Embryonic stem cells are grown in the laboratory from a small group of cells found in the very early embryo. Human embryonic stem cells are obtained from embryos that are 5-6 days old. At this stage, the embryo is called a blastocyst and is a ball of around 100 to 150 cells.
Like all other stem cells, embryonic stem cells can both self-renew (copy themselves) and differentiate (produce more specialized types of cells). However, embryonic stem cells are particularly powerful because they are pluripotent – they can form all the different types of cell in the body. Watch our film A Stem Cell Story to find out more.
Read more: Fact sheet on embryonic stem cells
Human embryonic stem cells (hESCs) can be used in research to:
Researchers can use hESCs to produce specialized cells like nerve or heart cells in the lab. These specialized cells can be studied in detail to understand more about the causes and development of diseases. They can also be used to reveal how our cells react to, or could be treated with potential new drugs. This is particularly useful for studying types of cells that cannot easily be obtained by taking tissue from patients, e.g. brain cells.
Recently, hESCs have been produced that meet the strict quality requirements for use in medical treatments. These ‘clinical grade’ hESCs have been approved for use in a small number of early clinical trials. One example is a trial led by The London Project to Cure Blindness, using hESCs to produce a particular type of eye cell for treatment of patients with the eye disease age-related macular degeneration. The biotechnology company ACT is also using human ESCs to make cells for patients with an eye disease: Stargardt’s macular dystrophy.
No, researchers do not need to start from a new embryo for every study they carry out. Cells taken from one embryo can be made to multiply in the laboratory to create a ‘cell line’ that is able to produce an almost infinite number of embryonic stem cells, all with the same genetic make-up. Many embryonic stem cell lines are kept in non-profit stem cell banks that can be accessed by researchers all over the world. Existing cell lines are also exchanged at no cost between laboratories in the context of research programmes, under tight legal controls.
Different European countries have different laws and regulations about embryonic stem cell research. Some only permit use of existing cell lines, whilst others do allow the creation of new cell lines from embryos. In both cases, researchers must first obtain a licence from their national regulatory agency. Licensing procedures include checks by ethicists, and the researcher must show that the requested use of embryos or embryonic stem cells is necessary to answer relevant research questions.
In the EU (except the UK), new human embryonic stem cell lines are always made from embryos that were created for fertility treatment, but not used. Most embryos used in UK stem cell research are also obtained in this way, although UK regulation does permit creation of embryos in the laboratory for research, under very strict licensing conditions.
Researchers use existing embryonic stem cell lines wherever possible. Since a cell line always produces cells with the same genes, it is sometimes necessary to create a new cell line to answer key research questions about the roles of different genes in development or disease. The way a cell line is created can also have an impact on how the cells behave. A new cell line created under new experimental conditions or grown in different culture media may produce stem cells that are more able to generate a sophisticated array of specialized cells with the properties needed for a particular study or type of application. Researchers may also need to generate new cell lines to study the very first steps of growth and development of the cells, to learn more about how this process works and which genes are involved. Those studying the very early steps of human embryo development need to look at these processes carefully using the cells that are involved in nature; iPS cells are not able to answer these questions.
Exampls of registries or banks containing human embryonic stem cell lines:
Much progress has been made since the first human embryonic stem cells were grown in the lab in 1998. The first clinical trials testing the safety of using specialised cells grown from hESCs are just beginning. However, scientists are still learning how to control the differentiation of embryonic stem cells into specialised cells. It is not yet possible to make pure and fully functional specialised cells of every type found in the body, starting from hESCs in the lab. hESCs do already give scientists access to cell types that would be difficult or impossible to obtain in other ways, such as the nerve cells that are affected in Parkinson’s disease. But more work is needed to understand and control hESCs if they are to fulfill their potential for use in future treatments. Further clinical research will also be vital to establish how cells made from hESCs (or any other stem cell type) survive and behave after transplantation into patients.
Some groups have raised objections to the use of human embryonic stem cells in research, on moral, ethical or religious grounds. Some of the views on this issue are described in our fact sheet on the embryonic stem cell research and ethics.
