What are Stem Cells and Where Do They Come From?
Stem cells are a special type of cell found in almost every tissue of your body. They can repair tissues and replace damaged cells. Because of this ability, stem cell transplants are an important part of treating certain cancers.
What are stem cells?
Stem cells are a special type of cell that can be found in almost every tissue of your body, including your bone marrow (the spongy center of certain bones).
Stem cells have the power to make more of themselves. They help repair and maintain tissues and organs in your body.
What are blood-forming stem cells?
Stem cells that live in your bone marrow are called hematopoietic or blood-forming stem cells. These are the type of stem cells used in stem cell or bone marrow transplants.
Stem cells are very young and not fully developed. All blood-forming stem cells start out the same, but they can mature into any type of blood cell, depending on what your body needs at the time. This skill is called multipotent (powerful).
Your bone marrow is where blood-forming stem cells divide to make new blood cells.
When blood-forming stem cells mature, they become either:
- Red blood cells (RBCs)
- White blood cells (WBCs) or
- Platelets
They leave your bone marrow and enter your bloodstream. A small number of the immature stem cells also get into your bloodstream. These are called peripheral blood stem cells.
What is the job of blood-forming stem cells?
All the blood cells in your body start out as blood-forming stem cells. As they mature, they turn into red blood cells, white blood cells, or platelets. These blood cells are important to keep you alive.
To work correctly, your body needs every type of blood cell. It also needs enough of each type.
Red blood cells, also known as erythrocytes, carry oxygen away from your lungs to all the cells in your body. They bring carbon dioxide from the cells back to your lungs to be exhaled.
They also carry a protein called hemoglobin. Hemoglobin contains iron and helps your red blood cells hold oxygen in place as they move throughout your body.
A blood test called a hematocrit shows how much of your blood is made up of red blood cells. The normal range is about 35% to 50% for adults. People whose hematocrit is below this level have anemia. Anemia can make you look pale and feel weak, tired, and short of breath.
White blood cells help fight infections caused by bacteria, viruses, and fungi. There are different types of WBCs.
Neutrophils
Neutrophils are the most important type of white blood cell for fighting infections. They are the first cells to respond to an injury or when germs enter your body. When they are low, you have a higher risk of infection.
The absolute neutrophil count (ANC) is a measure of the number of neutrophils in your blood. When your ANC drops below a certain level, you have neutropenia. ?The lower your ANC, the greater your risk of infection.
Lymphocytes
Lymphocytes are another type of white blood cell. There are different kinds of lymphocytes, including:
- T lymphocytes (T cells)
- B lymphocytes (B cells)
- Natural killer (NK) cells
Some lymphocytes make antibodies to help fight infections. Your body depends on lymphocytes to recognize its own cells and reject cells that don¡¯t belong, such as invading germs or cells that are transferred from someone else.
Platelets are small blood cells that help stop bleeding by sealing damaged blood vessels and helping blood to clot. They are also called thrombocytes.
A normal platelet count is usually between 150,000/cubic mm and 450,000/cubic mm, depending on the lab that does the test. A platelet count that drops below normal is called thrombocytopenia. This may cause you to bruise more easily, bleed longer, and have nosebleeds or bleeding gums.
Spontaneous bleeding (bleeding with no known injury) can happen if your platelet count drops lower than 20,000/mm3. This can be dangerous if bleeding happens in your brain, or if blood begins to leak into your intestines or stomach.
Learn more about blood counts and what the numbers mean in Understanding Your Lab Test Results.
Where do stem cells for transplants come from?
There are 3 possible sources of stem cells for transplants. The source used depends on the type of transplant you need.
- Bone marrow (from you or someone else)
- The bloodstream (peripheral blood ¨C from you or someone else)
- Umbilical cord blood (from newborns)
Stem cells from bone marrow
Bone marrow is the spongy liquid tissue in the center of some bones. It has a rich supply of stem cells, and its main job is to make blood cells that circulate in your body.
The bones of your pelvis (hip) and thigh have the most marrow and contain 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 amount of healthy stem cells.
The bone marrow is harvested (removed) in the operating room while the donor is under general anesthesia (medications that put a person into a deep sleep).
During bone marrow harvest:
- A large needle is put through the skin into the back of the pelvic (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 What Is It Like to Donate Stem Cells?
The harvested marrow is filtered, stored in a special solution in bags, and then frozen. When it¡¯s time to use the marrow, it is thawed and put into the recipient¡¯s blood through a vein, like a blood transfusion.
The stem cells travel to the bone marrow, where they engraft or ¡°take¡± and start to make new blood cells. Signs of the new blood cells can usually be checked with blood tests in a few weeks.
Stem cells from peripheral blood
Normally, people have very few stem cells in their blood. But giving stem cell donors injections of hormone-like substances makes their stem cells grow faster and move from the bone marrow into the blood. This is done a few days before the harvest.
For a peripheral blood stem cell transplant (PBSCT):
- The stem cells are taken from blood.
- A thin flexible tube (called a central line catheter) is put into a large vein in the donor and attached to tubing that carries the blood to a special machine, called a centrifuge.
- The machine separates the stem cells from the rest of the blood, which is returned to the donor during the same procedure. Usually, this takes several hours on a single day. Sometimes, a donor may be asked to return for a second day to get enough stem cells. This process is called apheresis.
- The stem cells are filtered, placed in bags, and stored until the recipient is ready for them.
When it¡¯s time to use the stem cells, they are put into the recipient¡¯s vein, like a blood transfusion. The stem cells travel to the bone marrow, engraft, and then start making new, normal blood cells. The new cells are usually found in the person¡¯s blood in about 4 weeks.
Stem cells from umbilical cord blood
The blood of newborn babies normally has large numbers of stem cells. Blood that¡¯s left behind in the placenta and umbilical cord after birth is known as cord blood. It can be frozen and stored for later use in a stem cell transplant.
A cord blood transplant uses blood that is normally thrown out. After the baby is born, specially trained members of the health care team make sure the cord blood is carefully collected. The baby is not harmed in any way.
Even though the blood of newborns has a lot of stem cells, there is only a small amount of blood in the placenta and umbilical cord. So, there are fewer stem cells in cord blood compared to other sources. This is a possible drawback to cord blood transplant. Also, cord blood transplants can take longer to engraft and start working.
Still, cord blood stem cells are a useful option. Cord blood is given into a person¡¯s blood, like a blood transfusion.
Learn more
- Learn more about how stem cell transplants work to treat cancer in Stem Cell or Bone Marrow Transplant.
- Information on donating stem cells (including umbilical cord blood) can be found in What Is It Like to Donate Stem Cells.
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- References
Developed by the ÁñÁ«ÊÓƵ medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).
American Society of Clinical Oncology (ASCO). What is a stem cell transplant (bone marrow transplant)? Accessed at cancer.net. Content is no longer available.
Mousaei Ghasroldasht M, Seok J, Park HS, et al. Stem cell therapy: from idea to clinical practice. International journal of molecular sciences. 2022 Mar 5;23(5):2850. Accessed at https://doi.org/10.3390/ijms23052850 on February 27, 2025.
Sieff CA. Overview of hematopoietic stem cells. UpToDate. 2024. Accessed at https://www.uptodate.com/contents/overview-of-hematopoietic-stem-cells on February 25, 2025.
Last Revised: July 10, 2025
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