The one-cell embryo develops into a fully formed human being through a process called cell differentiation. As an embryo develops, embryonic stem cells form the different cells that make up our bodies. As they go further along the developmental pathway, they become more and more specialised until they are only able to perform one function. For example, stem cells that specialise into muscle cells eventually lose the ability to do anything else.
Embryonic stem cells grown by scientists in the laboratory retain the power to become any cell in the human body through differentiation. This is known as pluripotency. One of the challenges for scientists is to direct this process. To be able to harness the therapeutic potential of stem cells, scientists need to learn how to direct them to differentiate appropriately, for example, to generate muscle cells for damaged hearts or neurones for damaged brains. Another challenge is to ensure they do not continue to multiply in an uncontrolled way and form a tumour.
Adult stem cells in our bodies replenish cells and tissues in their particular location. Because they are already more specialised than embryonic stem cells, it is more difficult to direct them to form different cell types. However, recent experiments have shown this might be possible. For example, researchers have induced adult bone marrow stem cells to form brain and kidney cells, and neural cells to generate blood cells. How this happens is not precisely understood yet.
Cell nuclear transfer
Scientists are also exploring a technique called cell nuclear transfer, which has the potential to create copies of healthy cells to replace or repair damaged or diseased tissues and organs. It involves removing the nucleus from a donated egg and fusing it with a healthy adult cell. The egg-cell combination is then stimulated to develop into a blastocyst, from which embryonic stem cells can be extracted after five days of growth. Obtaining stem cells for potential therapies this way is known as therapeutic cloning.
Cell nuclear transfer can also be used for reproductive cloning, although this is illegal in humans in many countries.
In 1996, scientists at the Roslin Institute in Edinburgh used cell nuclear transfer to produce a sheep from a mammary cell (a specialised cell) of a six-year-old sheep. They did this by inserting the nucleus of the mammary cell into a sheep egg cell that had had its nucleus removed. Factors in the egg reprogrammed the nucleus of the mammary cell and made it forget its original destiny. Instead, the nucleus behaved as if it were inside a one-cell embryo. The egg was then implanted into a sheep’s womb, where it grew into a fully formed lamb.
At the moment, doctors often use donated organs and tissues to replace ailing or destroyed tissue, but the need for transplantable tissues and organs far outweighs the available supply. Stem cells offer the possibility of a renewable source of replacement cells and tissues. These could be used to treat conditions such as Parkinson’s and Alzheimer’s diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, rheumatoid arthritis, and vision and hearing loss. Furthermore, if the new specialised cells were produced from the patient’s own stem cells, they would be genetically identical to the patient. This would greatly reduce the risk of rejection, a common problem in any form of transplant operation.
It may also be possible to use stem cells to screen new drugs and toxins in a far more effective manner than currently possible. Ideally the screening of drugs would be undertaken in conditions as close as possible to those encountered in the body. Currently this is not often achievable, and only in particular circumstances can cell lines can be used in this way – for example, immortal cancer cells are already used to screen potential anti-cancer drugs. Stem cells offer the potential for this cell-based approach to be more widely used, since in principle large numbers of specialised cells could be generated for drug toxicity testing, offering a potential alternative to the use of animals. However, to achieve this, scientists will need to first fully understand how to control the differentiation of stem cells into the specific type of cell needed for the tests.
Stem cells could also be study to increase our understanding of abnormal cell division and differentiation, which causes cancer and birth defects. But to reach that point, scientists must first identify the fundamental properties of stem cells.