Most scientists agree that research should continue on ALL types of stem cells. It is not yet clear which cells will be most useful for which types of treatments. No other type of stem cell can currently entirely replace human embryonic stem cells (hESCs) in today’s research:
Tissue stem cells are also called adult stem cells and exist in our bodies all our lives. They are restricted – they can only make the types of specialized cells that belong in their own tissue. Despite some claims to the contrary, no tissue stem cells have been shown to be pluripotent (able to make all the types of cells in the body). Skin and blood stem cells have been in use in the clinic for decades and tissue stem cells are undoubtedly valuable for future research and applications. However, scientists are still learning how to multiply, control and use different types of tissue stem cells. Tissue stem cells have not been obtained for every tissue of the body and are not always easily accessible. In addition, when placed in a dish they have a tendency to lose their ‘memory’ of what they are and what cells they are expected to produce. This means scientists need to develop systems for growing each type of tissue stem cell in a way that maintains the properties they would have in the body. Embryonic stem cells give researchers access to certain types of cells that are vital for disease research and are not available from tissue stem cells, such as neurons (nerve cells) for Parkinson’s disease research.
In 2006, scientist Shinya Yamanaka discovered that specialised adult cells can be ‘reprogrammed’ into cells that behave like embryonic stem cells, termed induced pluripotent stem cells (iPS cells). This discovery has led some people to suggest that work on hESCs is no longer needed. However, most scientists agree that side-by-side research on both hESCs and iPS cells is still required and will continue to be necessary for the foreseeable future. This is because:
iPS cells are not yet fully understood
Researchers do not yet know precisely how the process of reprogramming works, and there are important differences between the behaviour of hESCs and iPS cells that are not understood. It is necessary to continue to compare hESCs and iPS cells to establish how iPS cells work and what the advantages and limitations of each of these cell types are.
Safety questions still need to be resolved
iPS cells are created by reprogramming adult cells using laboratory techniques. They do not exist in our body nor appear during our development. The techniques used to make them have not yet been perfected and can produce abnormal, potentially unsafe cells. iPS cells can also undergo unpredictable changes in their genetic make-up when they are grown and multiplied over a period of time, which can affect their behaviour. As with hESCs, iPS cells can self-renew (copy themselves) indefinitely and this property must be turned off before use in therapies to avoid formation of tumours.
So iPS cells are very promising tools to investigate diseases and develop new drugs in the laboratory, but many more careful studies are needed to determine whether they will be safe to use in treatments for patients. As with hESCs, more work is also needed to understand how to control iPSCs to produce particular types of fully functional specialized cells. Continued comparison with hESCs will be important as research develops, and will aid progress since information about each of these types of cells can help improve understanding of the other.
iPS cells are not yet ready for the clinic
Cells that will eventually be transplanted into people must be prepared in a strictly controlled environment to ensure they are not contaminated and are of high quality and purity. Reaching this point on the road to the clinic is the result of years of research and development. Currently, no iPS cells are approved for clinical use because of the gene manipulation procedures required for their production. It will take time to overcome this hurdle and develop clinic-ready iPS cell lines. Most scientists think that the best use of iPS cells will be in drug discovery, certainly in the short- to medium term. Taking our adult cells backwards in life to an embryonic stage to produce iPS cells in a consistent and safe way may require a level of control over the DNA in the cells that we are far from understanding.
iPS technology is based on understanding human embryonic stem cells
The discovery of iPS cells was based on an understanding of hESC biology. It is not yet clear which cells will be most useful in which ways, and many questions remain to be answered. Further hES cell research could open up more unforeseen avenues of research and applications.
Other types of cell reprogramming are in their infancy
Recent research has shown that it is possible to convert adult cells directly from one type of specialized cell into another. This is termed direct reprogramming or transdifferentiation. However, it is not yet clear whether it will be possible to generate the large number of cells needed for treatments using this technique. As with iPS cells, the reprogramming process is not yet fully understood and many of the same questions apply to direct reprogramming as described above for iPS cells.