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Embryonic stem cells: where do they come from and what can they do?

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? 

What are researchers working on?

Researchers are learning how to create many different specialised cell types found in the body. They do this by exposing ESCs to different chemical signals and growth conditions. Many tissues, such as nerves and muscles, can now be grown from ESCs in a lab outside the body. 

Mouse ESCs are being used to learn about how the body develops, from early embryonic stages to the formation of complex organs. Scientists can study early developmental process outside the body by growing them in suspension (floating in a liquid containing nutrients). ESCs grown this way aggregate (form clumps of cells). Scientists can use these cell aggregates to recreate and investigate early developmental processes. 

These ESC aggregates can even undergo gastrulation. This is the process in which the cells start turning into different ‘types’ of tissue: ectoderm (neurons and skin), mesoderm (heart and muscle), and endoderm (gut). Researchers are learning how cells assemble into complex tissues, such as the layers of the brain, by attempting to grow primitive organs in the lab. Specific tissues grown from ESCs in the lab can be used to screen drugs for toxicity or other effects. 

Many studies are looking into how ESCs might be able to be used to treat numerous different diseases, from multiple sclerosis to blindness and diabetes.

Mouse stem cell basics

If an inner cell mass is taken from a mouse blastocyst and given the right nutrients and signals to prevent differentiation, the pluripotent cells can grow in the laboratory. The process of cell maturation and specialisation that would normally take place in the embryo stops. Instead, the cells multiply to make greater numbers of undifferentiated cells that resemble the cells of the inner cell mass. These laboratory-grown cells are called embryonic stem cells (ESCs). 

Diagram showing the paths of ESCs: self renewal and differentiation in the lab, or development into a foetus iif allowed to implant

Using mouse embryonic stem cells for science and medicine

Mouse ESCs grown in the lab can be injected back into a mouse blastocyst, and this blastocyst can then be returned to the uterus of a female mouse to develop into a foetus. The injected ESCs take part in the development of the foetus and the resulting pup is born with a mixture of cells: those from the host blastocyst and those that came from the injected ESCs. This new mouse with cells from two different origins is known as a chimaera.  

Researchers can precisely alter the genes of ESCs cells in the lab, put the cells back into blastocysts, and produce new chimaera mice that contain the modified genes. These chimaera mice can pass genes from ESCs on to their offspring, producing some mice which have only the altered gene. 

Scientists use genetically altered mice made from ESCs 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 cell to create genetically modified mice. 

What's next?

Human and mouse ESCs have some different properties. Scientists are trying to understand why this is and whether human cells can be obtained with the same properties as the mouse ESCs. 

Researchers are also working to expand and perfect methods for making specific adult cell types from ESCs in the lab. Controlling exactly how ESCs differentiate is still a major challenge. Even so, some scientists are already investigating whether ES cells can be used to make adult cells that could be transplanted into patients to help heal injured or diseased tissue. 

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