In reading the many stem cell diaries of late, there have been a few people who have asked about the basics of stem cells. Here I hope to provide the basic science of stem cells, define the terminology and lay out the foundations of argument over stem cells and their use.
This is my first diary, so I have no bells and whistles and hopefully won't bore you to death.
Introduction
Stem cells are cells that have the potential to become other kinds of cells. Most cells in our bodies are specific types, like skin cells, hair cells, heart cells, etc. These cells are differentiated - they have become the type they were fated to be. Cell fate is determined during development of a fertilized egg into an embryo. Once a cell has differentiated, that's the only kind of cell it can be. This can be a medical issue if say, an organ is damaged and no longer functions properly - your body can't make a new one.
Stem cells are undifferentiated cells. The ultimate stem cell is the fertilized egg, also called the zygote. This one cells will divide and become all the other different kinds of cells. A fertilized egg is called "totipotent" or all powerful because of this property.
Embryonic stem cells
So two people have that special moment together. The woman has, possibly unbeknownst to her, recently released an ovum into her fallopian tube and well, the guy's sperm find it. One lucky sperm burrows into the egg and voila - a fertilized egg is made.
After fertilization, the cell begins to divide. First there are 2 cells then 4 then 8. Up to this point, any of these cells is totipotent and can become a whole new organism. Division continues. Fairly soon, the embryo (the ball of cells is called that until week 8, when it becomes the fetus) starts to change shape. The embryo forms a hollow ball of cells called a blastula or blastocyst. The embryo is 5-6 days old and has not yet implanted in the uterus (it's still traveling down the fallopian tube). This is the beginning of differentiation, as there is now an inside, a middle, and an outside. These cells are "pluripotent" or multiple powerful and can become a variety of cells, though they are now limited to a certain subset depending on where they are found in the blastula.
As time goes on and development continues, the cells differentiate more and more and are fated to be certain things. These cells are no longer stem cells and can become only one kind of cell.
Adult stem cells
There are certain types of stem cells still found in adults. The classic example are stem cells found in the bone marrow. The thing about adult stem cells is that they are not totipotent and are fairly narrow in the types of cells that they produce. Bone marrow cells can make all the cells found in your blood, red blood cells, many categories of white blood cells, and the cells that become platelets. These cells are the basis of bone marrow transplants for individuals with blood related disorders including leukemia. This is a form of stem cell therapy and has been practiced for decades.
Stem cell potential
Stem cells may be the next great advance in medicine, on par with germ theory and the discovery and use of antibiotics. The potential uses of stem cells include organ replacement, nerve regeneration, degenerative disease treatment, and cancer treatment, just to name a few possibilities.
Imagine you have heart failure. A possible treatment is a heart transplant, but the difficulties of transplant include finding a healthy donor heart that matches yours in size and at least blood type, finding that donor heart in time to do you any good, and post-transplant rejection of the donor heart. Our bodies recognize our own cells and also recognize foreign cells. Foreign cells are attacked by the immune system as if they were disease. That's rejection. Using stem cells to grow you a new heart of your very own that would be recognized by your body as your own is a possible application of stem cell therapy.
Another possibility is repairing nerve tissue. A recent diary by 2kate2 discussed this possibility. Nerve cells don't divide, unlike all the other types of cells we have, so once they're damaged, that's it. With stem cell therapy, it would be possible to put new nerve cells where the damaged ones are and cure paralysis for instance.
Stem cell debate
The current controversies regarding stem cells center around stem cell research and the sources of stem cells. The possibilities of stem cells are huge, but research is being stalled in the US because of limitations on federal funding to research on stem cells lines. In 2001, President Bush restricted this funding of stem cell research to the 60+ existing stem cell lines.
The problem here is that stem cells WANT to differentiate - that's what they do, so these stem cells are being lost over time. Also many of those existing stem cell lines are from MICE - not humans. There is a potential for viral infection from mice to humans using these stem cells. There is more information here.
In order to explore the vast potential of stem cell research, we need more stem cells, preferably human ones. The most recent debate has involved the veto of a bill allowing the donation of frozen embryos, which would otherwise be destroyed, from fertility clinics to be used in this research.
President Bush states that destroying life to save life is morally reprehensible. This presupposes that these embryos are alive - tying this issue to the "pro-life" position regarding abortion. We know that not everyone agrees to this premise, but that some do and therefore equate the creation of stem cell lines to murder. We also know that the embryos in question are likely to be destroyed and perhaps a better use would be to use them to explore the possibilities of stem cell therapy.
The Future
It may be possible to develop pluripotent stem cells from adult stem cells or even differentiated cells - in fact that's what Dolly the sheep was all about. Dolly was created by de-differentiating an adult cell. BUT human cloning is banned and Dolly had issues that may be due to being a clone. And while this may be possible in the future, research on stem cell therapies would be stalled until then.
Conclusion
It was my aim to answer the few people who expressed a desire to know more about stem cells. I hope this helps. I have avoided going into a lot of detail and glossed over many things. I welcome any questions, requests for clarifications, corrections, additions, etc.