Q1. How are human embryonic stem cells obtained for research, and at what stage of development?
A. Human embryonic stem cells are obtained for research in two ways. Some come from surplus fertilised eggs donated by couples undergoing in vitro fertilisation (IVF) treatment. These would otherwise be destroyed. Alternatively, embryonic stem cells can be obtained from embryos created using cell nuclear transfer (CNT).
Stem cells can be extracted from embryos after five days of growth – in many countries that allow research on embryos, these can only be worked with until they are 14 days old.
Q2. How are embryonic stem cells grown in the laboratory?
A. Human embryonic stem cells are isolated by transferring the inner cell mass from an early embryo into a dish containing a nutrient broth known as culture medium. The cells divide and spread over the surface of the dish. The inner surface of the culture dish is typically coated with mouse embryonic skin cells that have been treated so they will not divide. This coating layer of cells is called a feeder layer: it gives the inner cell mass cells a sticky surface to which they can attach. The feeder cells also release nutrients into the culture medium. Scientists have begun to investigate ways of growing embryonic stem cells without the mouse feeder cells. This is a significant scientific advancement because of the risk that viruses or other macromolecules in the mouse cells may be transmitted to the human cells.
Over the course of several days, the cells of the inner cell mass grow and divide and are removed and plated into several fresh culture dishes. The process of replating the cells is repeated many times and for many months, and is called subculturing. After six months or longer, the original 30 cells of the inner cell mass yield millions of embryonic stem cells, referred to as an embryonic stem cell line.
Once cell lines are established, batches can be frozen and stored or shipped to other laboratories for further growth and experimentation.
Q3. Why not use human adult stem cells instead of human embryonic stem cells in research?
A. Embryonic stem cells are pluripotent, which means they have the ability to form all cell types of the body. Adult stem cells, however, are generally limited to differentiating into cell types of their tissue of origin. However, there is now evidence to suggest that some adult stem cells, referred to as multipotent, may be able to create a limited number of specialised cell types.
Large numbers of cells are needed for stem cell replacement therapies. While large numbers of embryonic stem cells can be grown relatively easily in culture, adult stem cells are rare in mature tissues and methods for growing them in cell culture have yet to be discovered.
Q4. Have stem cells been used to treat any human diseases yet?
A. Adult stem cells such as blood-forming stem cells in bone marrow (hematopoietic stem cells or HSCs) are currently the only type of stem cell commonly used to treat human disease. Doctors have been transferring HSCs in bone marrow transplants for over 40 years. More advanced techniques for collecting HSCs are now used to treat leukaemia, lymphoma and several other blood disorders.
The clinical potential of adult stem cells has also been demonstrated in the treatment of other diseases, including diabetes, Parkinson’s disease and advanced kidney cancer. For example, in 1999 scientists in the USA removed 10 to 15 neural stem cells from a Parkinson’s disease patient and used them to reproduce 6 million dopaminergic neural stem cells in culture. These were reintroduced into the patient’s brain tissue, producing a 62% increase in dopamine uptake and a 40–50% improvement in certain motor tasks. However, these newer uses of adult somatic stem cells have involved studies with very limited numbers of patients.
Embryonic stem cells have not been used to treat any human diseases yet, but are thought to offer potential cures and therapies for many devastating diseases. Scientists only began studying embryonic stem cells in the late 1990s, so much of the research is still at an early stage
Q5. How are embryonic stem cells stimulated to differentiate into specialised cells?
A. As long as embryonic stem cells are grown under certain conditions, they remain unspecialised. However, if they are allowed to clump together to form embryoid bodies, they begin to differentiate spontaneously, forming specialised muscle cells, nerve cells, and many other cell types. Although this spontaneous differentiation is a good indication that a culture of embryonic stem cells is healthy, it is not an efficient way to produce cultures of specific cell types.
To generate specific types of differentiated cells – for example, heart muscle cells, blood cells or nerve cells – researchers need to learn how to control and direct the specialisation of embryonic stem cells. They can change the chemical composition of the culture medium, alter the surface of the culture dish, or modify the cells by inserting specific genes. Through years of experimentation scientists have established some basic protocols or ‘recipes’ for directing embryonic stem cells to become some specific cell types. However, more research is still needed before scientists can direct embryonic stem cell specialisation reliably enough, or under the correct conditions, to use the resulting cells to treat human diseases.