Embryonic stem cells are cells found in very early human embryos that can develop into virtually any cell type in the body. They come from the inner cell mass of a blastocyst, a hollow ball of roughly 75 to 145 cells that forms four to seven days after fertilization. What makes them remarkable, and controversial, is this combination: they can copy themselves indefinitely in a lab dish while retaining the ability to become heart cells, nerve cells, blood cells, or any of the 200-plus specialized cell types a human body needs.
Where They Come From
After a sperm fertilizes an egg, the resulting cell divides repeatedly. By about day five, it forms a blastocyst: a tiny sphere with an outer shell (which would normally become the placenta) and a small cluster of cells on the inside called the inner cell mass. That inner cluster is the source of embryonic stem cells. Left alone in the body, these cells would go on to form the entire fetus. In a lab, scientists remove them from the blastocyst and place them in culture dishes, where they can grow and divide for months or even years without losing their potential.
The embryos used are almost always extras from in vitro fertilization (IVF) clinics, donated by people who no longer need them for fertility treatment. Researchers found that isolating the inner cell mass on day six after fertilization, when the blastocyst contains roughly 75 to 145 cells, produces the most reliable stem cell lines.
What Makes Them Pluripotent
The defining feature of embryonic stem cells is pluripotency: the ability to become any cell type in the body. This isn’t random. Inside these cells, a set of three core proteins work together to keep the cell in its flexible, undifferentiated state. These proteins act like master switches, activating the genes needed for self-renewal while keeping the genes for specialization turned off. When any one of these switches is disrupted, the cells lose their stem cell identity and begin transforming into more specialized types.
Pluripotency is maintained by a broader network too, including signaling molecules from the cell’s environment and chemical modifications to its DNA packaging. This means researchers can nudge embryonic stem cells toward becoming specific cell types by changing the signals they receive in a culture dish. Add certain growth factors and you get beating heart cells. Change the recipe and you get neurons, insulin-producing pancreatic cells, or blood cells.
How They Differ From Other Stem Cells
Not all stem cells are equal. Adult stem cells exist throughout the body in tissues like bone marrow, skin, and the gut, but they’re limited. A blood stem cell can make more blood cells, but it won’t become a brain cell. Embryonic stem cells face no such restriction.
Induced pluripotent stem cells (iPSCs) are the closest alternative. These are ordinary adult cells, often skin or blood cells, that scientists reprogram back into a stem-cell-like state by activating the same core set of genes found in embryonic stem cells. iPSCs share the same theoretical potential: unlimited self-renewal and the ability to become any cell type. They also carry a major practical advantage. Because they can be made from a patient’s own cells, they’re less likely to be rejected by the immune system. Embryonic stem cells, by contrast, come from a donor embryo, so any therapy derived from them faces the same immune mismatch problems as an organ transplant.
In practice, though, iPSCs don’t always perform identically. Studies have found that iPSCs show more variability when coaxed into becoming nerve cells or heart cells, producing lower and less consistent yields compared to embryonic stem cells. This unpredictability has made embryonic stem cells the more reliable benchmark for many research applications.
A Brief History
Scientists had worked with mouse embryonic stem cells since the early 1980s, but the human version proved far more difficult to isolate. The breakthrough came in 1998, when a team led by James Thomson at the University of Wisconsin successfully derived five human embryonic stem cell lines from donated IVF embryos. The first line, called H1, was established on January 22, 1998, and the results were published in Science that November. The H9 line from that original batch remains one of the most widely used cell lines in stem cell research worldwide.
Today, 503 human embryonic stem cell lines are approved for federally funded research in the United States through the NIH registry, including more than 200 lines carrying mutations linked to specific diseases. These disease-specific lines let researchers study conditions like Huntington’s disease or cystic fibrosis in a dish, without needing tissue from patients.
Growing Them in the Lab
Embryonic stem cells are finicky to maintain. They need very specific conditions to keep dividing without spontaneously turning into specialized cells. Traditionally, researchers grew them on top of a layer of inactive mouse cells, called feeder cells, which provided the right chemical signals and physical support. This worked, but the presence of animal material made the cells less suitable for human therapies.
Modern labs increasingly use feeder-free systems. These rely on culture dishes coated with proteins like laminin, which has been shown to keep embryonic stem cells pluripotent for over 130 rounds of cell division. Fully synthetic coatings made from engineered peptides or polymers can also do the job, supporting cell growth for months in chemically defined conditions with no animal-derived ingredients at all. These advances have made it more practical to produce clinical-grade cells suitable for use in patients.
Current Medical Applications
Embryonic stem cell therapies are still largely experimental, but several have reached human clinical trials. The first trial approved by the FDA, sponsored by Geron Corporation, tested cells derived from embryonic stem cells in patients with thoracic spinal cord injuries. The product consisted of young nerve-support cells (oligodendrocyte progenitors) intended to help repair damaged spinal tissue.
The area with the most clinical activity is eye disease. Multiple trials in the U.S. and China have used pigment cells derived from embryonic stem cells to treat age-related macular degeneration, both the dry and wet forms. These cells are transplanted into the back of the eye to replace the retinal pigment layer that deteriorates in the disease. Trials have tested doses ranging from 50,000 to one million cells per eye, and results have been promising enough that larger Phase II trials are now underway.
The Tumor Risk
The same property that makes embryonic stem cells powerful also makes them dangerous. Any undifferentiated stem cells left in a batch of specialized cells can, once transplanted, form a type of benign tumor called a teratoma. These tumors contain a chaotic mix of tissues (teeth, hair, bone) growing where they don’t belong. In animal studies, as few as 200,000 residual undifferentiated cells injected into the bloodstream were enough to trigger tumor formation.
This means purification is critical. Before any embryonic stem cell-derived therapy can be given to a patient, researchers must ensure that essentially zero undifferentiated cells remain. Several strategies are in development. One approach uses “suicide genes” engineered into the stem cells that can be activated by a drug to kill any remaining undifferentiated cells. Another targets a survival protein that undifferentiated cells depend on. Blocking this protein at very low concentrations killed residual stem cells in lab experiments without harming the desired specialized cells, and it completely prevented tumor formation in mice. No single method is foolproof yet, but the combination of better purification and sensitive detection is steadily reducing this risk.
The Ethical Landscape
Embryonic stem cell research has been ethically contentious since 1998 because extracting the cells destroys the embryo. People who believe life begins at fertilization view this as morally equivalent to ending a human life. Others argue that a five-day-old blastocyst, smaller than a grain of sand and lacking any nervous system, does not have the same moral status as a person, especially when the embryos would otherwise be discarded by IVF clinics.
The International Society for Stem Cell Research (ISSCR) maintains guidelines that serve as the global ethical framework for the field. These guidelines require specialized oversight for any research involving human embryos and gametes, and they prohibit growing stem cell-based embryo models to the point of potential viability. The guidelines don’t override national laws, which vary widely. Some countries ban embryonic stem cell research entirely, others permit it with strict regulation, and a few have relatively permissive frameworks. In the U.S., federal funding rules have shifted with different administrations, creating an uneven policy landscape that researchers have navigated for over two decades.