The process of collecting (harvesting) stem cells for transplant depends on the source of the stem cells. Stem cells can be collected from bone marrow, circulating (peripheral) blood or umbilical cord blood.
Stem cells from bone marrow
Stem cells are 10–100 times more concentrated in bone marrow than in peripheral blood. The hip (pelvic) bone contains the largest amount of active marrow in the body and large numbers of stem cells. Harvesting stem cells from the bone marrow is usually done in the operating room.
- The person will be given an epidural (spinal) or general anesthetic.
- The doctor makes several small punctures in the skin over the pelvic bone.
- A special needle attached to a syringe is inserted through these punctures and into the bone marrow.
- The doctor draws marrow and blood out of the bone through the syringe. This process is repeated until enough stem cells are collected for the transplant. It usually takes 1–2 hours.
- The specific amount of marrow and blood removed varies from person to person. It depends on the donor’s weight and the concentration of stem cells in the marrow.
- About 500–1000 mL (1–2 pints) of marrow is removed from an adult, which is about 3%–5% of the marrow.
- After the procedure, the doctor covers the puncture sites with bandages or a pressure dressing.
- The cells collected are filtered to remove bone fragments and fat particles.
- The donor recovers from the anesthetic in the recovery room. The healthcare team will check for signs of bleeding, pain or any other side effects from the procedure. The donor can usually leave the hospital a few hours after the procedure. Sometimes the donor will need to stay in the hospital overnight to recover from the procedure.
- The donor’s hip area may be sore for a few days. Medicines can be taken to relieve pain.
Few risks are associated with donating bone marrow. The body usually replaces these cells within a few weeks. The doctor may suggest iron supplements until blood cell counts increase.
Sometimes blood is collected from the donor a few weeks before the bone marrow harvest. The blood is stored and given back to the donor after the stem cells are removed. This blood transfusion helps prevent anemia.
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Stem cells from peripheral blood
Harvesting stem cells from peripheral (circulating) blood is done on an outpatient basis. No anesthetic is needed.
Preparation before donation may be slightly different for allogeneic and autologous stem cell transplants.
- Before donating stem cells, the person may be given a hematopoietic growth factor drug called a colony-stimulating factor (CSF).
- CSFs cause the bone marrow to make more stem cells and release them into the bloodstream (stem cell mobilization).
- They are usually given as an injection into the fatty tissue under the skin every day for about 4–6 days.
- CSFs can cause some side effects, such as bone and muscle pain, fever, chills and headaches.
- Before an autologous stem cell transplant (which uses the recipient’s own stem cells), the person may be given chemotherapy.
- Chemotherapy causes bone marrow suppression so the bone marrow doesn’t produce as many blood cells as it normally does. The body responds to this by producing more stem cells.
- Sometimes both growth factors and chemotherapy are used to produce enough stem cells.
- Stem cells are collected about 2 weeks after chemotherapy.
The harvesting process takes 3–4 hours. Depending on the amount of stem cells collected, this process may be repeated daily for 1–5 days to collect enough stem cells for a transplant. The healthcare team checks blood cell counts daily during the donation process to make sure that levels do not fall too much below normal. Low blood cell counts can cause problems such as anemia.
- A catheter (thin tube) is placed in a large vein in the donor’s arm.
- Blood travels through the catheter from the body to a special machine. The machine carries out a process called apheresis, which separates and collects stem cells from the blood.
- Once the stem cells are removed from the blood, it is returned to the donor.
Occasionally donors may have side effects during the collection process. Some people experience chills and light-headedness. Donors may also have numbness around the lips and in the fingers and toes or cramping of the hands. These side effects are due to low blood calcium levels, which is caused by the blood-thinning agent used during harvesting. They go away quickly when treated with calcium supplements.
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Stem cells from umbilical cord blood
Blood is collected from the umbilical cord shortly after a baby is born. The volume of stem cells collected per donation is quite small, so these cells are usually used for children or small adults.
- A machine separates and collects stem cells from the cord blood.
- The stem cells are frozen and stored by cord blood banks or programs until they are needed.
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Number of stem cells needed
Blood cells have a special antigen on their surface called CD34+. This antigen is important in stem cell transplant. The healthcare team can calculate the number of stem cells collected during harvesting using a method called the CD34+ assay. The number of stem cells needed for a transplant has not been formally determined, but it is based on the recipient’s weight.
Different transplant centres may have different standards for the number of stem cells needed. The range is 1–5 million CD34+ cells per kilogram of body weight. Less than 2 million CD34+ cells per kilogram of body weight can slow down engraftment.
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