widen the donor pool, but it is still considered experimental. One limitation is that the number of stem cells found in cord blood is small because babies are small. "As a result, engraftment is slower than if you obtain stem cells from the bone marrow or the peripheral blood," Litwin says. Again, growth factors can help increase the number of cells. Under another FDA-sponsored study, the National Marrow Donor Program has invited cord blood banks to apply for membership to its registry. The program receives federal funding from the Health Resources and Services Administration to manage the registry.

FDA and the National Heart, Lung, and Blood Institute, part of the National Institutes of Health, have co-sponsored a series of workshops on stem cells over the past several years. With the discovery of alternative sources, FDA proposed in 1997 to regulate stem cells collected from peripheral and umbilical cord blood. The proposed approach centers on preventing the transmission of communicable diseases and assuring that stem cell procedures are safe and effective. FDA continues to work on developing the best methods and practices to prevent contamination of tissue and preserve stem cell integrity.

The newest alternative source of stem cells involves taking existing cells — from either the bone marrow, peripheral blood, or cord blood — and expanding them in a lab. This procedure is already being used but is highly experimental and challenging. "So far, it's been easier to get stem cells to mature than to self-replicate," Confer says, "but the potential is tremendous. We want to be able to turn a million stem cells into 10 million."

Increasing the Donor Pool

About 30 percent of people who need a transplant have siblings who, because they are a tissue match, are suitable donors. But many people must look to registries of unrelated donors in order to survive. Increasing the donor pool has been a key part of advancing the search process, Confer says. The National Marrow Donor Program, which helps locate unrelated donors in the United States, had 8,000 registered donors when the program began in 1987. Now the registry is up to about 4 million.

Global cooperation has also been key, giving patients access to donors wherever they reside. "No one could have guessed 10 years ago that there would be 6 million international donors today," Confer says.

Donor shortages, however, do exist in racial and ethnic minority populations, including African Americans, Hispanics, Asians/Pacific Islanders, and American Indians/Alaska Natives. "It's not that an African American has to have an African American donor," Confer says, "but the best chance [for a match] would be within your group." The National Marrow Donor Program continues to conduct outreach and recruitment strategies for these populations. The program receives funds from the U.S. Navy for advancing the science of human leukocyte antigens and increasing diversity of the donor pool.

The National Marrow Donor Program is looking at ways to make the search for donors faster and more efficient. A search takes 100 days on average, but it can be as short as 20 to 30 days or as long as years, Confer points out. "But too many people have diseases that just won't stand for a long search."

Becoming a Donor

Mary Halet, of Minneapolis, Minn., says that being a bone marrow donor was one of the most important events in her life. After giving bone marrow in 1993, she arranged for a donor center coordinator to pass along an anonymous note to the recipient. "I wished her well and told her that this is a small part of me that I hope can be a big help to her," Halet says.

Unlike heart or lung transplants, bone marrow transplants don't involve surgery. Doctors remove marrow with a needle that is inserted into the hip bones in the pelvis, the most marrow-rich site in the body. The sternum, the bone in the middle of the chest, is another possible site. Marrow is then delivered intravenously to the recipient like a blood transfusion.

The removal of marrow for transplant is usually an outpatient procedure that takes about an hour and is performed under general or regional anesthesia. "It's important for donors to know and understand the risks," Halet says, "but many safety precautions are taken." Donors may experience back discomfort for three to five days. "I had the procedure on a Friday, and I was out riding my bike on Monday," Halet says. "If the goal is to help someone have a chance at life, then I can endure a few days of discomfort." Lost bone marrow replenishes itself in a few weeks.

In accordance with confidentiality rules, Halet only knew that her recipient was a 23- year-old female with chronic myelogenous leukemia. "Even though we never met, I have a kindred connection with her," says Halet, a former bone marrow researcher who now works as a search coordinator for the National Marrow Donor Program. Her recipient died 11 months after the transplant, and Halet still keeps in touch with her family. One year after a transplant is performed, donor and recipient families are allowed to meet.

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