DEAR DOCTOR K:
People have been talking about stem cells as a revolutionary technique for a long time. Has anything come of it? And why has it been so controversial?
You’re right; people have been excited about stem cells for nearly 25 years. Yet progress was quite slow, and some people had major ethical concerns. But in the past eight years, progress has accelerated and the ethical issues largely have been circumvented.
Theoretically, stem cells could be used to replace cells that are killed by disease — heart cells killed in a heart attack, or brain cells killed by Alzheimer’s disease. But to explain the exciting potential of stem cells, I need first to define some terms and concepts.
Each human being is made of trillions of cells. They are grouped together into organs, such as the eye and the stomach. Each organ contains types of specialized cells, and each type looks and acts differently.
Each of these very different cells came from just one original cell: the fertilized egg. The egg divides and keeps dividing until one cell has become trillions. Inside every human cell are a little over 20,000 genes — the same 20,000-plus genes. The genes inside a light-sensing cell in the eye are the same as the genes in an acid-making cell in the stomach. So if both cells have the same genes, why are these two different types of specialized cells so different from each other?
What defines a specialized cell is what genes inside the cell are turned on. The eye cell and the stomach cell have different genes turned on.
OK, back to the fertilized egg: About five days after the egg is fertilized, it has divided into several hundred cells. Some of these cells are called embryonic stem cells. They have the ability to turn into any type of specialized cell in the body. Indeed, they promptly begin to do that, and the embryo starts to form organs.
With a few exceptions, once a cell has become specialized, it will stay that way until it dies. For decades we believed that an embryonic stem cell could turn into a specialized cell, but not vice versa.
For example, suppose you needed new heart cells because of a heart attack. Until 2007, there were two problems with using embryonic stem cells to do this. First, the embryonic stem cells would come from the embryo of another human being. Your immune system might well recognize those cells as foreign and attack them. Second, many of those embryos would have come from induced abortions, raising ethical questions.
If only you could use your own embryonic stem cells, that would solve both problems. But of course your own embryonic stem cells existed only briefly, a long time ago, and you can’t turn back the clock.
Then, in 2007, a research breakthrough changed everything. The breakthrough basically showed that you can turn back the clock! Keep a copy of today’s column, and tune in tomorrow to see how.