Stem cells are your body’s raw materials, the cells responsible for building, maintaining, and repairing tissues throughout your life. They do two things no other cell can: they copy themselves through a process called self-renewal, and they transform into specialized cells like blood cells, bone cells, nerve cells, or skin cells. This combination makes them essential for everything from healing a cut to replacing the roughly 10 billion red blood cells your body uses up every single hour.
How Stem Cells Work
Every stem cell faces the same basic choice: make another copy of itself or become something more specialized. When a stem cell divides into two identical daughter cells, that’s self-renewal, and it keeps the supply of stem cells from running out. When it instead transforms into a muscle cell, a neuron, or a white blood cell, that’s differentiation.
Not all stem cells have the same range of options. A fertilized egg is totipotent, meaning it can produce every cell type in the body plus the placenta. Embryonic stem cells are pluripotent, capable of becoming virtually any cell type in the body but not placental tissue. Most of the stem cells in your adult body are multipotent, meaning they’re restricted to producing cells within a specific tissue. Blood-forming stem cells in your bone marrow, for example, can generate all types of blood cells but won’t produce skin or brain cells. Some stem cells are even more limited, producing only a single cell type.
This hierarchy matters because it determines what each type of stem cell can actually do, both in your body and in medicine.
What Stem Cells Do in Your Body Every Day
Adult stem cells live in nearly every tissue and organ, tucked into specialized environments called niches. Their daily job is quiet but relentless: replacing cells that wear out, get damaged, or die. Without them, tissues would degrade within days or weeks.
Your bone marrow is the most productive stem cell hub. Blood-forming stem cells there generate approximately 10 billion red blood cells and 100 million white blood cells every hour throughout your life. That continuous cycle of production, called hematopoiesis, is the reason a healthy person can lose blood and recover, fight off infections, and deliver oxygen to every organ. Bone marrow also houses stem cells that support bone formation and cells that can differentiate into the endothelial lining of blood vessels.
Your skin constantly sheds its outer layer and rebuilds from stem cells in deeper layers. Your intestinal lining, one of the fastest-renewing tissues in the body, replaces itself roughly every five days using stem cells anchored at the base of tiny finger-like projections in the gut wall. Even your brain, once thought incapable of regeneration, contains stem cells in specific regions that produce new neurons in limited quantities.
Healing Goes Beyond Replacement
For a long time, scientists assumed stem cells repaired damage simply by turning into new cells to fill the gap. That picture has expanded significantly. Stem cells also act as tiny chemical factories, releasing a cocktail of signaling molecules that influence the cells around them.
These secreted molecules do several things at once. Some stimulate nearby cells to multiply, speeding up tissue repair. Others protect existing cells from dying by neutralizing harmful oxygen molecules and blocking cell-death pathways. Stem cells also release signals that promote the growth of new blood vessels, which is critical for getting oxygen and nutrients to injured tissue. At the same time, they produce molecules that prevent excessive scarring by keeping the structural scaffolding of tissue in balance.
This signaling role is especially important in organs with limited regenerative ability, like the heart. After a heart attack, transplanted stem cells have been shown to reduce the expansion of damaged heart tissue, not primarily by becoming new heart muscle cells, but by releasing protective molecules that keep surviving heart cells alive and promote new blood vessel growth in oxygen-starved areas.
Stem Cells and the Immune System
One of the more surprising stem cell functions is immune regulation. Certain stem cells, particularly a type found in bone marrow, fat, and placental tissue, can dial inflammation up or down depending on the signals they receive from their environment.
These cells interact with the immune system in two main ways. Through direct contact, they can slow the multiplication of overactive immune cells that drive autoimmune conditions. They also flip a switch in immune cells called macrophages, converting them from a pro-inflammatory state to an anti-inflammatory state. Through their secreted molecules, they encourage the body to produce more regulatory immune cells, the type that keeps immune responses from spiraling out of control.
This immunomodulatory ability is why stem cells are being studied as treatments for conditions involving chronic, misdirected inflammation, from graft-versus-host disease to autoimmune disorders.
What Happens to Stem Cells as You Age
Aging takes a measurable toll on stem cells. Tissues throughout the body show a progressive decline in their ability to maintain and repair themselves, and much of that decline traces back to changes in the stem cell population. With age, stem cells become less responsive to injury signals, less capable of dividing on cue, and more likely to produce an imbalanced mix of daughter cells.
