Stem cells as we know are those cells in the body that can grow into any other cells as and when required. Unfortunately adult humans have a very limited set of these cells in specific locations like liver and bone marrow.
In animals like salamander for instance stem cells grow back an entire limb that has been cut off!
The question here is how do stem cells know how much of a limb has been cut off? For instance in salamanders when 1/4 of a limb is cut off, exactly the missing 1/4 grows back into place. If 3/4 of a limb is cut off, exactly the missing 3/4 part grows back into place!! So where is the information stored about what portion has to be grown back?
Well, the obvious region should be the place of amputation where the limb has been cut off. There should be some indicator here which informs the stem cells WHAT EXISTED AT THE CUT OFF REGION.
Researchers have discovered an indicator for this. It is a protein called CD59. The concentration of this protein varies in different parts of the body. For instance in a limb this protein has highest concentration at the place where the limb is attached to the main body and slowly decreases as you move down away from the main body and is the lowest at the ends of a limb where there are fingers.
So CD59 are like address specifiers in the body whose concentration tells the surrounding cells as to in which part of the body those cells are located exactly! An excellent design/evolution!!
So far so fine. But what about some exceptions? What if CD59 concentration changes in some part accidentally? What happens if there is an amputation in that place??
For instance what if at the fingers of a salamander CD59 was increased to the level that is found at the place where its limb is attached to its body, and then we cut off the fingers?? Well, it was observed that usually such variations result in the growth of deformed body parts!!!
In other words natural growth of stem cells into body parts demands that there be no manipulations artificial or otherwise in the level of CD59 concentration. There are no exceptions that are handled here! As a software rule says ‘Garbage In -> Garbage Out’
Once we master the use of stem cells, people without major or minor organ or body part loss need not worry anymore. Stem cells can be used to grow back any part of the body that has been lost or damaged. People with diabetes for instance can remove their non functional pancreas and replace it with a new functional pancreas and start enjoying sweet dishes. Those who have lost their legs/hands in accidents can grow them back!! Smokers who have their lungs damaged can get a new pair of lungs (of course for them brain and blood stream continue to be an issue!)
So provided there are no nuclear wars that annihilate human civilization on earth in the near future, we will be moving towards an interesting era of medical excellence. Still, a long way to move towards becoming a Type I civilization 
