What are 'stem cells'?
A "stem cell' is defined as a cell which can divide in such a way that at least one of the daughter cells retains just as much ability to continue dividing as the original mother cell had. And generally, exactly one of the two cells retains that property. And the other one becomes a little bit more, what we call, differentiated and starts down a pathway of commitment to becoming a particular type of normal cell like - you know, a blood cell, or a liver cell, or whatever. And most of our tissues are made up predominately of the differentiated cells, but we have a small minority of the stem cells that are able to divide to give rise to the replacement of any cells that die. Different tissues have different numbers of stems cells. And different types of stem cells divide at different frequencies or in different circumstances. So there are many different types of stem cell. In particular, very early on in life, early in pregnancy, we have particular types of stem cell that are very versatile; very able to become lots of different other types of cell. Those are called embryonic stem cells. And we really don't have any of them in the body at the moment. We would like to be able to take cells that we do have in adult bodies and revert them to this embryonic stem cell state - and we're getting much better at that. There are now reports of being able to do that rather well. Which would be great if we could do it, because then we wouldn't need to destroy embryos in order to extract embryonic stem cells from them, and that would solve a whole bunch of ethical issues. And they are of course, very well-known in the United States. The main problem with harvesting embryonic stem cells from an adult body is that we would have to reverse a whole bunch of changes that spontaneously happen in the process of development from an embryo through to being a fetus and then a baby, and then an adult. And those processes just cause all of the embryonic stem cells to become more committed cells - stem cells that are less powerful. That's the main problem. Embryonic stem cells however, are really useful because we sometimes need cells that can be developed into all manner of different types of cell. And we just - the easiest way to get them, the most effective way to get them is by taking them from early embryo. There is a method called, "stem cell nuclear transfer," which is part of this process. And it's a trick that allows the embryonic stem cells we derived to avoid being rejected by our immune system after they have been differentiated into a different type of cell and put into the body to do some particular job.
