Stem Cells - What Are They? (cont.)
To give you some perspective on the amount of activity that happens as a result of stem cell differentiation (turning into one of the more specialized cells of the blood), one can calculate how many cells are required or are produced by this system every day of your life. There are approximately 2-3 million red cells produced every second of your life. There are approximately 50 billion granulocytes required to fight infection every day of your life. These cells are vital to survival. Without them, you would die from anemia or infection.
The formation of mature blood cells from immature committed stem cells has been very well studied over the past 30 years. It provides an excellent paradigm, or model, for other organs.
It is now believed that all of the other organs possess similar, or committed, adult stem cells within them. Hence, there are now believed to be 20 or more different types of adult stem cells that are potentially amenable to manipulation.
One of the exciting discoveries of the last couple of years has been the overturning of a long-held scientific belief that an adult stem cell was a completely committed stem cell. We used to believe that a hematopoietic, or blood-forming stem cell, could only create other blood cells and could never become another type of stem cell. That belief is probably incorrect. There is now evidence that some of these apparently committed adult stem cells may be able to change direction to become a stem cell in a different organ.
For example, there are some models of bone marrow transplantation in rats with damaged livers in which the liver partially re-grows with cells that are derived from transplanted bone marrow. Similar studies can be done showing that many different cell types can be derived from each other. It appears that heart cells can be grown from bone marrow stem cells, that bone marrow cells can be grown from stem cells derived from muscle, and that brain stem cells can turn into almost anything.
An extended metaphor may help to illuminate this area. When I was in elementary school, I had the potential to become whatever I wanted to be (probably not an artist, but then a human zygote can't become a zebra). As I went through high school, I became more involved, or differentiated, towards biological sciences. I then went to medical school, did an internal medicine residency, followed by hematology training, and ultimately bone marrow transplant training. Currently, I am one of the most specialized specialists that you can come across.
Fifty years ago I would have stayed in that professional role for life. Now, however, I don't have to remain that way. I could be re-educated and become a history professor if I wished.
A similar process appears to be possible for the specialized stem cells in the body. It appears that they could be reeducated and turned into something different.
There is currently much enthusiastic research being carried out in this area of adult stem cell biology. It would clearly be more attractive to be able to use adult stem cells rather than embryonic stem cells to repair patients' hearts or treat their Alzheimer's or Parkinson's disease. This is because none of the ethical, philosophical, and religious problems that arise with embryonic stem cells occur with adult stem cells.
One of the great difficulties with this entire area
currently is that, first, no one is quite sure whether human embryonic stem
cells and human adult stem cells are capable of being manipulated in quite the
specific fashion that can be done in mice and rats. Second, it is not at all
clear whether one stem cell source may be superior to the other. Hence, there is a very strong scientific need to pursue active research using both sources of stem cells.