Embryonic stem cells come from human embryos created through in vitro fertilization (IVF) that are no longer needed for reproduction. These are commonly called “surplus” or “spare” embryos. An estimated 1.2 million frozen embryos are currently in storage in the United States alone, and a small fraction of these are donated by couples for scientific research.
Why IVF Embryos Are the Primary Source
During IVF treatment, clinics typically create more embryos than can be safely transferred at one time. The extras are frozen for potential future use. When couples become pregnant, decide their family is complete, or choose not to continue treatment, they face a decision about what to do with their remaining embryos. The options are generally to keep them frozen, discard them, donate them to another couple, or donate them for research.
In Europe, roughly one-third of couples never use their surplus frozen embryos. In the U.S., federal guidelines permit funding only for stem cell research using embryos that were originally created for reproductive purposes and later donated voluntarily. Embryos created solely for research are excluded from federal funding.
The Blastocyst Stage: What Scientists Actually Use
Stem cells aren’t extracted from a fully formed embryo. They come from a very early structure called a blastocyst, which forms about five to six days after fertilization. At this point, the embryo is a hollow ball of roughly 100 to 200 cells, smaller than a grain of sand.
Inside the blastocyst is a cluster of cells called the inner cell mass. This is the part that would eventually develop into a fetus if the embryo were implanted in a uterus. Scientists isolate this inner cell mass, typically on day six after fertilization, when the cell cluster is most distinct and yields the best results. Research published in Cell Stem Cell found that isolating cells on day six produced a derivation efficiency of about 50%, meaning half of attempts resulted in a viable stem cell line.
The outer layer of the blastocyst, which would normally become the placenta, is removed during the process. This means the embryo does not remain intact or viable after the inner cell mass is extracted.
What Makes These Cells Valuable
Embryonic stem cells have two properties that set them apart from almost every other cell in the body. First, they can divide and copy themselves indefinitely in a lab, a trait called self-renewal. Second, they are pluripotent, meaning they can become virtually any cell type: heart muscle, neurons, insulin-producing pancreatic cells, blood cells, and more.
These capabilities make them distinct from adult stem cells, which exist in specific tissues and can only produce a limited range of cell types. Induced pluripotent stem cells, created by reprogramming ordinary adult cells, share some of these abilities but are generated through a fundamentally different process. Embryonic stem cells remain the benchmark against which other stem cell types are measured.
Embryo Quality and Stem Cell Success
Not every donated embryo produces a usable stem cell line. The quality of the embryo matters significantly, and labs assess embryos using grading systems that evaluate the size and health of the inner cell mass and outer cell layer. Embryos with large, compact inner cell masses (graded A or B) are the best candidates for successful derivation.
Many donated embryos are ones that clinics have flagged as lower quality for reproductive purposes, perhaps because they developed slowly, showed fragmented cells, or had fertilization abnormalities. These “poor quality” embryos can still produce stem cell lines, but at lower rates, typically between 4% and 25% efficiency. Even among lower-quality embryos, reaching the blastocyst stage and having a good inner cell mass are the strongest predictors of success.
Consent and Ethical Requirements
Donating embryos for stem cell research is a voluntary process with specific ethical safeguards. Under NIH guidelines, donors must provide written informed consent. No payments of any kind can be offered for donated embryos. The quality of a patient’s fertility care cannot be affected by their decision to donate or refuse. And the fertility doctor treating the couple generally cannot be the same person conducting the research, creating a clear separation between clinical care and science.
Donors can withdraw consent at any point before the embryos are actually used to create a stem cell line or before their identifying information is removed. They must also be told that the research will not benefit them directly and that the resulting stem cells may benefit anyone, without restriction.
Despite these standards, transparency gaps exist. A survey of 66 U.S. IVF clinics found that only 30% of egg donor consent forms mentioned the possibility that embryos could be used for research. Just 8% of clinics that allow embryo donation for stem cell work actually informed their egg donors of that possibility.
Embryos Created Through Cloning
A small number of embryonic stem cell lines have been created from embryos produced through a cloning technique called somatic cell nuclear transfer. In this process, scientists remove the DNA from a donated egg cell and replace it with DNA from an adult patient’s skin or blood cell. The resulting embryo is genetically matched to the patient, which could theoretically allow researchers to create personalized stem cells for studying disease or developing therapies without immune rejection.
This approach was first achieved in humans in 2013, after more than a decade of failed attempts. Researchers were able to derive stable stem cell lines from cloned blastocysts using as few as two high-quality donated eggs. However, the technique remains technically difficult and is not widely used. Several countries, including the United Kingdom and China, permit it under regulated conditions. U.S. federal funding does not cover stem cell lines derived from embryos created specifically for research, which limits the practical application of this method domestically.
How Policies Differ Around the World
The types of embryos permitted for stem cell research vary by country. The UK allows destruction of embryos for stem cell derivation and permits cloning-based techniques, provided the research aims to increase knowledge about embryo development or serious disease. Singapore authorizes the use of embryos up to two weeks old for therapeutic research. China has some of the least restrictive policies globally, permitting surplus IVF embryos, fetal cells from abortions, cloned embryos, and voluntarily donated reproductive cells.
Brazil takes a more conservative approach, allowing only IVF embryos that have been frozen for more than three years. Australia restricts research to embryos left over from assisted reproduction before a specific 2002 cutoff date. These differences reflect varying cultural, religious, and political perspectives on the moral status of early-stage embryos, and they shape which stem cell lines are available to researchers in each country.