Embryonic stem cells come from a very early-stage embryo called a blastocyst, specifically from a cluster of cells inside it known as the inner cell mass. This structure forms between 3 and 7 days after fertilization, when the entire embryo is only about 150 cells. In practice, nearly all embryonic stem cell lines used in research today were derived from surplus embryos created during in vitro fertilization (IVF) and donated by patients who no longer needed them.
Inside the Blastocyst
After a human egg is fertilized, it divides rapidly. By about day 3 to 5, it forms a hollow ball of cells called a blastocyst. The outer layer of the blastocyst will eventually become the placenta. Tucked inside is a small group of roughly 30 cells called the inner cell mass, and this is where embryonic stem cells are found.
These inner cells are remarkable because they are pluripotent: they can become virtually any cell type in the human body. That includes nerve cells, heart muscle, blood cells, bone, skin, and everything else. More specifically, they can give rise to all three fundamental tissue layers that form during early development (endoderm, mesoderm, and ectoderm), which together produce every organ and tissue. They can also copy themselves indefinitely without losing this ability, a trait called self-renewal. No other naturally occurring human cell combines both of these properties to the same degree.
IVF Clinics: The Practical Source
The blastocysts used to create embryonic stem cell lines are not taken from pregnancies. They come from fertility clinics. During IVF, multiple embryos are typically created, and patients often end up with more than they need. These surplus embryos can be frozen, discarded, or, with the donor’s informed consent, donated for research.
When researchers derive a new stem cell line, they extract the inner cell mass from a donated blastocyst and place those cells in a lab dish with nutrients that encourage them to keep dividing. If the cells successfully grow and maintain their pluripotency, they become an established cell line that can be shared with labs around the world. A single line can supply cells for decades of experiments, meaning new embryos do not need to be used each time a scientist needs embryonic stem cells.
Donors who provide embryos for this purpose go through a consent process that covers how the cells will be used. Some donors have described their decision as a way to give purpose to embryos they would otherwise discard, and researchers have documented these perspectives to better understand the ethics of the donation process.
Stem Cell Banks and Registries
Most scientists working with embryonic stem cells do not derive their own lines. Instead, they obtain established lines from centralized repositories. The NIH historically maintained a Human Embryonic Stem Cell Registry listing lines approved for use in federally funded research. However, as of early 2025 the NIH paused review and approval of new lines for the registry. Previously approved lines can still be used in NIH-funded studies, but no new additions are being processed.
Outside the United States, stem cell banks in the UK, South Korea, Japan, and other countries distribute established lines to qualified researchers. These banks verify the quality and genetic identity of each line, ensuring consistency across experiments worldwide.
How They Differ From Other Stem Cells
Embryonic stem cells are not the only stem cells in the body, but they are the most versatile. Adult stem cells exist in bone marrow, fat tissue, blood, and other organs, yet they are generally limited to producing the cell types found in their home tissue. A blood stem cell, for instance, can make red and white blood cells but cannot become a brain cell under normal conditions.
Fetal stem cells, found in developing fetuses later in pregnancy, fall somewhere in between. They retain broader potential than adult stem cells but are already more specialized than embryonic ones. The key distinction is timing: embryonic stem cells are isolated from a days-old blastocyst before any organs or tissues have begun to form, which is why their range of possibilities is so wide.
There is also a lab-made alternative called induced pluripotent stem cells (iPSCs). Scientists can reprogram ordinary adult cells, such as skin or blood cells, back into a pluripotent state that closely mimics embryonic stem cells. iPSCs have become widely used in research, though subtle differences between the two types remain an active area of investigation.
Where Embryonic Stem Cells Are Being Used
The main areas of active clinical testing involve eye diseases, nervous system conditions, and cancer. The first embryonic stem cell product ever cleared for a clinical trial in the United States targeted spinal cord injury. Geron Corporation developed a treatment based on nerve-supporting cells derived from embryonic stem cells, and a phase I trial launched in 2010.
Eye diseases have become the most common target. At least 21 products aimed at conditions like dry age-related macular degeneration and Stargardt macular dystrophy have entered clinical trials. In these procedures, researchers grow retinal pigment cells from embryonic stem cells and transplant them into the back of the eye. Some patients in early trials gained measurable improvements in vision, with one Korean study reporting gains of up to 19 letters on an eye chart at the one-year mark. The eye was an appealing starting point because it requires a relatively small number of transplanted cells and is partially shielded from immune rejection.
Beyond direct therapies, embryonic stem cells serve as a foundational research tool. Scientists use them to model diseases in a dish, study the earliest stages of human development, and screen potential drugs before testing them in people. Disease-specific lines derived from embryos identified through preimplantation genetic testing offer a window into conditions like cystic fibrosis or Huntington’s disease at the cellular level, long before symptoms would appear in a person.