Stem cells come from several places in the body and at different stages of life. The major sources include early-stage embryos, bone marrow, umbilical cord blood, fat tissue, amniotic fluid, and even reprogrammed adult skin cells. Each source produces stem cells with different capabilities, and understanding those differences helps explain why certain types are used for specific medical purposes.
Embryonic Stem Cells
Embryonic stem cells are extracted from a structure called the inner cell mass of a blastocyst, which is an early-stage embryo that forms between the 4th and 7th day after fertilization. At this point, the embryo is a hollow ball of roughly 200 to 300 cells, and the inner cell mass is the small cluster inside that would eventually develop into all the tissues of a human body. When removed and placed in laboratory conditions, these cells can multiply indefinitely.
What makes embryonic stem cells so valuable to researchers is their pluripotency. They can become virtually any cell type in the body: nerve cells, heart muscle, bone, skin, blood cells, and more. This broad potential is what distinguishes them from stem cells found in adults, which are generally limited to producing cell types related to the tissue they live in. The blastocysts used in research typically come from unused embryos created during in vitro fertilization (IVF), donated with consent from the individuals involved.
Bone Marrow and Blood
The most widely used stem cells in medicine come from bone marrow, the spongy tissue inside your larger bones. These are called hematopoietic stem cells, and they produce every type of blood and immune cell your body needs: red blood cells, white blood cells, and platelets. Your body relies on them continuously throughout life to replace blood cells that wear out or get destroyed.
Bone marrow is also home to a second population called mesenchymal stem cells, which can give rise to bone, cartilage, and fat cells. Both types sit in specialized environments within the marrow, sometimes called niches, that regulate when they divide and what they become. Hematopoietic stem cell transplants from bone marrow remain the only stem cell therapy routinely approved by the FDA, used primarily to treat blood cancers like leukemia and lymphoma, as well as other disorders affecting the blood and immune system.
Umbilical Cord Blood and Tissue
When a baby is born, the blood remaining in the umbilical cord and placenta is rich in hematopoietic stem cells. Cord blood has been used as a source for stem cell transplants for over 25 years and contains a relatively high concentration of early-stage blood-forming cells. It also carries immune cells, including T cells and natural killer cells, that are being explored for cancer immunotherapy.
The cord tissue itself, specifically a jelly-like substance called Wharton’s jelly that surrounds the blood vessels, is a separate source of mesenchymal stem cells. These cord-derived mesenchymal cells multiply faster than those taken from adult bone marrow or fat tissue, which makes them attractive for research. Collection is simple and painless, since the cord is biological waste after delivery. This is one reason umbilical cord tissue has become one of the fastest-growing sources of mesenchymal stem cells worldwide.
Amniotic Fluid
The fluid surrounding a developing fetus contains its own population of stem cells. These can be collected through amniocentesis, a procedure already performed routinely during pregnancy to screen for genetic conditions. The collection poses no additional harm to the mother or the fetus.
Amniotic fluid stem cells occupy a middle ground between embryonic and adult stem cells. They can differentiate into a wider range of cell types than most adult stem cells, including nerve, heart, lung, kidney, bone, and cartilage cells. Unlike embryonic stem cells, they don’t form tumors when transplanted into animals, which removes one of the major safety concerns associated with embryonic cells. They also grow in the lab without some of the specialized support layers that embryonic cells require.
Fat Tissue and Other Adult Sources
Mesenchymal stem cells can be isolated from nearly any tissue in the body, but adipose (fat) tissue has become a major source alongside bone marrow and umbilical cord. Fat is abundant, and collecting it through liposuction is relatively straightforward compared to a bone marrow biopsy. Other adult tissues harbor their own resident stem cell populations. Your skin constantly regenerates from stem cells in its deeper layers. The lining of your intestines replaces itself every few days, driven by stem cells tucked into tiny pockets called crypts. Even the brain, once thought incapable of producing new cells, contains small populations of neural stem cells in specific regions.
These adult stem cells are multipotent rather than pluripotent, meaning they typically produce only the cell types relevant to their home tissue. A blood-forming stem cell in bone marrow won’t spontaneously become a nerve cell under normal conditions. This narrower range is a practical limitation, but it also makes adult stem cells more predictable and safer for transplantation.
Reprogrammed Cells (iPSCs)
In 2006, a Japanese research team led by Shinya Yamanaka demonstrated that ordinary adult cells, like skin cells, could be reprogrammed back into a stem cell state. The technique involves introducing four specific proteins into the cell that essentially reset its identity, returning it to a pluripotent state similar to an embryonic stem cell. These reprogrammed cells are called induced pluripotent stem cells, or iPSCs.
The implications were enormous. iPSCs can become nearly any cell type in the body, just like embryonic stem cells, but they don’t require an embryo. They can be made from a patient’s own cells, which in theory reduces the risk of immune rejection after transplantation. Yamanaka received the Nobel Prize in 2012 for the discovery. Today, iPSCs are widely used in drug testing and disease modeling, where researchers create specific cell types from a patient’s own reprogrammed cells to study what goes wrong in conditions like Parkinson’s disease or heart failure.
How Stem Cell Types Compare
The fundamental difference between stem cell sources comes down to potency, or how many cell types they can produce:
- Totipotent cells can become any cell in the body plus the placenta. Only the fertilized egg and its first few divisions qualify.
- Pluripotent cells can become any cell type in the body but not placental tissue. Embryonic stem cells and iPSCs fall here.
- Multipotent cells can produce several related cell types. Hematopoietic stem cells in bone marrow, for example, generate all blood cell types but not nerve or muscle cells.
- Unipotent cells produce only one cell type, serving as a dedicated replacement supply for a single tissue.
In practice, pluripotent cells offer the widest therapeutic potential but come with greater complexity in controlling what they become. Multipotent cells from bone marrow and cord blood are more limited but better understood and already in clinical use. The only FDA-approved stem cell therapy remains hematopoietic stem cell transplantation for blood cancers and immune disorders, though clinical trials are testing cells from nearly every source for conditions ranging from spinal cord injuries to type 1 diabetes.