Re: Why can't we regrow body parts?

That is a very good and difficult question. Unfortunately, science has not been able to provide a good answer as of yet. Throughout the animal kingdom, there are examples where organisms can regenerate body parts when they are lost. Examples include parts of different kinds of worms, fish fins, limbs of some amphibians, and arms of seastars (as you mentioned). In rare cases, baby mammals (i.e. the opossum) can partially regenerate a limb if they lose it before a certain age. In humans, we can regenerate parts of our liver, and of course, we constantly produce new skin and intestinal lining. Also, as you noted, our bodies can heal its wounds. However, it is important to note that healing a wound and rebuilding an arm are very different processes. As an embryo develops, it follows an innate program, or blueprint for properly building a body with its different parts. The simplest way to think about any regenerative process is that when an animal loses a body part, it can still recall the blueprint for that body part and can therefore rebuild it. Meanwhile, in humans, our bodies cannot recall the blueprint for an limb after embryogenesis, and so we can't rebuild it if we lose it.

Now, let me see if I can explain a bit about how the blueprint is actually recalled. In an animal's body, there are many many cells. Cells can have a variety of different roles. Some are committed to be bone cells, some muscle cells, some nerve cells, etc.. For a body part to be regenerated, you need some cells that are not committed to any one job (called "stem cells"). Now it is true that if you cut off your arm right now, you have stem cells for all the tissue you would need. You have stem cells for muscle, bone, skin, blood, etc. all in your arm right now. The blueprint/program can only be recalled by stem cells. The problem is that certain stem cells, like the ones in your arm, don't know how to coordinate themselves to initiate the "blueprint". The blueprint for building any body part is not a one-cell job. It is an extremely complex and beautiful example of teamwork between many different kinds of cells. While the stem cells in the arm of a newt may be able to organize and "talk" amongst themselves to regrow the limb, yours and mine simply aren't that talented.

It is true that we can heal cuts and other wounds. This is due to the ability of a limited group of stem cells (mostly skin and blood) to organize and follow the blueprint for healthy skin. Why they can do this and not regrow the whole arm is the subject of lots of research that has been going on for well over a hundred years.

Good question, and I hope I helped a little!