This has been documented in multiple tissues. Muscle stem cells, neural stem cells, and reproductive stem cells all decline in number or function with age. Blood-forming stem cells illustrate the problem well: older adults still have them, but those cells tend to overproduce certain types of immune cells (myeloid cells) while underproducing others (lymphoid cells, which include the immune cells that fight viruses). The result is a weaker, less balanced immune system.
The environment around stem cells deteriorates too. The niches that once protected and regulated them become less supportive, and systemic signals in the bloodstream shift in ways that further suppress stem cell activity. This combination of fewer functional stem cells and a less hospitable environment is a core driver of age-related tissue breakdown, slower wound healing, and increased vulnerability to disease.
Stem Cell Therapies in Use Today
The most established stem cell therapy is the bone marrow transplant, used for decades to treat blood cancers like leukemia and lymphoma, as well as blood disorders like sickle cell disease. The procedure works by replacing a patient’s diseased blood-forming stem cells with healthy ones, either from a donor or from the patient’s own body after collection and storage.
The process has several stages. First, medications are given to push stem cells out of the bone marrow and into the bloodstream, where they’re easier to collect. Alternatively, bone marrow is harvested directly from the hip bone under anesthesia. The patient then receives high-dose chemotherapy or radiation to destroy their existing bone marrow, followed by an infusion of the collected stem cells. Engraftment, the point at which transplanted cells begin producing new blood cells, typically takes about two weeks with cells collected from the bloodstream and a few days longer with bone marrow-derived cells.
Beyond transplants, the FDA has approved several other cell-based products. These include cord blood products (stem cells collected from umbilical cord blood after birth) used for blood disorders, cultured cartilage cells implanted to repair knee damage, a skin substitute made from cultured skin cells for burns, and a mesenchymal cell product for graft-versus-host disease in children.
Risks and Limitations
Stem cell therapies carry real risks that scale with how powerful the cells are. The very flexibility that makes pluripotent stem cells so promising also makes them harder to control. When undifferentiated embryonic or reprogrammed stem cells are transplanted, they can form teratomas, a type of tumor containing a disorganized mix of tissues like hair, teeth, and bone. In animal studies, teratoma formation rates range from 33% to 100% depending on how the cells are prepared and where they’re implanted.
Reprogrammed stem cells (iPSCs), made by reverting adult cells back to a pluripotent state, carry an additional concern. The genetic tools used to reprogram them can sometimes activate cancer-promoting genes or disrupt normal ones, raising the risk of malignant transformation. One advantage of iPSCs is that because they’re made from a patient’s own cells, immune rejection is not a concern, which is a significant issue with donor-derived therapies.
Even the more commonly used multipotent stem cells have limitations. Their immune-suppressing properties, while therapeutically useful, can backfire in patients with cancer. Transplanted stem cells have been shown to migrate toward tumors, where they can suppress the immune response that would otherwise fight the cancer and promote the growth of new blood vessels that feed the tumor. This dual nature, helpful in autoimmune disease, potentially harmful in cancer, is one of the central challenges in bringing more stem cell treatments from the lab to the clinic.
Reprogrammed Cells and Lab-Grown Tissues
A major advance came in 2006 when researchers discovered that ordinary adult cells could be reprogrammed into a stem cell-like state by activating just four genes. These induced pluripotent stem cells (iPSCs) behave almost identically to embryonic stem cells in their gene activity and their ability to become virtually any cell type, but they can be made from a simple skin or blood sample.
This opened doors that were previously closed by both biology and ethics. Researchers can now take cells from a patient with Parkinson’s disease, reprogram them into stem cells, turn those into dopamine-producing neurons, and study the disease in a dish using the patient’s own genetics. The same approach is being tested for macular degeneration, a leading cause of vision loss. The National Eye Institute is running a trial that converts a patient’s blood cells into iPSCs, differentiates them into retinal pigment cells, and transplants them into the eye. That trial is currently focused on proving the approach is safe rather than restoring vision, given the advanced disease state of participants, but it represents the kind of personalized cell therapy that iPSCs make possible.
iPSCs are also widely used in drug testing, allowing pharmaceutical companies to screen compounds on human heart cells, liver cells, or neurons grown in the lab rather than relying solely on animal models. This has made drug development faster and more predictive of how a compound will behave in the human body.