Embryonic stem cells have captured the imagination of scientists and non-scientists alike. But what are they, where do they come from and why are researchers all over the world working hard to understand more about them?
Embryonic stem cells are derived from very early embryos called blastocysts; the diameter of a human blastocyst is roughly four times that of a human hair
A mouse blastocyst aged 3.5 days; the inner cell mass is coloured green and the trophectoderm is coloured red
A chimeric mouse and his offspring; the offspring have a gene for black hair from the ES cells used to make their father
Embryonic stem cells are grown from cells found in the embryo when it is just a few days old. In humans, mice and other mammals, the embryo is a ball of approximately 100 cells at this stage. It is known as a blastocyst and has two parts:
The cells of the inner cell mass are pluripotent: they can make every type of cell in the body.
If an inner cell mass is taken from a mouse blastocyst and given the right nutrients, it can be grown in the laboratory. The process of cell maturation and specialization that would normally take place in the embryo stops. Instead, the cells multiply to make more undifferentiated cells that resemble the cells of the inner cell mass. These laboratory-grown cells are called embryonic stem (ES) cells.
Mouse ES cells can be put back into a mouse blastocyst and this blastocyst can then be returned to the uterus of a female mouse to grow. The injected ES cells take part in the development of the embryo and the resulting pup is born with a mixture of cells, (a) from the host blastocyst and (b) cells that came from the injected ES cells. This new mouse with cells from two different origins is known as a chimera.
Chimeras can pass on genes from embryonic stem cells to their offspring. Researchers can therefore alter the genes of ES cells in the lab, put the cells back into blastocysts, and produce new mice that have the altered genes, or ‘mutations’. Scientists use genetically altered mice made from ES cells to study genes involved in many human diseases. For example, they have made mice with mutations found in human cancers. These mice can be studied to learn more about how cancers grow and to test potential drugs. The immense value of this technique for science and medicine was recognized in 2007 when the Nobel Prize in Medicine was awarded to three scientists who pioneered the use of ES cells to create genetically modified mice.
Scientists have also made ES cells from the inner cell masses of human embryos by employing the same technique used to isolate mouse ES cells. You can learn more about the human embryos used in this research and where they come from in the factsheet, Embyronic stem cell research: an ethical dilemma. While it is not allowed to inject human ES cells back into blastocysts, these cells can be differentiated to make many different specialized cell types in the laboratory. By using human ES cells to produce specialized cells like neurons (nerve cells) or heart cells in the lab, scientists can gain access to adult human cells without taking tissue from patients. They can then study these specialized adult cells in detail to see what goes wrong in certain diseases, or to study how the cells respond to potential new drugs.
So far, scientists have made ES cells from mice and rats, humans and other primates. In the future, they hope to make ES cells from other species too. Researchers are also working to expand and perfect methods for making particular adult cell types from ES cells in the lab. Controlling exactly how ES cells differentiate is still a major challenge. Even so, some scientists are beginning to investigate whether ES cells could one day be used to make adult cells that could be transplanted into patients to help heal injured or diseased tissue.
EuroStemCell film, "A Stem Cell Story"
EuroStemCell FAQ on human embryonic stem cells and their use in research
EuroStemCell factsheet on the ethics of using embryos in research
EuroStemCell factsheet on ethical issues relating to the sources of embryonic stem cells
Public information on the Nobel Prize 2007, awarded to 3 scientists for work on ES cells
Diagram showing how chimeric mouse are made, from NobelPrize.org
Human blastocyst image (lead image) by Mila Roode. Mouse blastocystphotograph by Maria Elena Torres Padilla. Chimeric mouse photograph by Jenny Nichols. Neuron photograph by Oliver Bruestle. All other images and diagrams by Kate Blair.